Returns an observable that, at the moment of subscription, will synchronously emit all values provided to this operator, then subscribe to the source and mirror all of its emissions to subscribers.

startWith<T, D>(...values: D[]): OperatorFunction<T, T | D>

Parameters

values

D[]

Items you want the modified Observable to emit first.

Returns

OperatorFunction<T, T | D>: A function that returns an Observable that synchronously emits provided values before subscribing to the source Observable.

Description

This is a useful way to know when subscription has occurred on an existing observable.

First emits its arguments in order, and then any emissions from the source.

Examples

Emit a value when a timer starts.

import { timer, map, startWith } from 'rxjs';

timer(1000)
  .pipe(
    map(() => 'timer emit'),
    startWith('timer start')
  )
  .subscribe(x => console.log(x));

// results:
// 'timer start'
// 'timer emit'

Overloads

startWith(value: null): OperatorFunction<T, T | null>

Parameters

value

null

Returns

OperatorFunction<T, T | null>

startWith(value: undefined): OperatorFunction<T, T | undefined>

Parameters

value

undefined

Returns

OperatorFunction<T, T | undefined>

startWith(...valuesAndScheduler: [...A, SchedulerLike]): OperatorFunction<T, T | ValueFromArray<A>>

Parameters

valuesAndScheduler

[...A, SchedulerLike]

Returns

OperatorFunction<T, T | ValueFromArray<A>>

startWith(...values: A): OperatorFunction<T, T | ValueFromArray<A>>

Parameters

values

A

Returns

OperatorFunction<T, T | ValueFromArray<A>>

See Also

• endWith
• finalize
• concat