first

signature: first(predicate: function, select: function)

Emit the first value or first to pass provided expression.

The counterpart to first is last!

Examples

( example tests )

Example 1: First value from sequence

( StackBlitz | jsBin | jsFiddle )

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

const source = from([1, 2, 3, 4, 5]);
//no arguments, emit first value
const example = source.pipe(first());
//output: "First value: 1"
const subscribe = example.subscribe(val => console.log(`First value: ${val}`));

Example 2: First value to pass predicate

( StackBlitz | jsBin | jsFiddle )

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

const source = from([1, 2, 3, 4, 5]);
//emit first item to pass test
const example = source.pipe(first(num => num === 5));
//output: "First to pass test: 5"
const subscribe = example.subscribe(val =>
  console.log(`First to pass test: ${val}`)
);

Example 3: Utilizing default value

( StackBlitz | jsBin | jsFiddle )

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

const source = from([1, 2, 3, 4, 5]);
//no value will pass, emit default
const example = source.pipe(first(val => val > 5, 'Nothing'));
//output: 'Nothing'
const subscribe = example.subscribe(val => console.log(val));

Additional Resources

• first - Official docs
• Filtering operator: take, first, skip - André Staltz

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