filter

signature: filter(select: Function, thisArg: any): Observable

Emit values that pass the provided condition.

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💡 If you would like to complete an observable when a condition fails, check out takeWhile!

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Why use filter?

This operator is your go-to when you need to sift out unwanted values from an observable stream. Think of it as a fisherman's net, catching only the types of fish you desire while allowing others to slip through.

The critical point to remember is that filter will only emit values that meet the specified condition. If no values in the observable satisfy the condition, nothing gets emitted. It's a strict bouncer at a club's entrance, only letting in those who fit the criteria.

Also, for scenarios where you not only want to filter values but also transform them, map is an ideal companion to filter. Use them in tandem to both shape and refine your data streams.

Examples

Example 1: filter for even numbers

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { from } from 'rxjs';
import { filter } from 'rxjs/operators';

//emit (1,2,3,4,5)
const source = from([1, 2, 3, 4, 5]);
//filter out non-even numbers
const example = source.pipe(filter(num => num % 2 === 0));
//output: "Even number: 2", "Even number: 4"
const subscribe = example.subscribe(val => console.log(`Even number: ${val}`));

Example 2: filter objects based on property

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { from } from 'rxjs';
import { filter } from 'rxjs/operators';

//emit ({name: 'Joe', age: 31}, {name: 'Bob', age:25})
const source = from([
  { name: 'Joe', age: 31 },
  { name: 'Bob', age: 25 }
]);
//filter out people with age under 30
const example = source.pipe(filter(person => person.age >= 30));
//output: "Over 30: Joe"
const subscribe = example.subscribe(val => console.log(`Over 30: ${val.name}`));

Example 3: filter for number greater than specified value

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { interval } from 'rxjs';
import { filter } from 'rxjs/operators';

//emit every second
const source = interval(1000);
//filter out all values until interval is greater than 5
const example = source.pipe(filter(num => num > 5));
/*
  "Number greater than 5: 6"
  "Number greater than 5: 7"
  "Number greater than 5: 8"
  "Number greater than 5: 9"
*/
const subscribe = example.subscribe(val =>
  console.log(`Number greater than 5: ${val}`)
);

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Additional Resources

• filter 📰 - Official docs

• Adding conditional logic with filter 🎥 💵 - John Linquist

• Filtering operator: filter 🎥 💵 - André Staltz

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📁 Source Code: https://github.com/ReactiveX/rxjs/blob/master/packages/rxjs/src/internal/operators/filter.ts