Why and How to Create an Impure Filter Pipe in Angular (2023)


Understand why Angular doesn’t ship with filtering and sorting pipes and how you can create a custom filter pipe

Why and How to Create an Impure Filter Pipe in Angular (1)

Angular offers several built-in pipes for the most common scenarios.

One of the big absences is a filter pipe. There is a very good reason why Angular doesn’t offer a built-in filter pipe.

Pipes for filtering and sorting lists are not available in Angular, even though AngularJS used to offer filterand orderBy.

This isn’t a mistake.

Because such pipelines perform badly and prohibit aggressive minification, Angular does not offer them out of the box. Filtering is costly.

Even though computing power grows every year, data and information also grow constantly!

Why and How to Create an Impure Filter Pipe in Angular (2)

Generally speaking, parameters that correspond to object attributes are required for filtering. However, some scenarios require pipes to be impure, meaning that Angular calls impure pipes practically every time a change-detection cycle occurs.

As a consequence, filtering and sorting become costly operations, especially when the amount of data is significant.

(Video) Creating Filter Pipe in Angular | Angular Routing | Angular 13+

When Angular invokes these pipes multiple times per second, the user experience for even moderate-sized lists might suffer significantly.

First of all, let’s have a look at the template and the class.

AppComponent and Data

In the template, we have an input element that uses two-way binding to store the user input in a property called filterBy. Remember to import FormsModule in AppModule to use two-way binding in your app.

Below, we use NgFor to list the name of some users from usersList. Here’s the code:

The list of users usersList is an array of ten items, e.g., users. It is stored in the class, and you can find it on jsonplaceholder or in the StackBlitz of this app.

Each user is an object with several properties. For our purpose, it suffices to know that one of the properties is name.

We want to filter users by name.

The initial app looks like this:

Why and How to Create an Impure Filter Pipe in Angular (3)

Let’s start by using the Angular CLI command ng generate pipe filter, where filter is the pipe name that will be used in template bindings.

As a side note, you can also use ng g p pipe.

Angular will take care of creating a file, populating some fields, and importing it correctly in app.module.ts.

In the transform method, value is the value on the left side of the pipe in the template while args?is an optional argument that we will use to filter value.

(Video) Data Filtering in Angular Component | Angular Pipes | Angular 12+

To use this filter, we can simply add it to the template as follows:

<div *ngFor="let user of usersList | filter">

Naturally, nothing happens. However, note that usersList is the property that goes through the pipe and is associated with thevalue argument in the transform method.

We will now work inside the transform method in FilterPipe.

The arguments of the transform method

First, we update the arguments of the transform method with this code:

transform(value: User[], filterString: string, property: string)

We discussed value above. The type of value is an array of objects that are shaped like the User interface. You can see that in model.ts in StackBlitz.

The second argument is filterString of type string. The value of filterString has to come from the template. Therefore, we will add a parameter to the pipe. The parameter will be filterBy that is the user input, e.g., the string the user wants to use as a filter.

The third and last argument is property. Since our user objects contain several properties, we need to specify which property we want to use. For the sake of simplicity, I will hard code the string name as the second parameter of the filter pipe in the template.

This is an example. Usually, you shouldn’t hard code values in your code.

In the template, the pipe becomes:

<div *ngFor="let user of usersList | filter: filterBy:'name'">
<li>{{ user.name }}</li>

where filteris the name of the pipe, filterByis the property we get from the input element thanks to two-way binding, and 'name' is the hardcoded property in the user objects.

The number of parameters you need is optional, according to your needs.

Filtering logic in the transform method

Now we work inside the transform method.

We start by adding some code to return every value when there is no filter.

if (value.length === 0 || !filterString) {
return value;

Then, we add the logic to filter based on the user’s string.

(Video) Pure vs Impure Pipes | Angular Interview Concepts

let filteredUsers: User[] = [];for (let user of value) { if (user[property].includes(filterString)) {
}return filteredUsers;

Remember that property is hardcoded and it means name. value is the list of users from usersListand filterString is the string that a person wants to use as a filter, e.g., filterBy.

Therefore, you can read this code as “for each user from userList, if the user’s name includes the filterBystring, add this user to the filteredUsersarray” that is eventually returned.

This is FilterPipe so far:

At this point, the pipe allows us to filter names just fine.

Note that I added the JavaScript toLowerCase() method so that the filter is case insensitive. This is totally optional of course.

What about the expensive filtering operations?

Let’s take it one step further.

We add a simple button that adds a new user to usersList with this code:

<button (click)="onAddUser()">Add user</button>

The method looks like this:

onAddUser() {
id: Math.floor(Math.random() * 10000),
name: 'Leanne Graham',
username: 'Bret',

On every click, one user gets pushed to usersList.

Here is the problem!

If you filter for Leanne and then you click on the button, the user is added to usersList but the filtered list is not rendered. Therefore, you won’t see the new users in the filtered list, even though you will see them as soon as you remove the filter.

Angular doesn’t rerun the pipe on the data every time composite arrays or objects change.

Pure and impure pipes

To be more precise, we need to talk about pure and impure pipes.

(Video) Impure Pipes in Angular | Angular Pipes | Angular 13+

We have a pure pipe when there is a pure change to the input value. A pure change can be one of the following:

  • a change to a primitive input value (String, Number, Boolean, Symbol)
  • a change to an object reference (Date, Array, Function, Object)

A pure pipe uses pure functions. Given the same input, pure functions should always return the same output.

By default, pipes are pure because Angular ignores changes inside objects or arrays. Therefore, it won’t call a pure pipe when we add to an input array, like in our case, or update an input object property.

As you can guess, an object reference check is faster than a deep check for differences. Furthermore, when Angular skips the pipe execution, it also skips the view update.

For this reason, a pure pipe is preferable when the regular change detection strategy is fine.

When we need a deep check, like in the current example, we need to use impure pipes.

An impure pipe runs during every component change detection cycle. For instance, it could run for every keystroke or mouse move.

As you can guess, this become very expensive fast.

Having said that, implementing an impure pipe is as simple as declaring pure: false in the Pipe decorator.

name: 'filter',
pure: false,

By adding the pure property to the Pipe decorator, we force FilterPipe to update every time data changes.

Add a log inside the pipe to visualize the frequency.

Using impure pipes might lead to performance and aggressive minimization issues. So use it carefully and only when really needed.

Find the code on StackBlitz or clone the app from GitHub.

As a side note, it is worth mentioning that the async pipe is impure. Roughly speaking, we could argue that “a pipe is impure when it is always checking for new input data.”

(Video) Angular impure pipe

While the Angular async pipe optimizes change detection, it also removes the necessity to use subscribe and unsubscribe manually.

Furthermore, it is a good way to use an RxJS Declarative Pattern in Angular.


Why we use impure pipes in Angular? ›

An impure pipe is called for every change detection cycle no matter whether the value or parameter(s) changes. when the pipe injects a service to get access to other values, Angular doesn't recognize if they have changed.

Why Async pipe is impure? ›

Because of the way Promise s work, Angular's async pipe has to be impure (meaning that it can return different outputs without any change in input). The transform method on Pipe s is synchronous, so when an async pipe gets a Promise , the pipe adds a callback to the Promise and returns null.

What impure pipes? ›

Impure Pipes:

Impure pipes in angular are the pipes that execute when it detects an impure change in the input value. An impure change is when the change detection cycle detects a change to composite objects, such as adding an element to the existing array.

How do you specify impure pipes using pure property? ›

From the above code, the isPure method will check whether a pipe is pure or impure by looking at the pure property in @Pipe decorator. For impure pipes Angular calls the transform method on every change detection. For any input change to the pure pipe, it will call transform function.

What is difference between pipes and filters in Angular? ›

In Angular 1, we had filter which helped format, sort or transform how data was displayed in our templates. Filters can be used with a binding expression or a directive. In Angular 2, we have a similar feature but renamed to Pipes. This rename was to better align of what the feature does.

Why Async pipe is used in Angular? ›

The async pipe subscribes to an Observable or Promise and returns the latest value it has emitted. When a new value is emitted, the async pipe marks the component to be checked for changes. When the component gets destroyed, the async pipe unsubscribes automatically to avoid potential memory leaks.

What is impure method? ›

An impure function is a function that contains one or more side effects. It mutates data outside of its lexical scope and does not predictably produce the same output for the same input.

What is difference between observable and observer in Angular? ›

Observable are just that — things you wish to observe and take action on. Angular uses the Observer pattern which simply means — Observable objects are registered, and other objects observe (in Angular using the subscribe method) them and take action when the observable object is acted on in some way.

What is pure and impure procedure? ›

An impure function is a function that contains one or more side effects. A pure function is a function without any side effects.

What are different types of pipes in Angular? ›

Angular provides the following set of built-in pipes.
  • CurrencyPipe. This pipe is used for formatting currencies. ...
  • DatePipe. This pipe is used for the transformation of dates. ...
  • DecimalPipe. This pipe is used for transformation of decimal numbers. ...
  • JsonPipe. ...
  • LowerCasePipe. ...
  • UpperCasePipe. ...
  • PercentPipe. ...
  • SlicePipe.

Are Angular pipes pure by default? ›

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 ).

What does .pipe do in Angular? ›

Ans. The pipe method in AngularJS is used to combine multiple functions into a single one. It can take single or multiple functions as the parameters and then transforms the arguments into various collections.

Is Async pipe pure? ›

Async is an example of an impure pipe. It is always checking for new input data. Pure will be true if not specified. The pure property tells Angular whether or not the value should be recomputed when its input changes.

Which character is used for chaining multiple pipes in Angular? ›

What are Angular Pipes?
  • Pipes are defined using the pipe “|” symbol.
  • Pipes can be chained with other pipes.
  • Pipes can be provided with arguments by using the colon (:) sign.
16 Aug 2022

Can you name some built-in Angular pipes? ›

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.

What is the advantage of pipe and filters? ›

Decompose a task that performs complex processing into a series of separate elements that can be reused. This can improve performance, scalability, and reusability by allowing task elements that perform the processing to be deployed and scaled independently.

What are the different types of filters in Angular? ›

AngularJS Filters
  • currency Format a number to a currency format.
  • date Format a date to a specified format.
  • filter Select a subset of items from an array.
  • json Format an object to a JSON string.
  • limitTo Limits an array/string, into a specified number of elements/characters.
  • lowercase Format a string to lower case.

What is difference between stateful and stateless pipes in Angular? ›

Most pipes are stateless. The CurrencyPipe we used and the length pipe we created are examples of a stateless pipe. Stateful pipes are those which can manage the state of the data they transform. A pipe that creates an HTTP request, stores the response and displays the output, is a stateful pipe.

Which is faster sync or async? ›

Sync is blocking — it will only send the server one request at a time and will wait for that request to be answered by the server. Async increases throughput because multiple operations can run at the same time. Sync is slower and more methodical.

Is async faster than threading? ›

So why asyncio is faster than multi-threading if they both belong to asynchronous programming? It's because asyncio is more robust with task scheduling and provides the user with full control of code execution.

Is async pure or impure? ›

A good example of impure pipe is the AsyncPipe from @angular/common package. This pipe has internal state that holds an underlying subscription created by subscribing to the observable passed to the pipe as a parameter.

What do you mean by impure? ›

: containing something unclean : foul. impure water. : mixed or impregnated with an extraneous and usually unwanted substance. an impure chemical. : lewd, unchaste.

Does impure method return a value? ›

An impure function does not return anything.

What is the difference between impurity and purity? ›

An impure substance is a combination or mixture of two or more different substances that are not chemically bonded together. Purity is how pure a substance is or the degree to which a substance is free from contamination. An impurity is a substance that is mixed with, and contaminates, a desired substance.

Is Observable sync or async? ›

An observable produces values over time. An array is created as a static set of values. In a sense, observables are asynchronous where arrays are synchronous. In the following examples, → implies asynchronous value delivery.

Is Observable hot or cold? ›

An Observable is cold when data is produced inside the Observable and the Observable is hot when the data is produced outside the Observable. As we just saw the hot Observable is able to share data between multiple subscribers. We call this behaviour “multicasting”.

Is Observable better than promise? ›

Both observables and promises help us work with asynchronous functionality in JavaScript. Promises deal with one asynchronous event at a time, while observables handle a sequence of asynchronous events over a period of time. Emit multiple values over a period of time.

What is the difference between pure and impure spectrum? ›

A spectrum in which there is overlapping of different colours is called an impure Spectrum. In other words in an impure spectrum the colours are not distinct and independent but they overlap. On the other hand a spectrum with each colour distinct and independent is called a pure spectrum.

What are the two types of purity? ›

A brief general introduction to the concept of purity in Islam and its division into two types: physical purity and spiritual purity.

What are the two types of pure? ›

Pure substances can be divided into two categories, elements and compounds.

What are the two types of pipelines? ›

There are two types of oil pipeline: crude oil pipeline and product pipeline.

What are the three types of pipes? ›

There are five main types of plumbing pipe materials that are still in use today: copper, galvanized steel, polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS), and cross-linked polyethylene (PEX).

Can JSON used in pipes Angular? ›

Angular Json Pipe converts a value or object into JSON formatted string. Usually we will get the data from the server in JSON format, and we will bind it to HTML. To know the exact JSON data coming from the server we can use network tab in browser developer tools.

What is difference between pipe and directive in Angular? ›

Pipes are for formatting data, directives are to alter the behavior/appearance of an element.

Is there filter pipe in Angular? ›

Let's start by using the Angular CLI command ng generate pipe filter , where filter is the pipe name that will be used in template bindings. As a side note, you can also use ng g p pipe . Angular will take care of creating a file, populating some fields, and importing it correctly in app.

Can I use pipe in ngModel? ›

Use pipes to transform strings, currency amounts, dates, and other data for display. This method results in an error, however: “You cannot have a pipe in an action expression.” Since I was using two-way data binding on ngModel, I was basically trying to format an input rather than an output.

What is lazy loading in Angular? ›

Lazy loading is an approach to limit the modules that are loaded to the ones that the user currently needs. This can improve your application's performance and reduce the initial bundle size. By default, Angular uses eager loading to load modules.

What is the use of pipe in observable? ›

RxJS' pipe() is both a standalone function and a method on the Observable interface that can be used to combine multiple RxJS operators to compose asynchronous operations. The pipe() function takes one or more operators and returns an RxJS Observable.

Can Angular pipe return observable? ›

Angular expects pipes to be synchronous. They should return a resolved value, not a Promise or an Observable . To use a pipe that returns an unresolved value, you can use Angular's async pipe. If that's not an option, you can resolve the asynchronous value inside the pipe if you make it impure, as I describe below.

Is Angular synchronous or asynchronous? ›

Angular makes use of observables as an interface to handle a variety of common asynchronous operations. For example: The HTTP module uses observables to handle AJAX requests and responses.

What is difference between pure and impure pipe in Angular? ›

Pure pipes are the pipes which are executed only when a “PURE CHANGE” to the input value is detected. So impure pipe executes everytime irrespective of source has changed or not.

Can we add two pipes in Angular? ›

No Filter Pipe Or OrderBy Pipe:

Luckily, now these two pipes are not available in Angular 2 and instead Angular allows us to write custom pipes so we can write the custom pipes according to the logic we like to use. So let's see how to create the custom pipes.

Can two components have same name in Angular? ›

Two or more components use the same element selector. Because there can only be a single component associated with an element, selectors must be unique strings to prevent ambiguity for Angular.

What is impure pipe in Angular? ›

An impure pipe is called on every change detection cycle in Angular. It is called on every digest cycle irrespective of the change in the input or value. If we need some pipe to be called on every change detection, mark the pipe as impure.

How many pipes are available in Angular? ›

The last yet important thing I want to mention is that there are two types of pipes in Angular, pure and impure pipes. A pure pipe (the default) is only called when Angular detects a change in the value or the parameters passed to a pipe.

Why do we need Named Pipes? ›

Named pipes allow transfer of data between processes in a FIFO manner and synchronization of process execution. Use of a named pipe allows processes to communicate even though they do not know what processes are on the other end of the pipe.

What is the use of custom Pipes? ›

Create custom pipes to encapsulate transformations that are not provided with the built-in pipes. 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.

What is the impure function? ›

An impure function is a function that contains one or more side effects. It mutates data outside of its lexical scope and does not predictably produce the same output for the same input.

What is impure function example? ›

First, an impure function is a function that contains one or more side effects. In the snippet above, updateMyName() is an impure function because it contains code ( myNames ) that mutates an external state — which gives updateMyName() some side effects.

Is RESTful stateless or stateful? ›

As per the REST architecture, a RESTful Web Service should not keep a client state on the server. This restriction is called Statelessness. It is the responsibility of the client to pass its context to the server and then the server can store this context to process the client's further request.

Why is stateless better than stateful? ›

A stateless system sends a request to the server and relays the response (or the state) back without storing any information. On the other hand, stateful systems expect a response, track information, and resend the request if no response is received.

Is stateless or stateful faster? ›

Stateless firewalls deliver fast performance. Heavy traffic is no match for stateless firewalls, which perform well under pressure without getting caught up in the details. Stateless firewalls have historically been cheaper to purchase, although these days stateful firewalls have significantly come down in price.

What are the impure pipe in Angular example? ›

A good example of impure pipe is the AsyncPipe from @angular/common package. This pipe has internal state that holds an underlying subscription created by subscribing to the observable passed to the pipe as a parameter.

What is Pipetransform in Angular? ›


An interface that is implemented by pipes in order to perform a 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.

Can we create custom pipes in Angular? ›

Creating Custom Pipes

Angular makes provision to create custom pipes that convert the data in the format that you desire. Angular Pipes are TypeScript classes with the @Pipe decorator. The decorator has a name property in its metadata that specifies the Pipe and how and where it is used.

What are custom pipes in Angular? ›

The most basic of pipe transforms a single value, into a new value. This value can be anything you like, a string, array, object, etc.


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