Reference · 20

first

Emits the first value (optionally matching a predicate) and completes.

Signature

first(predicate?: (v: T, i: number) => boolean, defaultValue?: T): OperatorFunction

Marble diagram

in:           --1--2--3--|
first(>1):       -----2|

What it does

With no arguments it's the same as take(1). With a predicate it takes the first value that satisfies the condition. If the stream completes without a match, it throws EmptyError (or emits the default, if one is given).

When to use

Get the first click, wait for an element to appear in the stream, get the first successful response. When you want the strict "otherwise error" semantics; otherwise take(1) is preferable.

Gotcha

first(predicate) and filter(predicate).pipe(take(1)) differ: the former throws EmptyError, the latter just completes quietly.

first

first

Signature

Marble diagram

What it does

When to use

See also