Operator: map / select

Syntax#

Rx.Observable.prototype.map(selector, [thisArg])
Rx.Observable.prototype.select(selector, [thisArg])

Parameters#

| Parameter, Type | Details | | --- | --- | --- | | selector, Function or Object | Transform function to apply to each source element or an element to yield. If selector is a function, it is called with the following information: 1. the value of the element, 2. the index of the element, 3. the Observable object being subscribed. | | [thisArg], Any | Object to use as this when executing the predicate. |

Remarks#

map and select are aliases.

They produce an observable sequence emitting one element every time the source observable emits an element.

If selector is not a function, its value is emitted for each source element.

If selector is a function, the emitted element is the result of running selector on the source element, and can possibly use its position.

Using an element to yield

const md = Rx.Observable.fromEvent(document, 'mousedown').map(true);
// `md` will emit `true` whenever the mouse is pressed
const mu = Rx.Observable.fromEvent(document, 'mouseup').map(false);
// `mu` will emit `false` whenever the mouse is depressed

Using a transform function

const source = Rx.Observable.range(1, 3)
  .map(x => x * x);

const subscription = source.subscribe(
  x => console.log(`Next: ${x}`),
  err => console.log(`Error: ${err}`),
  () => console.log(`Completed`)
);

// => Next: 1
// => Next: 4
// => Next: 9
// => Completed

Using a transform function and the element index

const source = Rx.Observable.range(1, 3)
  .map((x, idx, obs) => `Element ${x} was at position ${idx}`);

const subscription = source.subscribe(
  x => console.log(`Next: ${x}`),
  err => console.log(`Error: ${err}`),
  () => console.log(`Completed`)
);

// => Next: Element 1 was at position 0
// => Next: Element 2 was at position 1
// => Next: Element 3 was at position 2
// => Completed