Pipes are simple functions to use in [template expressions](/guide/glossary#template-expression "Definition of template expression") 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.
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:
* For a complete list of built-in pipes, see the [pipes API documentation](/api/common#pipes "Pipes API reference summary").
* To learn more about using pipes for internationalization (i18n) efforts, see [formatting data based on locale](/guide/i18n#i18n-pipes "Formatting data based on locale").
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`](api/common/DatePipe).
For example, `{{ amount | currency:'EUR':'Euros '}}` adds the second parameter, the string literal `'Euros '`, to the output string. 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`](/api/common/SlicePipe "API reference for 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`.
* The `app.component.html` template uses a format parameter for the [`DatePipe`](api/common/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
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**.
To mark a class as a pipe and supply configuration metadata, apply the [`@Pipe`](/api/core/Pipe "API reference for Pipe") [decorator](/guide/glossary#decorator--decoration "Definition for decorator") to the class.
Use [UpperCamelCase](guide/glossary#case-types "Definition of case types") (the general convention for class names) for the pipe class name, and [camelCase](guide/glossary#case-types "Definition of case types") for the corresponding `name` string.
Do not use hyphens in the `name`.
For details and more examples, see [Pipe names](guide/styleguide#pipe-names "Pipe names in the Angular coding style guide").
* 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 application (<live-example></live-example>). For details, see [NgModules](guide/ngmodules "NgModules introduction").
* Register your custom pipes. The [Angular CLI](cli "CLI Overview and Command Reference") [`ng generate pipe`](cli/generate#pipe "ng generate pipe in the CLI Command Reference") command registers the pipe automatically.
Implement the [`PipeTransform`](/api/core/PipeTransform "API reference for 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.
* 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.
You use [data binding](/guide/glossary#data-binding "Definition of 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.
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.
Angular looks for changes to data-bound values in a [change detection](guide/glossary#change-detection "Definition of 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.
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`).
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.
Replace the array with a new array containing the newly changed elements, and then input the new array to the pipe.
In the preceding example, 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 keep your component 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.
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.
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`](/api/common/AsyncPipe "API description of 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.
To [communicate with backend services using HTTP](/guide/http "Communicating with backend services using HTTP"), the `HttpClient` service uses observables and offers the `HttpClient.get()` method to fetch data from a server.
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.
* 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.
The built-in [JsonPipe](api/common/JsonPipe "API description for JsonPipe") provides a way to diagnose a mysteriously failing data binding or to inspect an object for future binding.
