rxjs/src/internal/observable/from.ts

Reactive Extensions for modern JavaScript

import { Observable } from '../Observable';
import { ObservableInput, SchedulerLike, ObservedValueOf } from '../types';
import { scheduled } from '../scheduled/scheduled';
import { innerFrom } from './innerFrom';

export function from<O extends ObservableInput<any>>(input: O): Observable<ObservedValueOf<O>>;
/** @deprecated The `scheduler` parameter will be removed in v8. Use `scheduled`. Details: https://rxjs.dev/deprecations/scheduler-argument */
export function from<O extends ObservableInput<any>>(input: O, scheduler: SchedulerLike | undefined): Observable<ObservedValueOf<O>>;

/**
 * Creates an Observable from an Array, an array-like object, a Promise, an iterable object, or an Observable-like object.
 *
 * <span class="informal">Converts almost anything to an Observable.</span>
 *
 * ![](from.png)
 *
 * `from` converts various other objects and data types into Observables. It also converts a Promise, an array-like, or an
 * <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#iterable" target="_blank">iterable</a>
 * object into an Observable that emits the items in that promise, array, or iterable. A String, in this context, is treated
 * as an array of characters. Observable-like objects (contains a function named with the ES2015 Symbol for Observable) can also be
 * converted through this operator.
 *
 * ## Examples
 *
 * Converts an array to an Observable
 *
 * ```ts
 * import { from } from 'rxjs';
 *
 * const array = [10, 20, 30];
 * const result = from(array);
 *
 * result.subscribe(x => console.log(x));
 *
 * // Logs:
 * // 10
 * // 20
 * // 30
 * ```
 *
 * Convert an infinite iterable (from a generator) to an Observable
 *
 * ```ts
 * import { from, take } from 'rxjs';
 *
 * function* generateDoubles(seed) {
 *    let i = seed;
 *    while (true) {
 *      yield i;
 *      i = 2 * i; // double it
 *    }
 * }
 *
 * const iterator = generateDoubles(3);
 * const result = from(iterator).pipe(take(10));
 *
 * result.subscribe(x => console.log(x));
 *
 * // Logs:
 * // 3
 * // 6
 * // 12
 * // 24
 * // 48
 * // 96
 * // 192
 * // 384
 * // 768
 * // 1536
 * ```
 *
 * With `asyncScheduler`
 *
 * ```ts
 * import { from, asyncScheduler } from 'rxjs';
 *
 * console.log('start');
 *
 * const array = [10, 20, 30];
 * const result = from(array, asyncScheduler);
 *
 * result.subscribe(x => console.log(x));
 *
 * console.log('end');
 *
 * // Logs:
 * // 'start'
 * // 'end'
 * // 10
 * // 20
 * // 30
 * ```
 *
 * @see {@link fromEvent}
 * @see {@link fromEventPattern}
 *
 * @param input A subscription object, a Promise, an Observable-like,
 * an Array, an iterable, or an array-like object to be converted.
 * @param scheduler An optional {@link SchedulerLike} on which to schedule the emission of values.
 * @return An Observable converted from {@link ObservableInput}.
 */
export function from<T>(input: ObservableInput<T>, scheduler?: SchedulerLike): Observable<T> {
  return scheduler ? scheduled(input, scheduler) : innerFrom(input);
}