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first

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

Emit the first value or first to pass provided expression.

💡 The counterpart to first is last!
💡 First will deliver an EmptyError to the Observer's error callback if the Observable completes before any next notification was sent. If you don't want this behavior, use take(1) instead.
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Examples

Example 1: First value from sequence
// 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
// 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
// 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

Last modified 3yr ago