Function: first()

first<T>(predicate?): Operator<T, T>

Defined in: operators/first.ts:22

Creates a stream operator that emits only the first element from the source stream that matches an optional predicate.

This operator is designed to find a specific value and then immediately terminate.

• If a predicate function is provided, the operator will emit the first value for which the predicate returns a truthy value.
• If no predicate is provided, it will simply emit the very first value from the source.

After emitting a single value, the operator completes. If the source stream completes before a matching value is found, an error is thrown.

Type Parameters

T

T = any

The type of the values in the source stream.

Parameters

predicate?

(value) => CallbackReturnType<boolean>

An optional function to test each value. It receives the value and should return true to indicate a match.

Returns

Operator<T, T>

An Operator instance that can be used in a stream's pipe method.

Throws

Throws an error with the message "No elements in sequence" if no matching value is found before the source stream completes.

Examples

From first.spec.ts:4

const testStream = from([1, 2, 3, 4]);
const firstStream = testStream.pipe(first());
setTimeout(() => {
  firstStream.subscribe({
    next: (value) => {
      expect(value).toBe(1); // Should emit the first value even with a delay
      done();
    },
    error: (err) => done.fail(err),
    complete: () => {},
  });
}, 100);