Template Syntax

Our Angular application manages what the user sees and can do, achieving this through the interaction of a Component class instance (the component) and its user-facing template.

Many of us are familiar with the component/template duality from our experience with model-view-controller (MVC) or model-view-viewmodel (MVVM). In Angular, the component plays the part of the controller/viewmodel, and the template represents the view.

Let’s find out what it takes to write a template for our view. We’ll cover these basic elements of template syntax:

The demonstrates all of the syntax and code snippets described in this chapter.

HTML

HTML is the language of the Angular template. The QuickStart application has a template that is pure HTML:

Hello Angular

Almost all HTML syntax is valid template syntax. The <script> element is a notable exception; it is forbidden, eliminating the risk of script injection attacks. (In practice, <script> is simply ignored.)

Some legal HTML doesn’t make much sense in a template. The <html>, <body>, and <base> elements have no useful role in our repertoire. Pretty much everything else is fair game.

We can extend the HTML vocabulary of our templates with components and directives that appear as new elements and attributes. In the following sections we are going to learn how to get and set DOM (Document Object Model) values dynamically through data binding.

Let’s turn to the first form of data binding — interpolation — to see how much richer template HTML can be.

Interpolation

We met the double-curly braces of interpolation, {{ and }}, early in our Angular education.

<p>My current hero is {{currentHero.firstName}}</p>

We use interpolation to weave calculated strings into the text between HTML element tags and within attribute assignments.

<h3> {{title}} <img src="{{heroImageUrl}}" style="height:30px"> </h3>

The material between the braces is often the name of a component property. Angular replaces that name with the string value of the corresponding component property. In the example above, Angular evaluates the title and heroImageUrl properties and "fills in the blanks", first displaying a bold application title and then a heroic image.

More generally, the material between the braces is a template expression that Angular first evaluates and then converts to a string. The following interpolation illustrates the point by adding the two numbers within braces:

<!-- "The sum of 1 + 1 is 2" --> <p>The sum of 1 + 1 is {{1 + 1}}</p>

The expression can invoke methods of the host component, as we do here with getVal():

<!-- "The sum of 1 + 1 is not 4" --> <p>The sum of 1 + 1 is not {{1 + 1 + getVal()}}</p>

Angular evaluates all expressions in double curly braces, converts the expression results to strings, and links them with neighboring literal strings. Finally, it assigns this composite interpolated result to an element or directive property.

We appear to be inserting the result between element tags and assigning it to attributes. It's convenient to think so, and we rarely suffer for this mistake. Though this is not exactly true. Interpolation is a special syntax that Angular converts into a property binding, and is explained below.

But first, let's take a closer look at template expressions and statements.

Template expressions

A template expression produces a value. Angular executes the expression and assigns it to a property of a binding target; the target might be an HTML element, a component, or a directive.

We put a template expression within the interpolation braces when we wrote {{1 + 1}}. We’ll see template expressions again in the property binding section, appearing in quotes to the right of the = symbol as in [property]="expression".

We write template expressions in a language that looks like JavaScript. Many JavaScript expressions are legal template expressions, but not all.

JavaScript expressions that have or promote side effects are prohibited, including:

Other notable differences from JavaScript syntax include:

Expression context

Perhaps more surprising, template expressions cannot refer to anything in the global namespace. They can’t refer to window or document. They can’t call console.log or Math.max. They are restricted to referencing members of the expression context.

The expression context is typically the component instance, which is the source of binding values.

When we see title wrapped in double-curly braces, {{title}}, we know that title is a property of the data-bound component. When we see isUnchanged in [disabled]="isUnchanged", we know we are referring to that component's isUnchanged property.

The component itself is usually the expression context, in which case the template expression usually references that component.

The expression context can include objects other than the component. A template reference variable is one such alternative context object.

Expression guidelines

Template expressions can make or break an application. Please follow these guidelines:

The only exceptions to these guidelines should be in specific circumstances that you thoroughly understand.

No visible side effects

A template expression should not change any application state other than the value of the target property.

This rule is essential to Angular's "unidirectional data flow" policy. We should never worry that reading a component value might change some other displayed value. The view should be stable throughout a single rendering pass.

Quick execution

Angular executes template expressions more often than we think. They can be called after every keypress or mouse move. Expressions should finish quickly or the user experience may drag, especially on slower devices. Consider caching values computed from other values when the computation is expensive.

Simplicity

Although it's possible to write quite complex template expressions, we really shouldn't.

A property name or method call should be the norm. An occasional Boolean negation (!) is OK. Otherwise, confine application and business logic to the component itself, where it will be easier to develop and test.

Idempotence

An idempotent expression is ideal because it is free of side effects and improves Angular's change detection performance.

In Angular terms, an idempotent expression always returns exactly the same thing until one of its dependent values changes.

Dependent values should not change during a single turn of the event loop. If an idempotent expression returns a string or a number, it returns the same string or number when called twice in a row. If the expression returns an object (including an Array), it returns the same object reference when called twice in a row.

Template statements

A template statement responds to an event raised by a binding target such as an element, component, or directive.

We’ll see template statements in the event binding section, appearing in quotes to the right of the = symbol as in (event)="statement".

A template statement has a side effect. It's how we update application state from user input. There would be no point to responding to an event otherwise.

Responding to events is the other side of Angular's "unidirectional data flow". We're free to change anything, anywhere, during this turn of the event loop.

Like template expressions, template statements use a language that looks like JavaScript. The template statement parser is different than the template expression parser and specifically supports both basic assignment (=) and chaining expressions (with ; or ,).

However, certain JavaScript syntax is not allowed:

Statement context

As with expressions, statements can refer only to what's in the statement context — typically the component instance to which we're binding the event.

Template statements cannot refer to anything in the global namespace. They can’t refer to window or document. They can’t call console.log or Math.max.

The onSave in (click)="onSave()" is sure to be a method of the data-bound component instance.

The statement context may include an object other than the component. A template reference variable is one such alternative context object. We'll frequently see the reserved $event symbol in event binding statements, representing the "message" or "payload" of the raised event.

Statement guidelines

As with expressions, avoid writing complex template statements. A method call or simple property assignment should be the norm.

Now that we have a feel for template expressions and statements, we’re ready to learn about the varieties of data binding syntax beyond interpolation.

Binding syntax: An overview

Data binding is a mechanism for coordinating what users see with application data values. While we could push values to and pull values from HTML, the application is easier to write, read, and maintain if we turn these chores over to a binding framework. We simply declare bindings between binding sources and target HTML elements and let the framework do the work.

Angular provides many kinds of data binding, and we’ll discuss each of them in this chapter. First we'll take a high-level view of Angular data binding and its syntax.

We can group all bindings into three categories by the direction in which data flows. Each category has its distinctive syntax:

Data directionSyntaxBinding type
One-way
from data source
to view target
{{expression}} [target] = "expression" bind-target = "expression"Interpolation
Property
Attribute
Class
Style
One-way
from view target
to data source
(target) = "statement" on-target = "statement"Event
Two-way[(target)] = "expression" bindon-target = "expression"Two-way

Binding types other than interpolation have a target name to the left of the equal sign, either surrounded by punctuation ([], ()) or preceded by a prefix (bind-, on-, bindon-).

What is that target? Before we can answer that question, we must challenge ourselves to look at template HTML in a new way.

A new mental model

With all the power of data binding and our ability to extend the HTML vocabulary with custom markup, it is tempting to think of template HTML as HTML Plus.

Well, it is HTML Plus. But it’s also significantly different than the HTML we’re used to. We really need a new mental model.

In the normal course of HTML development, we create a visual structure with HTML elements, and we modify those elements by setting element attributes with string constants.

<div class="special">Mental Model</div> <img src="images/hero.png"> <button disabled>Save</button>

We still create a structure and initialize attribute values this way in Angular templates.

Then we learn to create new elements with components that encapsulate HTML and drop them into our templates as if they were native HTML elements.

<!-- Normal HTML --> <div class="special">Mental Model</div> <!-- Wow! A new element! --> <hero-detail></hero-detail>

That’s HTML Plus.

Now we start to learn about data binding. The first binding we meet might look like this:

<!-- Bind button disabled state to `isUnchanged` property --> <button [disabled]="isUnchanged">Save</button>

We’ll get to that peculiar bracket notation in a moment. Looking beyond it, our intuition tells us that we’re binding to the button's disabled attribute and setting it to the current value of the component’s isUnchanged property.

Our intuition is wrong! Our everyday HTML mental model is misleading us. In fact, once we start data binding, we are no longer working with HTML attributes. We aren't setting attributes. We are setting the properties of DOM elements, components, and directives.

HTML attribute vs. DOM property

The distinction between an HTML attribute and a DOM property is crucial to understanding how Angular binding works.

Attributes are defined by HTML. Properties are defined by the DOM (Document Object Model).

That last category can be especially confusing ... until we understand this general rule:

Attributes initialize DOM properties and then they are done. Property values can change; attribute values can't.

For example, when the browser renders <input type="text" value="Bob">, it creates a corresponding DOM node with a value property initialized to "Bob".

When the user enters "Sally" into the input box, the DOM element value property becomes "Sally". But the HTML value attribute remains unchanged as we discover if we ask the input element about that attribute: input.getAttribute('value') // returns "Bob"

The HTML attribute value specifies the initial value; the DOM value property is the current value.

The disabled attribute is another peculiar example. A button's disabled property is false by default so the button is enabled. When we add the disabled attribute, its presence alone initializes the button's disabled property to true so the button is disabled.

Adding and removing the disabled attribute disables and enables the button. The value of the attribute is irrelevant, which is why we cannot enable a button by writing <button disabled="false">Still Disabled</button>.

Setting the button's disabled property (say, with an Angular binding) disables or enables the button. The value of the property matters.

The HTML attribute and the DOM property are not the same thing, even when they have the same name.

This is so important, we’ll say it again.

Template binding works with properties and events, not attributes.

A world without attributes

In the world of Angular, the only role of attributes is to initialize element and directive state. When we data bind, we're dealing exclusively with element and directive properties and events. Attributes effectively disappear.

With this model firmly in mind, let's learn about binding targets.

Binding targets

The target of a data binding is something in the DOM. Depending on the binding type, the target can be an (element | component | directive) property, an (element | component | directive) event, or (rarely) an attribute name. The following table summarizes:

Binding typeTargetExamples
PropertyElement property
Component property
Directive property
<img [src] = "heroImageUrl"> <hero-detail [hero]="currentHero"></hero-detail> <div [ngClass] = "{selected: isSelected}"></div>
EventElement event
Component event
Directive event
<button (click) = "onSave()">Save</button> <hero-detail (deleteRequest)="deleteHero()"></hero-detail> <div (myClick)="clicked=$event">click me</div>
Two-wayEvent and property
<input [(ngModel)]="heroName">
AttributeAttribute (the exception)
<button [attr.aria-label]="help">help</button>
Classclass property
<div [class.special]="isSpecial">Special</div>
Stylestyle property
<button [style.color] = "isSpecial ? 'red' : 'green'">

Let’s descend from the architectural clouds and look at each of these binding types in concrete detail.

Property binding

We write a template property binding when we want to set a property of a view element to the value of a template expression.

The most common property binding sets an element property to a component property value. An example is binding the src property of an image element to a component’s heroImageUrl property:

<img [src]="heroImageUrl">

Another example is disabling a button when the component says that it isUnchanged:

<button [disabled]="isUnchanged">Cancel is disabled</button>

Another is setting a property of a directive:

<div [ngClass]="classes">[ngClass] binding to the classes property</div>

Yet another is setting the model property of a custom component (a great way for parent and child components to communicate):

<hero-detail [hero]="currentHero"></hero-detail>

One-way in

People often describe property binding as one-way data binding because it flows a value in one direction, from a component’s data property into a target element property.

We cannot use property binding to pull values out of the target element. We can't bind to a property of the target element to read it. We can only set it.

Nor can we use property binding to call a method on the target element.

If the element raises events we can listen to them with an event binding.

If we must read a target element property or call one of its methods, we'll need a different technique. See the API reference for ViewChild and ContentChild.

Binding target

An element property between enclosing square brackets identifies the target property. The target property in the following code is the image element’s src property.

<img [src]="heroImageUrl">

Some people prefer the bind- prefix alternative, known as the canonical form:

<img bind-src="heroImageUrl">

The target name is always the name of a property, even when it appears to be the name of something else. We see src and may think it’s the name of an attribute. No. It’s the name of an image element property.

Element properties may be the more common targets, but Angular looks first to see if the name is a property of a known directive, as it is in the following example:

<div [ngClass]="classes">[ngClass] binding to the classes property</div>

Technically, Angular is matching the name to a directive input, one of the property names listed in the directive’s inputs array or a property decorated with @Input(). Such inputs map to the directive’s own properties.

If the name fails to match a property of a known directive or element, Angular reports an “unknown directive” error.

Avoid side effects

As we've already discussed, evaluation of a template expression should have no visible side effects. The expression language itself does its part to keep us safe. We can’t assign a value to anything in a property binding expression nor use the increment and decrement operators.

Of course, our expression might invoke a property or method that has side effects. Angular has no way of knowing that or stopping us.

The expression could call something like getFoo(). Only we know what getFoo() does. If getFoo() changes something and we happen to be binding to that something, we risk an unpleasant experience. Angular may or may not display the changed value. Angular may detect the change and throw a warning error. Our general advice: stick to data properties and to methods that return values and do no more.

Return the proper type

The template expression should evaluate to the type of value expected by the target property. Return a string if the target property expects a string. Return a number if the target property expects a number. Return an object if the target property expects an object.

The hero property of the HeroDetail component expects a Hero object, which is exactly what we’re sending in the property binding:

<hero-detail [hero]="currentHero"></hero-detail>

Remember the brackets

The brackets tell Angular to evaluate the template expression. If we forget the brackets, Angular treats the string as a constant and initializes the target property with that string. It does not evaluate the string!

Don't make the following mistake:

<!-- ERROR: HeroDetailComponent.hero expects a Hero object, not the string "currentHero" --> <hero-detail hero="currentHero"></hero-detail>

One-time string initialization

We should omit the brackets when all of the following are true:

We routinely initialize attributes this way in standard HTML, and it works just as well for directive and component property initialization. The following example initializes the prefix property of the HeroDetailComponent to a fixed string, not a template expression. Angular sets it and forgets about it.

<hero-detail prefix="You are my" [hero]="currentHero"></hero-detail>

The [hero] binding, on the other hand, remains a live binding to the component's currentHero property.

Property binding or interpolation?

We often have a choice between interpolation and property binding. The following binding pairs do the same thing:

<p><img src="{{heroImageUrl}}"> is the <i>interpolated</i> image.</p> <p><img [src]="heroImageUrl"> is the <i>property bound</i> image.</p> <p><span>"{{title}}" is the <i>interpolated</i> title.</span></p> <p>"<span [innerHTML]="title"></span>" is the <i>property bound</i> title.</p>

Interpolation is a convenient alternative for property binding in many cases. In fact, Angular translates those interpolations into the corresponding property bindings before rendering the view.

There is no technical reason to prefer one form to the other. We lean toward readability, which tends to favor interpolation. We suggest establishing coding style rules and choosing the form that both conforms to the rules and feels most natural for the task at hand.

Content security

Imagine the following malicious content.

evilTitle = 'Template <script>alert("evil never sleeps")</script>Syntax';

Fortunately, Angular data binding is on alert for dangerous HTML. It sanitizes the values before displaying them. It will not allow HTML with script tags to leak into the browser, neither with interpolation nor property binding.

<p><span>"{{evilTitle}}" is the <i>interpolated</i> evil title.</span></p> <p>"<span [innerHTML]="evilTitle"></span>" is the <i>property bound</i> evil title.</p>

Interpolation handles the script tags differently than property binding but both approaches render the content harmlessly.

evil title made safe

Attribute, class, and style bindings

The template syntax provides specialized one-way bindings for scenarios less well suited to property binding.

Attribute binding

We can set the value of an attribute directly with an attribute binding.

This is the only exception to the rule that a binding sets a target property. This is the only binding that creates and sets an attribute.

We have stressed throughout this chapter that setting an element property with a property binding is always preferred to setting the attribute with a string. Why does Angular offer attribute binding?

We must use attribute binding when there is no element property to bind.

Consider the ARIA, SVG, and table span attributes. They are pure attributes. They do not correspond to element properties, and they do not set element properties. There are no property targets to bind to.

We become painfully aware of this fact when we try to write something like this:

<tr><td colspan="{{1 + 1}}">Three-Four</td></tr>

We get this error:

Template parse errors: Can't bind to 'colspan' since it isn't a known native property

As the message says, the <td> element does not have a colspan property. It has the "colspan" attribute, but interpolation and property binding can set only properties, not attributes.

We need attribute bindings to create and bind to such attributes.

Attribute binding syntax resembles property binding. Instead of an element property between brackets, we start with the prefix attr, followed by a dot (.) and the name of the attribute. We then set the attribute value, using an expression that resolves to a string.

Here we bind [attr.colspan] to a calculated value:

<table border=1> <!-- expression calculates colspan=2 --> <tr><td [attr.colspan]="1 + 1">One-Two</td></tr> <!-- ERROR: There is no `colspan` property to set! <tr><td colspan="{{1 + 1}}">Three-Four</td></tr> --> <tr><td>Five</td><td>Six</td></tr> </table>

Here's how the table renders:

One-Two
FiveSix

One of the primary use cases for attribute binding is to set ARIA attributes, as in this example:

<!-- create and set an aria attribute for assistive technology --> <button [attr.aria-label]="actionName">{{actionName}} with Aria</button>

Class binding

We can add and remove CSS class names from an element’s class attribute with a class binding.

Class binding syntax resembles property binding. Instead of an element property between brackets, we start with the prefix class, optionally followed by a dot (.) and the name of a CSS class: [class.class-name].

The following examples show how to add and remove the application's "special" class with class bindings. Here's how we set the attribute without binding:

<!-- standard class attribute setting --> <div class="bad curly special">Bad curly special</div>

We can replace that with a binding to a string of the desired class names; this is an all-or-nothing, replacement binding.

<!-- reset/override all class names with a binding --> <div class="bad curly special" [class]="badCurly">Bad curly</div>

Finally, we can bind to a specific class name. Angular adds the class when the template expression evaluates to truthy. It removes the class when the expression is falsey.

<!-- toggle the "special" class on/off with a property --> <div [class.special]="isSpecial">The class binding is special</div> <!-- binding to `class.special` trumps the class attribute --> <div class="special" [class.special]="!isSpecial">This one is not so special</div>

While this is a fine way to toggle a single class name, we generally prefer the NgClass directive for managing multiple class names at the same time.

Style binding

We can set inline styles with a style binding.

Style binding syntax resembles property binding. Instead of an element property between brackets, we start with the prefix style, followed by a dot (.) and the name of a CSS style property: [style.style-property].

<button [style.color] = "isSpecial ? 'red': 'green'">Red</button> <button [style.background-color]="canSave ? 'cyan': 'grey'" >Save</button>

Some style binding styles have unit extension. Here we conditionally set the font size in “em” and “%” units .

<button [style.font-size.em]="isSpecial ? 3 : 1" >Big</button> <button [style.font-size.%]="!isSpecial ? 150 : 50" >Small</button>

While this is a fine way to set a single style, we generally prefer the NgStyle directive when setting several inline styles at the same time.

Note that a style property name can be written in either dash-case, as shown above, or camelCase, such as fontSize.

Event binding

The bindings we’ve met so far flow data in one direction: from a component to an element.

Users don’t just stare at the screen. They enter text into input boxes. They pick items from lists. They click buttons. Such user actions may result in a flow of data in the opposite direction: from an element to a component.

The only way to know about a user action is to listen for certain events such as keystrokes, mouse movements, clicks, and touches. We declare our interest in user actions through Angular event binding.

Event binding syntax consists of a target event within parentheses on the left of an equal sign, and a quoted template statement on the right. The following event binding listens for the button’s click event, calling the component's onSave() method whenever a click occurs:

<button (click)="onSave()">Save</button>

Target event

A name between parentheses — for example, (click) — identifies the target event. In the following example, the target is the button’s click event.

<button (click)="onSave()">Save</button>

Some people prefer the on- prefix alternative, known as the canonical form:

<button on-click="onSave()">On Save</button>

Element events may be the more common targets, but Angular looks first to see if the name matches an event property of a known directive, as it does in the following example:

<!-- `myClick` is an event on the custom `ClickDirective` --> <div (myClick)="clickMessage=$event">click with myClick</div>

The myClick directive is further described in the section on aliasing input/output properties.

If the name fails to match an element event or an output property of a known directive, Angular reports an “unknown directive” error.

$event and event handling statements

In an event binding, Angular sets up an event handler for the target event.

When the event is raised, the handler executes the template statement. The template statement typically involves a receiver, which performs an action in response to the event, such as storing a value from the HTML control into a model.

The binding conveys information about the event, including data values, through an event object named $event.

The shape of the event object is determined by the target event. If the target event is a native DOM element event, then $event is a DOM event object, with properties such as target and target.value.

Consider this example:

<input [value]="currentHero.firstName" (input)="currentHero.firstName=$event.target.value" >

This code sets the input box value property by binding to the firstName property. To listen for changes to the value, the code binds to the input box's input event. When the user makes changes, the input event is raised, and the binding executes the statement within a context that includes the DOM event object, $event.

To update the firstName property, the changed text is retrieved by following the path $event.target.value.

If the event belongs to a directive (recall that components are directives), $event has whatever shape the directive decides to produce.

Custom events with EventEmitter

Directives typically raise custom events with an Angular EventEmitter. The directive creates an EventEmitter and exposes it as a property. The directive calls EventEmitter.emit(payload) to fire an event, passing in a message payload, which can be anything. Parent directives listen for the event by binding to this property and accessing the payload through the $event object.

Consider a HeroDetailComponent that presents hero information and responds to user actions. Although the HeroDetailComponent has a delete button it doesn't know how to delete the hero itself. The best it can do is raise an event reporting the user's delete request.

Here are the pertinent excerpts from that HeroDetailComponent:

app/hero-detail.component.ts (template)

template: ` <div> <img src="{{heroImageUrl}}"> <span [style.text-decoration]="lineThrough"> {{prefix}} {{hero?.fullName}} </span> <button (click)="delete()">Delete</button> </div>`

app/hero-detail.component.ts (deleteRequest)

// This component make a request but it can't actually delete a hero. deleteRequest = new EventEmitter<Hero>(); delete() { this.deleteRequest.emit(this.hero); }

The component defines a deleteRequest property that returns an EventEmitter. When the user clicks delete, the component invokes the delete() method, telling the EventEmitter to emit a Hero object.

Now imagine a hosting parent component that binds to the HeroDetailComponent's deleteRequest event.

<hero-detail (deleteRequest)="deleteHero($event)" [hero]="currentHero"></hero-detail>

When the deleteRequest event fires, Angular calls the parent component's deleteHero method, passing the hero-to-delete (emitted by HeroDetail) in the $event variable.

Template statements have side effects

The deleteHero method has a side effect: it deletes a hero. Template statement side effects are not just OK, but expected.

Deleting the hero updates the model, perhaps triggering other changes including queries and saves to a remote server. These changes percolate through the system and are ultimately displayed in this and other views.

Two-way binding

We often want to both display a data property and update that property when the user makes changes.

On the element side that takes a combination of setting a specific element property and listening for an element change event.

Angular offers a special two-way data binding syntax for this purpose, [(x)]. The [(x)] syntax combines the brackets of property binding, [x], with the parentheses of event binding, (x).

[( )] = banana in a box

Visualize a banana in a box to remember that the parentheses go inside the brackets.

The [(x)] syntax is easy to demonstrate when the element has a settable property called x and a corresponding event named xChange. Here's a SizerComponent that fits the pattern. It has a size value property and a companion sizeChange event:

app/sizer.component.ts

import { Component, EventEmitter, Input, Output } from '@angular/core'; @Component({ selector: 'my-sizer', template: ` <div> <button (click)="dec()" title="smaller">-</button> <button (click)="inc()" title="bigger">+</button> <label [style.font-size.px]="size">FontSize: {{size}}px</label> </div>` }) export class SizerComponent { @Input() size: number | string; @Output() sizeChange = new EventEmitter<number>(); dec() { this.resize(-1); } inc() { this.resize(+1); } resize(delta: number) { this.size = Math.min(40, Math.max(8, +this.size + delta)); this.sizeChange.emit(this.size); } }

The initial size is an input value from a property binding. Clicking the buttons increases or decreases the size, within min/max values constraints, and then raises (emits) the sizeChange event with the adjusted size.

Here's an example in which the AppComponent.fontSizePx is two-way bound to the SizerComponent:

<my-sizer [(size)]="fontSizePx"></my-sizer> <div [style.font-size.px]="fontSizePx">Resizable Text</div>

The AppComponent.fontSizePx establishes the initial SizerComponent.size value. Clicking the buttons updates the AppComponent.fontSizePx via the two-way binding. The revised AppComponent.fontSizePx value flows through to the style binding, making the displayed text bigger or smaller. Try it in the .

The two-way binding syntax is really just syntactic sugar for a property binding and an event binding. Angular desugars the SizerComponent binding into this:

<my-sizer [size]="fontSizePx" (sizeChange)="fontSizePx=$event"></my-sizer>

The $event variable contains the payload of the SizerComponent.sizeChange event. Angular assigns the $event value to the AppComponent.fontSizePx when the user clicks the buttons.

Clearly the two-way binding syntax is a great convenience compared to separate property and event bindings.

We'd like to use two-way binding with HTML form elements like <input> and <select>. Sadly, no native HTML element follows the x value and xChange event pattern.

Fortunately, the Angular NgModel directive is a bridge that enables two-way binding to form elements.

Two-way binding with NgModel

When developing data entry forms, we often want to both display a data property and update that property when the user makes changes.

Two-way data binding with the NgModel directive makes that easy. Here's an example:

<input [(ngModel)]="currentHero.firstName">
FormsModule is Required to use ngModel

Before we can use the ngModel directive in a two-way data binding, we must import the FormsModule and add it to the Angular module's imports list. Learn more about the FormsModule and ngModel in the Forms chapter.

Here's how to import the FormsModule to make [(ngModel)] available.

app/app.module.ts (FormsModule import)

import { NgModule } from '@angular/core'; import { BrowserModule } from '@angular/platform-browser'; import { FormsModule } from '@angular/forms'; import { AppComponent } from './app.component'; @NgModule({ imports: [ BrowserModule, FormsModule ], declarations: [ AppComponent ], bootstrap: [ AppComponent ] }) export class AppModule { }

Inside [(ngModel)]

Looking back at the firstName binding, it's important to note that we could have achieved the same result with separate bindings to the <input> element's value property and input event.

<input [value]="currentHero.firstName" (input)="currentHero.firstName=$event.target.value" >

That’s cumbersome. Who can remember which element property to set and which element event emits user changes? How do we extract the currently displayed text from the input box so we can update the data property? Who wants to look that up each time?

That ngModel directive hides these onerous details behind its own ngModel input and ngModelChange output properties.

<input [ngModel]="currentHero.firstName" (ngModelChange)="currentHero.firstName=$event">

The ngModel data property sets the element's value property and the ngModelChange event property listens for changes to the element's value.

The details are specific to each kind of element and therefore the NgModel directive only works for specific form elements, such as the input text box, that are supported by a ControlValueAccessor.

We can't apply [(ngModel)] to a custom component until we write a suitable value accessor, a technique that is beyond the scope of this chapter. That's something we might want to do for an Angular component or a WebComponent whose API we can't control.

It's completely unnecessary for an Angular component that we do control ... because we can name the value and event properties to suit Angular's basic two-way binding syntax and skip NgModel altogether.

Separate ngModel bindings is an improvement over binding to the element's native properties. We can do better.

We shouldn't have to mention the data property twice. Angular should be able to capture the component’s data property and set it with a single declaration — which it can with the [(ngModel)] syntax:

<input [(ngModel)]="currentHero.firstName">

Is [(ngModel)] all we need? Is there ever a reason to fall back to its expanded form?

The [(ngModel)] syntax can only set a data-bound property. If we need to do something more or something different, we need to write the expanded form ourselves.

Let's try something silly like forcing the input value to uppercase:

<input [ngModel]="currentHero.firstName" (ngModelChange)="setUpperCaseFirstName($event)">

Here are all variations in action, including the uppercase version:

NgModel variations

Built-in directives

Earlier versions of Angular included over seventy built-in directives. The community contributed many more, and countless private directives have been created for internal applications.

We don’t need many of those directives in Angular. Quite often we can achieve the same results with the more capable and expressive Angular binding system. Why create a directive to handle a click when we can write a simple binding such as this?

<button (click)="onSave()">Save</button>

We still benefit from directives that simplify complex tasks. Angular still ships with built-in directives; just not as many. We'll write our own directives, just not as many.

This segment reviews some of the most frequently used built-in directives.

NgClass

We typically control how elements appear by adding and removing CSS classes dynamically. We can bind to NgClass to add or remove several classes simultaneously.

A class binding is a good way to add or remove a single class.

<!-- toggle the "special" class on/off with a property --> <div [class.special]="isSpecial">The class binding is special</div>

The NgClass directive may be the better choice when we want to add or remove many CSS classes at the same time.

A good way to apply NgClass is by binding it to a key:value control object. Each key of the object is a CSS class name; its value is true if the class should be added, false if it should be removed.

Consider a component method such as setClasses that manages the state of three CSS classes:

setClasses() { let classes = { saveable: this.canSave, // true modified: !this.isUnchanged, // false special: this.isSpecial, // true }; return classes; }

Now we can add an NgClass property binding that calls setClasses and sets the element's classes accordingly:

<div [ngClass]="setClasses()">This div is saveable and special</div>

NgStyle

We can set inline styles dynamically, based on the state of the component. Binding to NgStyle lets us set many inline styles simultaneously.

A style binding is an easy way to set a single style value.

<div [style.font-size]="isSpecial ? 'x-large' : 'smaller'" > This div is x-large. </div>

The NgStyle directive may be the better choice when we want to set many inline styles at the same time.

We apply NgStyle by binding it to a key:value control object. Each key of the object is a style name; its value is whatever is appropriate for that style.

Consider a component method such as setStyles that returns an object defining three styles:

setStyles() { let styles = { // CSS property names 'font-style': this.canSave ? 'italic' : 'normal', // italic 'font-weight': !this.isUnchanged ? 'bold' : 'normal', // normal 'font-size': this.isSpecial ? '24px' : '8px', // 24px }; return styles; }

Now we just add an NgStyle property binding that calls setStyles and sets the element's styles accordingly:

<div [ngStyle]="setStyles()"> This div is italic, normal weight, and extra large (24px). </div>

NgIf

We can add an element subtree (an element and its children) to the DOM by binding an NgIf directive to a truthy expression.

<div *ngIf="currentHero">Hello, {{currentHero.firstName}}</div>

Don't forget the asterisk (*) in front of ngIf. For more information, see * and <template>.

Binding to a falsey expression removes the element subtree from the DOM.

<!-- because of the ngIf guard `nullHero.firstName` never has a chance to fail --> <div *ngIf="nullHero">Hello, {{nullHero.firstName}}</div> <!-- Hero Detail is not in the DOM because isActive is false--> <hero-detail *ngIf="isActive"></hero-detail>

Visibility and NgIf are not the same

We can show and hide an element subtree (the element and its children) with a class or style binding:

<!-- isSpecial is true --> <div [class.hidden]="!isSpecial">Show with class</div> <div [class.hidden]="isSpecial">Hide with class</div> <!-- HeroDetail is in the DOM but hidden --> <hero-detail [class.hidden]="isSpecial"></hero-detail> <div [style.display]="isSpecial ? 'block' : 'none'">Show with style</div> <div [style.display]="isSpecial ? 'none' : 'block'">Hide with style</div>

Hiding a subtree is quite different from excluding a subtree with NgIf.

When we hide the element subtree, it remains in the DOM. Components in the subtree are preserved, along with their state. Angular may continue to check for changes even to invisible properties. The subtree may tie up substantial memory and computing resources.

When NgIf is false, Angular physically removes the element subtree from the DOM. It destroys components in the subtree, along with their state, potentially freeing up substantial resources and resulting in better performance for the user.

The show/hide technique is probably fine for small element trees. We should be wary when hiding large trees; NgIf may be the safer choice. Always measure before leaping to conclusions.

NgSwitch

We bind to NgSwitch when we want to display one element tree (an element and its children) from a set of possible element trees, based on some condition. Angular puts only the selected element tree into the DOM.

Here’s an example:

<span [ngSwitch]="toeChoice"> <span *ngSwitchCase="'Eenie'">Eenie</span> <span *ngSwitchCase="'Meanie'">Meanie</span> <span *ngSwitchCase="'Miney'">Miney</span> <span *ngSwitchCase="'Moe'">Moe</span> <span *ngSwitchDefault>other</span> </span>

We bind the parent NgSwitch directive to an expression returning a switch value. The value is a string in this example, but it can be a value of any type.

In this example, the parent NgSwitch directive controls a set of child <span> elements. A <span> is either pegged to a match value expression or marked as the default.

At any particular moment, at most one of these spans is in the DOM.

If the span’s match value equals the switch value, Angular adds the <span> to the DOM. If none of the spans is a match, Angular adds the default span to the DOM. Angular removes and destroys all other spans.

We could substitute any element for the span in this example. That element could be a <div> with a vast subtree of its own elements. Only the matching <div> and its subtree would appear in the DOM; the others would be removed.

Three collaborating directives are at work here:

  1. ngSwitch: bound to an expression that returns the switch value
  2. ngSwitchCase: bound to an expression returning a match value
  3. ngSwitchDefault: a marker attribute on the default element

Do not put the asterisk (*) in front of ngSwitch. Use the property binding instead.

Do put the asterisk (*) in front of ngSwitchCase and ngSwitchDefault. For more information, see * and <template>.

NgFor

NgFor is a repeater directive — a way to customize data display.

Our goal is to present a list of items. We define a block of HTML that defines how a single item should be displayed. We tell Angular to use that block as a template for rendering each item in the list.

Here is an example of NgFor applied to a simple <div>:

<div *ngFor="let hero of heroes">{{hero.fullName}}</div>

We can also apply an NgFor to a component element, as in this example:

<hero-detail *ngFor="let hero of heroes" [hero]="hero"></hero-detail>

Don't forget the asterisk (*) in front of ngFor. For more information, see * and <template>.

The text assigned to *ngFor is the instruction that guides the repeater process.

NgFor microsyntax

The string assigned to *ngFor is not a template expression. It’s a microsyntax — a little language of its own that Angular interprets. In this example, the string "let hero of heroes" means:

Take each hero in the heroes array, store it in the local hero variable, and make it available to the templated HTML for each iteration.

Angular translates this instruction into a new set of elements and bindings.

In the two previous examples, the ngFor directive iterates over the heroes array returned by the parent component’s heroes property, stamping out instances of the element to which it is applied. Angular creates a fresh instance of the template for each hero in the array.

The let keyword before hero creates a template input variable called hero.

A template input variable is not the same as a template reference variable!

We use this variable within the template to access a hero’s properties, as we’re doing in the interpolation. We can also pass the variable in a binding to a component element, as we're doing with hero-detail.

NgFor with index

The ngFor directive supports an optional index that increases from 0 to the length of the array for each iteration. We can capture the index in a template input variable and use it in our template.

The next example captures the index in a variable named i, using it to stamp out rows like "1 - Hercules Son of Zeus".

<div *ngFor="let hero of heroes; let i=index">{{i + 1}} - {{hero.fullName}}</div>

Learn about other special index-like values such as last, even, and odd in the NgFor API reference.

NgForTrackBy

The ngFor directive has the potential to perform poorly, especially with large lists. A small change to one item, an item removed, or an item added can trigger a cascade of DOM manipulations.

For example, we could refresh the list of heroes by re-querying the server. The refreshed list probably contains most, if not all, of the previously displayed heroes.

We know this because the id of each hero hasn't changed. But Angular sees only a fresh list of new object references. It has no choice but to tear down the old list, discard those DOM elements, and re-build a new list with new DOM elements.

Angular can avoid this churn if we give it a tracking function that tells it what we know: that two objects with the same hero.id are the same hero. Here is such a function:

trackByHeroes(index: number, hero: Hero) { return hero.id; }

Now set the NgForTrackBy directive to that tracking function.

<div *ngFor="let hero of heroes; trackBy:trackByHeroes">({{hero.id}}) {{hero.fullName}}</div>

The tracking function doesn't eliminate all DOM changes. Angular may have to update the DOM element if the same-hero properties have changed. But if the properties haven't changed — and most of the time they will not have changed — Angular can leave those DOM elements alone. The list UI will be smoother and more responsive.

Here is an illustration of the NgForTrackBy effect.

NgForTrackBy

* and <template>

When we reviewed the NgFor, NgIf, and NgSwitch built-in directives, we called out an oddity of the syntax: the asterisk (*) that appears before the directive names.

The * is a bit of syntactic sugar that makes it easier to read and write directives that modify HTML layout with the help of templates. NgFor, NgIf, and NgSwitch all add and remove element subtrees that are wrapped in <template> tags.

We didn't see the <template> tags because the * prefix syntax allowed us to skip those tags and focus directly on the HTML element that we are including, excluding, or repeating.

In this section we go under the hood and see how Angular strips away the * and expands the HTML into the <template> tags for us.

Expanding *ngIf

We can do what Angular does ourselves and expand the * prefix syntax to template syntax. Here's some code with *ngIf:

<hero-detail *ngIf="currentHero" [hero]="currentHero"></hero-detail>

The currentHero is referenced twice, first as the true/false condition for NgIf and again as the actual hero passed into the HeroDetailComponent.

The first expansion step transports the ngIf (without the * prefix) and its contents into an expression assigned to a template directive.

<hero-detail template="ngIf:currentHero" [hero]="currentHero"></hero-detail>

The next (and final) step unfolds the HTML into a <template> tag and [ngIf] property binding:

<template [ngIf]="currentHero"> <hero-detail [hero]="currentHero"></hero-detail> </template>

Notice that the [hero]="currentHero" binding remains on the child <hero-detail> element inside the template.

Remember the brackets!

Don’t make the mistake of writing ngIf="currentHero"! That syntax assigns the string value "currentHero" to ngIf. In JavaScript a non-empty string is a truthy value, so ngIf would always be true and Angular would always display the hero-detail … even when there is no currentHero!

Expanding *ngSwitch

A similar transformation applies to *ngSwitch. We can unfold the syntax ourselves. Here's an example, first with *ngSwitchCase and *ngSwitchDefault and then again with <template> tags:

<span [ngSwitch]="toeChoice"> <!-- with *NgSwitch --> <span *ngSwitchCase="'Eenie'">Eenie</span> <span *ngSwitchCase="'Meanie'">Meanie</span> <span *ngSwitchCase="'Miney'">Miney</span> <span *ngSwitchCase="'Moe'">Moe</span> <span *ngSwitchDefault>other</span> <!-- with <template> --> <template [ngSwitchCase]="'Eenie'"><span>Eenie</span></template> <template [ngSwitchCase]="'Meanie'"><span>Meanie</span></template> <template [ngSwitchCase]="'Miney'"><span>Miney</span></template> <template [ngSwitchCase]="'Moe'"><span>Moe</span></template> <template ngSwitchDefault><span>other</span></template> </span>

The *ngSwitchCase and *ngSwitchDefault expand in exactly the same manner as *ngIf, wrapping their former elements in <template> tags.

Now we can see why the ngSwitch itself is not prefixed with an asterisk (*). It does not define content. It's job is to control a collection of templates.

In this case, it governs two sets of ngSwitchCase and NgSwitchDefault directives. We should expect it to display the values of the selected template twice, once for the (*) prefixed version and once for the expanded template version. That's exactly what we see in this example:

NgSwitch

Expanding *ngFor

The *ngFor undergoes a similar transformation. We begin with an *ngFor example:

<hero-detail *ngFor="let hero of heroes; trackBy:trackByHeroes" [hero]="hero"></hero-detail>

Here's the same example after transporting the ngFor to the template directive:

<hero-detail template="ngFor let hero of heroes; trackBy:trackByHeroes" [hero]="hero"></hero-detail>

And here it is expanded further into a <template> tag wrapping the original <hero-detail> element:

<template ngFor let-hero [ngForOf]="heroes" [ngForTrackBy]="trackByHeroes"> <hero-detail [hero]="hero"></hero-detail> </template>

The NgFor code is a bit more complex than NgIf because a repeater has more moving parts to configure. In this case, we have to remember to create and assign the NgForOf directive that identifies the list and the NgForTrackBy directive. Using the *ngFor syntax is much easier than writing out this expanded HTML ourselves.

Template reference variables

A template reference variable is a reference to a DOM element or directive within a template.

It can be used with native DOM elements but also with Angular components — in fact, it will work with any custom web component.

Referencing a template reference variable

We can refer to a template reference variable anywhere in the current template.

Do not define the same variable name more than once in the same template. The runtime value will be unpredictable.

Here are two other examples of creating and consuming a Template reference variable:

<!-- phone refers to the input element; pass its `value` to an event handler --> <input #phone placeholder="phone number"> <button (click)="callPhone(phone.value)">Call</button> <!-- fax refers to the input element; pass its `value` to an event handler --> <input ref-fax placeholder="fax number"> <button (click)="callFax(fax.value)">Fax</button>

The hash (#) prefix to "phone" means that we're defining a phone variable.

Folks who don't like using the # character can use its canonical alternative, the ref- prefix. For example, we can declare the our phone variable using either #phone or ref-phone.

How a variable gets its value

Angular sets the variable's value to the element on which it was defined. We defined these variables on the input elements. We’re passing those input element objects across to the button elements, where they're used in arguments to the call methods in the event bindings.

NgForm and template reference variables

Let's look at one final example: a form, the poster child for template reference variables.

The HTML for a form can be quite involved, as we saw in the Forms chapter. The following is a simplified example — and it's not simple at all.

<form (ngSubmit)="onSubmit(theForm)" #theForm="ngForm"> <div class="form-group"> <label for="name">Name</label> <input class="form-control" name="name" required [(ngModel)]="currentHero.firstName"> </div> <button type="submit" [disabled]="!theForm.form.valid">Submit</button> </form>

A template reference variable, theForm, appears three times in this example, separated by a large amount of HTML.

<form (ngSubmit)="onSubmit(theForm)" #theForm="ngForm"> <button type="submit" [disabled]="!theForm.form.valid">Submit</button> </form>

What is the value of theForm?

It would be the HTMLFormElement if Angular hadn't taken it over. It's actually ngForm, a reference to the Angular built-in NgForm directive that wraps the native HTMLFormElement and endows it with additional superpowers such as the ability to track the validity of user input.

This explains how we can disable the submit button by checking theForm.form.valid and pass an object with rich information to the parent component's onSubmit method.

Input and output properties

So far, we’ve focused mainly on binding to component members within template expressions and statements that appear on the right side of the binding declaration. A member in that position is a data binding source.

This section concentrates on binding to targets, which are directive properties on the left side of the binding declaration. These directive properties must be declared as inputs or outputs.

Remember: All components are directives.

We're drawing a sharp distinction between a data binding target and a data binding source.

The target of a binding is to the left of the =. The source is on the right of the =.

The target of a binding is the property or event inside the binding punctuation: [], () or [()]. The source is either inside quotes (" ") or within an interpolation ({{}}).

Every member of a source directive is automatically available for binding. We don't have to do anything special to access a directive member in a template expression or statement.

We have limited access to members of a target directive. We can only bind to properties that are explicitly identified as inputs and outputs.

In the following example, iconUrl and onSave are members of a component that are referenced within quoted syntax to the right of the =.

<img [src]="iconUrl"/> <button (click)="onSave()">Save</button>

They are neither inputs nor outputs of the component. They are data sources for their bindings.

Now look at HeroDetailComponent when it is the target of a binding.

<hero-detail [hero]="currentHero" (deleteRequest)="deleteHero($event)"> </hero-detail>

Both HeroDetailComponent.hero and HeroDetailComponent.deleteRequest are on the left side of binding declarations. HeroDetailComponent.hero is inside brackets; it is the target of a property binding. HeroDetailComponent.deleteRequest is inside parentheses; it is the target of an event binding.

Declaring input and output properties

Target properties must be explicitly marked as inputs or outputs.

When we peek inside HeroDetailComponent, we see that these properties are marked with decorators as input and output properties.

@Input() hero: Hero; @Output() deleteRequest = new EventEmitter<Hero>();

Alternatively, we can identify members in the inputs and outputs arrays of the directive metadata, as in this example:

@Component({ inputs: ['hero'], outputs: ['deleteRequest'], })

We can specify an input/output property either with a decorator or in a metadata array. Don't do both!

Input or output?

Input properties usually receive data values. Output properties expose event producers, such as EventEmitter objects.

The terms input and output reflect the perspective of the target directive.

Inputs and outputs

HeroDetailComponent.hero is an input property from the perspective of HeroDetailComponent because data flows into that property from a template binding expression.

HeroDetailComponent.deleteRequest is an output property from the perspective of HeroDetailComponent because events stream out of that property and toward the handler in a template binding statement.

Aliasing input/output properties

Sometimes we want the public name of an input/output property to be different from the internal name.

This is frequently the case with attribute directives. Directive consumers expect to bind to the name of the directive. For example, when we apply a directive with a myClick selector to a <div> tag, we expect to bind to an event property that is also called myClick.

<div (myClick)="clickMessage=$event">click with myClick</div>

However, the directive name is often a poor choice for the name of a property within the directive class. The directive name rarely describes what the property does. The myClick directive name is not a good name for a property that emits click messages.

Fortunately, we can have a public name for the property that meets conventional expectations, while using a different name internally. In the example immediately above, we are actually binding through the myClick alias to the directive's own clicks property.

We can specify the alias for the property name by passing it into the input/output decorator like this:

@Output('myClick') clicks = new EventEmitter<string>(); // @Output(alias) propertyName = ...

We can also alias property names in the inputs and outputs arrays. We write a colon-delimited (:) string with the directive property name on the left and the public alias on the right:

@Directive({ outputs: ['clicks:myClick'] // propertyName:alias })

Template expression operators

The template expression language employs a subset of JavaScript syntax supplemented with a few special operators for specific scenarios. We'll cover two of these operators: pipe and safe navigation operator.

The pipe operator ( | )

The result of an expression might require some transformation before we’re ready to use it in a binding. For example, we might want to display a number as a currency, force text to uppercase, or filter a list and sort it.

Angular pipes are a good choice for small transformations such as these. Pipes are simple functions that accept an input value and return a transformed value. They're easy to apply within template expressions, using the pipe operator (|):

<div>Title through uppercase pipe: {{title | uppercase}}</div>

The pipe operator passes the result of an expression on the left to a pipe function on the right.

We can chain expressions through multiple pipes:

<!-- Pipe chaining: convert title to uppercase, then to lowercase --> <div> Title through a pipe chain: {{title | uppercase | lowercase}} </div>

And we can also apply parameters to a pipe:

<!-- pipe with configuration argument => "February 25, 1970" --> <div>Birthdate: {{currentHero?.birthdate | date:'longDate'}}</div>

The json pipe is particularly helpful for debugging our bindings:

<div>{{currentHero | json}}</div>

The generated output would look something like this

{ "firstName": "Hercules", "lastName": "Son of Zeus", "birthdate": "1970-02-25T08:00:00.000Z", "url": "http://www.imdb.com/title/tt0065832/", "rate": 325, "id": 1 }

The safe navigation operator ( ?. ) and null property paths

The Angular safe navigation operator (?.) is a fluent and convenient way to guard against null and undefined values in property paths. Here it is, protecting against a view render failure if the currentHero is null.

The current hero's name is {{currentHero?.firstName}}

Let’s elaborate on the problem and this particular solution.

What happens when the following data bound title property is null?

The title is {{title}}

The view still renders but the displayed value is blank; we see only "The title is" with nothing after it. That is reasonable behavior. At least the app doesn't crash.

Suppose the template expression involves a property path, as in this next example where we’re displaying the firstName of a null hero.

The null hero's name is {{nullHero.firstName}}

JavaScript throws a null reference error, and so does Angular:

TypeError: Cannot read property 'firstName' of null in [null].

Worse, the entire view disappears.

We could claim that this is reasonable behavior if we believed that the hero property must never be null. If it must never be null and yet it is null, we've made a programming error that should be caught and fixed. Throwing an exception is the right thing to do.

On the other hand, null values in the property path may be OK from time to time, especially when we know the data will arrive eventually.

While we wait for data, the view should render without complaint, and the null property path should display as blank just as the title property does.

Unfortunately, our app crashes when the currentHero is null.

We could code around that problem with NgIf.

<!--No hero, div not displayed, no error --> <div *ngIf="nullHero">The null hero's name is {{nullHero.firstName}}</div>

Or we could try to chain parts of the property path with &&, knowing that the expression bails out when it encounters the first null.

The null hero's name is {{nullHero && nullHero.firstName}}

These approaches have merit but can be cumbersome, especially if the property path is long. Imagine guarding against a null somewhere in a long property path such as a.b.c.d.

The Angular safe navigation operator (?.) is a more fluent and convenient way to guard against nulls in property paths. The expression bails out when it hits the first null value. The display is blank, but the app keeps rolling without errors.

<!-- No hero, no problem! --> The null hero's name is {{nullHero?.firstName}}

It works perfectly with long property paths such as a?.b?.c?.d.

Summary

We’ve completed our survey of template syntax. Now it's time to put that knowledge to work as we write our own components and directives.

Next Step

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