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first
signature: first(predicate: function, select: function)
first(predicate: function, select: function)Emit the first value or first to pass provided expression.
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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.Examples
Example 1: First value from sequence
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// RxJS v6+
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import { from } from 'rxjs';
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import { first } from 'rxjs/operators';
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const source = from([1, 2, 3, 4, 5]);
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//no arguments, emit first value
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const example = source.pipe(first());
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//output: "First value: 1"
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const subscribe = example.subscribe(val => console.log(`First value: ${val}`));
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Example 2: First value to pass predicate
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// RxJS v6+
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import { from } from 'rxjs';
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import { first } from 'rxjs/operators';
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const source = from([1, 2, 3, 4, 5]);
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//emit first item to pass test
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const example = source.pipe(first(num => num === 5));
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//output: "First to pass test: 5"
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const subscribe = example.subscribe(val =>
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console.log(`First to pass test: ${val}`)
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);
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Example 3: Utilizing default value
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// RxJS v6+
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import { from } from 'rxjs';
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import { first } from 'rxjs/operators';
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const source = from([1, 2, 3, 4, 5]);
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//no value will pass, emit default
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const example = source.pipe(first(val => val > 5, 'Nothing'));
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//output: 'Nothing'
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const subscribe = example.subscribe(val => console.log(val));
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Additional Resources
- 🎥 💵 - André Staltz
Last modified 2yr ago