Reference · 06

defer

Creates an Observable "lazily" — the factory runs on every subscription.

Signature

defer(observableFactory: () => ObservableInput): Observable

What it does

Defers creating the Observable until subscribe. Each subscriber gets a "fresh" Observable from the factory. This makes values that would otherwise be fixed at creation time (Date.now, new Promise(...), random) truly lazy.

When to use

You need a new Promise/HTTP request per subscriber. You need data that's current at subscription time, not at pipe-configuration time. You want to turn ordinary code into a cold Observable.

Gotcha

of(fetch('/api')) creates one Promise when the module loads. defer(() => fetch('/api')) creates one per subscription.

defer

defer

Signature

What it does

When to use

See also