filter

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

Emit values that pass the provided condition.

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

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}`)
);

Related Recipes

• HTTP Polling
• Game Loop

Additional Resources

• filter - Official docs
• Adding conditional logic with filter - John Linquist
• Filtering operator: filter - André Staltz

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