angular-docs-cn/aio/content/guide/pipes.md

Transforming Data Using Pipes

Use pipes to transform strings, currency amounts, dates, and other data for display. Pipes are simple functions you can use in template expressions to accept an input value and return a transformed value. Pipes are useful because you can use them throughout your application, while only declaring each pipe once. For example, you would use a pipe to show a date as April 15, 1988 rather than the raw string format.

For the sample app used in this topic, see the .

Angular provides built-in pipes for typical data transformations, including transformations for internationalization (i18n), which use locale information to format data. The following are commonly used built-in pipes for data formatting:

• DatePipe: Formats a date value according to locale rules.
• UpperCasePipe: Transforms text to all upper case.
• LowerCasePipe: Transforms text to all lower case.
• CurrencyPipe: Transforms a number to a currency string, formatted according to locale rules.
• DecimalPipe: Transforms a number into a string with a decimal point, formatted according to locale rules.
• PercentPipe: Transforms a number to a percentage string, formatted according to locale rules.

• For a complete list of built-in pipes, see the pipes API documentation.
• To learn more about using pipes for internationalization (i18n) efforts, see formatting data based on locale.

You can also create pipes to encapsulate custom transformations and use your custom pipes in template expressions.

Prerequisites

To use pipes you should have a basic understanding of the following:

• Typescript and HTML5 programming
• Templates in HTML with CSS styles
• Components

Using a pipe in a template

To apply a pipe, use the pipe operator (|) within a template expression as shown in the following code example, along with the name of the pipe, which is date for the built-in DatePipe. The tabs in the example show the following:

• app.component.html uses date in a separate template to display a birthday.
• hero-birthday1.component.ts uses the same pipe as part of an in-line template in a component that also sets the birthday value.

The component's birthday value flows through the pipe operator ( | ) to the date function.

{@a parameterizing-a-pipe}

Transforming data with parameters and chained pipes

Use optional parameters to fine-tune a pipe's output. For example, you can use the CurrencyPipe with a country code such as EUR as a parameter. The template expression {{ amount | currency:'EUR' }} transforms the amount to currency in euros. Follow the pipe name (currency) with a colon (:) and the parameter value ('EUR').

If the pipe accepts multiple parameters, separate the values with colons. For example, {{ amount | currency:'EUR':'Euros '}} adds the second parameter, the string literal 'Euros ', to the output string. You can use any valid template expression as a parameter, such as a string literal or a component property.

Some pipes require at least one parameter and allow more optional parameters, such as SlicePipe. For example, {{ slice:1:5 }} creates a new array or string containing a subset of the elements starting with element 1 and ending with element 5.

Example: Formatting a date

The tabs in the following example demonstrates toggling between two different formats ('shortDate' and 'fullDate'):

• The app.component.html template uses a format parameter for the DatePipe (named date) to show the date as 04/15/88.
• The hero-birthday2.component.ts component binds the pipe's format parameter to the component's format property in the template section, and adds a button for a click event bound to the component's toggleFormat() method.
• The hero-birthday2.component.ts component's toggleFormat() method toggles the component's format property between a short form ('shortDate') and a longer form ('fullDate').

Clicking the Toggle Format button alternates the date format between 04/15/1988 and Friday, April 15, 1988 as shown in Figure 1.

Figure 1. Clicking the button toggles the date format

For date pipe format options, see DatePipe.

Example: Applying two formats by chaining pipes

You can chain pipes so that the output of one pipe becomes the input to the next.

In the following example, chained pipes first apply a format to a date value, then convert the formatted date to uppercase characters. The first tab for the src/app/app.component.html template chains DatePipe and UpperCasePipe to display the birthday as APR 15, 1988. The second tab for the src/app/app.component.html template passes the fullDate parameter to date before chaining to uppercase, which produces FRIDAY, APRIL 15, 1988.

{@a Custom-pipes}

Creating pipes for custom data transformations

Create custom pipes to encapsulate transformations that are not provided with the built-in pipes. You can then use your custom pipe in template expressions, the same way you use built-in pipes—to transform input values to output values for display.

Marking a class as a pipe

To mark a class as a pipe and supply configuration metadata, apply the @Pipe decorator to the class. Use UpperCamelCase (the general convention for class names) for the pipe class name, and camelCase for the corresponding name string. Do not use hyphens in the name. For details and more examples, see Pipe names.

Use name in template expressions as you would for a built-in pipe.

• Include your pipe in the declarations field of the NgModule metadata in order for it to be available to a template. See the app.module.ts file in the example app (). For details, see NgModules.
• Register your custom pipes. The Angular CLI ng generate pipe command registers the pipe automatically.

Using the PipeTransform interface

Implement the PipeTransform interface in your custom pipe class to perform the transformation.

Angular invokes the transform method with the value of a binding as the first argument, and any parameters as the second argument in list form, and returns the transformed value.

Example: Transforming a value exponentially

In a game, you may want to implement a transformation that raises a value exponentially to increase a hero's power. For example, if the hero's score is 2, boosting the hero's power exponentially by 10 produces a score of 1024. You can use a custom pipe for this transformation.

The following code example shows two component definitions:

• The exponential-strength.pipe.ts component defines a custom pipe named exponentialStrength with the transform method that performs the transformation. It defines an argument to the transform method (exponent) for a parameter passed to the pipe.

• The power-booster.component.ts component demonstrates how to use the pipe, specifying a value (2) and the exponent parameter (10). Figure 2 shows the output.

Figure 2. Output from the exponentialStrength pipe

To examine the behavior the exponentialStrength pipe in the , change the value and optional exponent in the template.

{@a change-detection}

Detecting changes with data binding in pipes

You use data binding with a pipe to display values and respond to user actions. If the data is a primitive input value, such as String or Number, or an object reference as input, such as Date or Array, Angular executes the pipe whenever it detects a change for the input value or reference.

For example, you could change the previous custom pipe example to use two-way data binding with ngModel to input the amount and boost factor, as shown in the following code example.

The exponentialStrength pipe executes every time the user changes the "normal power" value or the "boost factor", as shown in Figure 3.

Figure 3. Changing the amount and boost factor for the exponentialStrength pipe

Angular detects each change and immediately runs the pipe. This is fine for primitive input values. However, if you change something inside a composite object (such as the month of a date, an element of an array, or an object property), you need to understand how change detection works, and how to use an impure pipe.

How change detection works

Angular looks for changes to data-bound values in a change detection process that runs after every DOM event: every keystroke, mouse move, timer tick, and server response. The following example, which doesn't use a pipe, demonstrates how Angular uses its default change detection strategy to monitor and update its display of every hero in the heroes array. The example tabs show the following:

• In the flying-heroes.component.html (v1) template, the *ngFor repeater displays the hero names.
• Its companion component class flying-heroes.component.ts (v1) provides heroes, adds heroes into the array, and resets the array.

Angular updates the display every time the user adds a hero. If the user clicks the Reset button, Angular replaces heroes with a new array of the original heroes and updates the display. If you add the ability to remove or change a hero, Angular would detect those changes and update the display as well.

However, executing a pipe to update the display with every change would slow down your app's performance. So Angular uses a faster change-detection algorithm for executing a pipe, as described in the next section.

{@a pure-and-impure-pipes}

Detecting pure changes to primitives and object references

By default, pipes are defined as pure so that Angular executes the pipe only when it detects a pure change to the input value. A pure change is either a change to a primitive input value (such as String, Number, Boolean, or Symbol), or a changed object reference (such as Date, Array, Function, or Object).

{@a pure-pipe-pure-fn}

A pure pipe must use a pure function, which is one that processes inputs and returns values without side effects. In other words, given the same input, a pure function should always return the same output.

With a pure pipe, Angular ignores changes within composite objects, such as a newly added element of an existing array, because checking a primitive value or object reference is much faster than performing a deep check for differences within objects. Angular can quickly determine if it can skip executing the pipe and updating the view.

However, a pure pipe with an array as input may not work the way you want. To demonstrate this issue, change the previous example to filter the list of heroes to just those heroes who can fly. Use the FlyingHeroesPipe in the *ngFor repeater as shown in the following code. The tabs for the example show the following:

• The template (flying-heroes.component.html (flyers)) with the new pipe.
• The FlyingHeroesPipe custom pipe implementation (flying-heroes.pipe.ts).

The app now shows unexpected behavior: When the user adds flying heroes, none of them appear under "Heroes who fly." This happens because the code that adds a hero does so by pushing it onto the heroes array:

The change detector ignores changes to elements of an array, so the pipe doesn't run.

The reason Angular ignores the changed array element is that the reference to the array hasn't changed. Since the array is the same, Angular does not update the display.

One way to get the behavior you want is to change the object reference itself. You can replace the array with a new array containing the newly changed elements, and then input the new array to the pipe. In the above example, you can create an array with the new hero appended, and assign that to heroes. Angular detects the change in the array reference and executes the pipe.

To summarize, if you mutate the input array, the pure pipe doesn't execute. If you replace the input array, the pipe executes and the display is updated, as shown in Figure 4.

Figure 4. The flyingHeroes pipe filtering the display to flying heroes

The above example demonstrates changing a component's code to accommodate a pipe.

To keep your component simpler and independent of HTML templates that use pipes, you can, as an alternative, use an impure pipe to detect changes within composite objects such as arrays, as described in the next section.

{@a impure-flying-heroes}

Detecting impure changes within composite objects

To execute a custom pipe after a change within a composite object, such as a change to an element of an array, you need to define your pipe as impure to detect impure changes. Angular executes an impure pipe every time it detects a change with every keystroke or mouse movement.

While an impure pipe can be useful, be careful using one. A long-running impure pipe could dramatically slow down your app.

Make a pipe impure by setting its pure flag to false:

The following code shows the complete implementation of FlyingHeroesImpurePipe, which extends FlyingHeroesPipe to inherit its characteristics. The example shows that you don't have to change anything else—the only difference is setting the pure flag as false in the pipe metadata.

FlyingHeroesImpurePipe is a good candidate for an impure pipe because the transform function is trivial and fast:

You can derive a FlyingHeroesImpureComponent from FlyingHeroesComponent. As shown in the code below, only the pipe in the template changes.

To confirm that the display updates as the user adds heroes, see the .

{@a async-pipe}

Unwrapping data from an observable

Observables let you pass messages between parts of your application. Observables are recommended for event handling, asynchronous programming, and handling multiple values. Observables can deliver single or multiple values of any type, either synchronously (as a function delivers a value to its caller) or asynchronously on a schedule.

For details and examples of observables, see the [Observables Overview](/guide/observables#using-observables-to-pass-values "Using observables to pass values"").

Use the built-in AsyncPipe to accept an observable as input and subscribe to the input automatically. Without this pipe, your component code would have to subscribe to the observable to consume its values, extract the resolved values, expose them for binding, and unsubscribe when the observable is destroyed in order to prevent memory leaks. AsyncPipe is an impure pipe that saves boilerplate code in your component to maintain the subscription and keep delivering values from that observable as they arrive.

The following code example binds an observable of message strings (message$) to a view with the async pipe.

{@a no-filter-pipe}

Caching HTTP requests

To communicate with backend services using HTTP, the HttpClient service uses observables and offers the HTTPClient.get() method to fetch data from a server. The asynchronous method sends an HTTP request, and returns an observable that emits the requested data for the response.

As shown in the previous section, you can use the impure AsyncPipe to accept an observable as input and subscribe to the input automatically. You can also create an impure pipe to make and cache an HTTP request.

Impure pipes are called whenever change detection runs for a component, which could be every few milliseconds for CheckAlways. To avoid performance problems, call the server only when the requested URL changes, as shown in the following example, and use the pipe to cache the server response. The tabs show the following:

• The fetch pipe (fetch-json.pipe.ts).
• A harness component (hero-list.component.ts) for demonstrating the request, using a template that defines two bindings to the pipe requesting the heroes from the heroes.json file. The second binding chains the fetch pipe with the built-in JsonPipe to display the same hero data in JSON format.

In the above example, a breakpoint on the pipe's request for data shows the following:

• Each binding gets its own pipe instance.
• Each pipe instance caches its own URL and data and calls the server only once.

The fetch and fetch-json pipes display the heroes as shown in Figure 5.

Figure 5. The fetch and fetch-json pipes displaying the heroes

The built-in JsonPipe provides a way to diagnose a mysteriously failing data binding or to inspect an object for future binding.