Revision 926455 of Promise.prototype.then()

<div>{{JSRef}} {{harmony}}</div>

<p>The <code><strong>then()</strong></code> method returns a <code>Promise</code>. It takes two arguments, both are callback functions for the success and failure cases of the <code>Promise</code>.</p>

<h2 id="Syntax">Syntax</h2>

<pre class="syntaxbox">
<var>p.then(onFulfilled, onRejected)</var>;

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

<h3 id="Parameters">Parameters</h3>

<dl>
 <dt>onFulfilled</dt>
 <dd>A {{jsxref("Function")}} called when the <code>Promise</code> is fulfilled. This function has one argument, the fulfillment <code>value</code>.</dd>
 <dt>onRejected</dt>
 <dd>A {{jsxref("Function")}} called when the <code>Promise</code> is rejected. This function has one argument, the rejection <code>reason</code>.</dd>
</dl>

<h2 id="Description">Description</h2>

<p>As the <code>then</code> and {{jsxref("Promise.prototype.catch()")}} methods return promises, they can be chained — an operation called <em>composition</em>.</p>

<h2 id="Examples">Examples</h2>

<h3 id="Using_the_then_method">Using the <code>then</code> method</h3>

<pre class="brush: js">
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!
});
</pre>

<h3 id="Chaining">Chaining</h3>

<p>Because the <code>then</code> method returns a <code>Promise</code>, you can easily chain <code>then</code> calls.</p>

<pre class="brush: js">
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
});
</pre>

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

<pre class="brush: js">
function fetch_current_data() {
  // The <a href="/en-US/docs/Web/API/GlobalFetch/fetch">fetch</a>() 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) =&gt; {
    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()
  });
}
</pre>

<h2 id="Specifications">Specifications</h2>

<table class="standard-table">
 <tbody>
  <tr>
   <th scope="col">Specification</th>
   <th scope="col">Status</th>
   <th scope="col">Comment</th>
  </tr>
  <tr>
   <td>{{SpecName('ES6', '#sec-promise.prototype.then', 'Promise.prototype.then')}}</td>
   <td>{{Spec2('ES6')}}</td>
   <td>Initial definition in an ECMA standard.</td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility</h2>

<p>{{CompatibilityTable}}</p>

<div id="compat-desktop">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Chrome</th>
   <th>Firefox (Gecko)</th>
   <th>Internet Explorer</th>
   <th>Opera</th>
   <th>Safari</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>32</td>
   <td>{{CompatGeckoDesktop(29.0)}} [1]</td>
   <td>{{CompatNo}}</td>
   <td>19</td>
   <td>7.1</td>
  </tr>
 </tbody>
</table>
</div>

<div id="compat-mobile">
<table class="compat-table">
 <tbody>
  <tr>
   <th>Feature</th>
   <th>Android</th>
   <th>Firefox Mobile (Gecko)</th>
   <th>IE Mobile</th>
   <th>Opera Mobile</th>
   <th>Safari Mobile</th>
   <th>Chrome for Android</th>
  </tr>
  <tr>
   <td>Basic support</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatGeckoMobile(29.0)}} [1]</td>
   <td>{{CompatNo}}</td>
   <td>{{CompatNo}}</td>
   <td>8</td>
   <td>32</td>
  </tr>
 </tbody>
</table>
</div>

<p>[1] Gecko 24 has an experimental implementation of <code>Promise</code>, under the initial name of <code>Future</code>. It got renamed to its final name in Gecko 25, but disabled by default behind the flag <code>dom.promise.enabled</code>. <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=918806">Bug 918806</a> enabled Promises by default in Gecko 29.</p>

<h2 id="See_also">See also</h2>

<ul>
 <li>{{jsxref("Promise")}}</li>
 <li>{{jsxref("Promise.prototype.catch()")}}</li>
</ul>