first.md

This is RxJS v 4. Find the latest version here

Rx.Observable.prototype.first([predicate], [thisArg], [defaultValue])

Rx.Observable.prototype.first([settings])

Ⓢ

Returns the first element of an observable sequence that satisfies the condition in the predicate, or a default value if no such element exists. If no default value is given, then onError will be called.

Arguments

Rx.Observable.prototype.first([predicate], [thisArg], [defaultValue])

1. [predicate] (Function): A predicate function to evaluate for elements in the source sequence. The callback is called with the following information:
   1. the value of the element
   2. the index of the element
   3. the Observable object being subscribed
2. [thisArg] (Any): Object to use as this when executing the predicate.
3. [defaultValue] (Any): Default value if no such element exists.

Rx.Observable.prototype.first([settings])

1. [settings] (Object): An object with the following fields
   • [predicate] (Function): A predicate function to evaluate for elements in the source sequence. The callback 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.
   • [defaultValue] (Any): Default value if no such element exists.

Returns

(Observable): An observable sequence that contains elements from the input sequence that satisfy the condition.

Example

/* Without a predicate */
var source = Rx.Observable.range(0, 10).first();

var subscription = source.subscribe(
  function (x) {
    console.log('Next: %s', x);
  },
  function (err) {
    console.log('Error: %s', err);
  },
  function () {
    console.log('Completed');
  });

// => Next: 0
// => Completed

/* With a predicate */
var source = Rx.Observable.range(0, 10)
  .first(function (x, idx, obs) { return x % 2 === 1; });

var subscription = source.subscribe(
  function (x) {
    console.log('Next: %s', x);
  },
  function (err) {
    console.log('Error: %s', err);
  },
  function () {
    console.log('Completed');
  });

// => Next: 1
// => Completed

/* With a default value */
var source = Rx.Observable.range(0, 10)
  .first({
    predicate: function (x, idx, obs) { return x > 10; },
    defaultValue: 42
  });


var subscription = source.subscribe(
  function (x) {
    console.log('Next: %s', x);
  },
  function (err) {
    console.log('Error: %s', err);
  },
  function () {
    console.log('Completed');
  });

// => Next: 42
// => Completed

Location

File:

• /src/core/linq/observable/first.js

Dist:

• rx.all.js
• rx.all.compat.js
• rx.aggregates.js

Prerequisites:

• rx.js | rx.compat.js | rx.lite.js | rx.lite.compat.js

NPM Packages:

• rx

NuGet Packages:

• RxJS-All
• RxJS-Aggregates

Unit Tests:

• /tests/observable/first.js