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The then() method returns a Promise. It takes two arguments: callback functions for the success and failure cases of the Promise.

Syntax

p.then(onFulfilled, onRejected);

p.then(function(value) {
   // fulfillment
  }, function(reason) {
  // rejection
});

Parameters

onFulfilled
A Function called when the Promise is fulfilled. This function has one argument, the fulfillment value.
onRejected
A Function called when the Promise is rejected. This function has one argument, the rejection reason.

Return value

A Promise.

Description

As the then and Promise.prototype.catch() methods return promises, they can be chained — an operation called composition.

Examples

Using the then method

var p1 = new Promise(function(resolve, reject) {
  resolve("Success!");
  // or
  // reject ("Error!");
});

p1.then(function(value) {
  console.log(value); // Success!
}, function(reason) {
  console.log(reason); // Error!
});

Chaining

Because the then method returns a Promise, you can easily chain then calls. Values that is not a Promise returned from the onFulfilled or onRejected callback functions will be automatically wrapped in a resolved promise.

var p2 = new Promise(function(resolve, reject) {
  resolve(1);
});

p2.then(function(value) {
  console.log(value); // 1
  return value + 1;
}).then(function(value) {
  console.log(value); // 2
});

p2.then(function(value) {
  console.log(value); // 1
});

You can also use chaining to implement one function with a Promise-based API on top of another such function.

function fetch_current_data() {
  // The fetch() API returns a Promise.  This function
  // exposes a similar API, except the fulfillment
  // value of this function's Promise has had more
  // work done on it.
  return fetch("current-data.json").then((response) => {
    if (response.headers.get("content-type") != "application/json") {
      throw new TypeError();
    }
    var j = response.json();
    // maybe do something with j
    return j; // fulfillment value given to user of
              // fetch_current_data().then()
  });
}

If onFulfilled returns a promise, the return value of then will be resolved/rejected by the promise.

function resolveLater(resolve, reject) {
  setTimeout(function () {
    resolve(10);
  }, 1000);
}
function rejectLater(resolve, reject) {
  setTimeout(function () {
    reject(20);
  }, 1000);
}

var p1 = Promise.resolve("foo");
var p2 = p1.then(function() {
  // Return promise here, that will be resolved to 10 after 1 second
  return new Promise(resolveLater);
});
p2.then(function(v) {
  console.log("resolved", v);  // "resolved", 10
}, function(e) {
  // not called
  console.log("rejected", e);
});

var p3 = p1.then(function() {
  // Return promise here, that will be rejected with 20 after 1 second
  return new Promise(rejectLater);
});
p3.then(function(v) {
  // not called
  console.log("resolved", v);
}, function(e) {
  console.log("rejected", e); // "rejected", 20
});

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Promise.prototype.then' in that specification.
Standard Initial definition in an ECMA standard.
ECMAScript 2017 Draft (ECMA-262)
The definition of 'Promise.prototype.then' in that specification.
Draft  

Browser compatibility

Feature Chrome Edge Firefox Internet Explorer Opera Safari Servo
Basic Support32.0(Yes)29.0No support197.1No support
Feature Android Chrome for Android Edge Mobile Firefox for Android IE Mobile Opera Mobile Safari Mobile
Basic Support4.4.432.0(Yes)29No support(Yes)8.0

See also

Tag del documento e collaboratori

 Hanno collaborato alla realizzazione di questa pagina: arai, eduardoboucas, fscholz, RodH257, SphinxKnight, grabber24, Sheppy, DBaron, DenisIzmaylov, realityking
 Ultima modifica di: arai,