Promise.prototype.then()

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

Note: If one or both arguments are omitted or are provided non-functions, then then will be missing the handler(s), but will not generate any errors. If the Promise that then is called on adopts a state (fulfillment or rejection) for which then has no handler, the returned promise adopts the final state of the original Promise on which then was called.

Syntax

p.then(onFulfilled[, onRejected]);

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

Parameters

onFulfilled Optional

A Function called if the Promise is fulfilled. This function has one argument, the fulfillment value. If it is not a function, it is internally replaced with an "Identity" function (it returns the received argument).

onRejected Optional

A Function called if the Promise is rejected. This function has one argument, the rejection reason. If it is not a function, it is internally replaced with a "Thrower" function (it throws an error it received as argument).

Return value

Once a Promise is fulfilled or rejected, the respective handler function (onFulfilled or onRejected) will be called asynchronously (scheduled in the current thread loop). The behavior of the handler function follows a specific set of rules. If a handler function:

  • returns a value, the promise returned by then gets resolved with the returned value as its value.
  • doesn't return anything, the promise returned by then gets resolved with an undefined value.
  • throws an error, the promise returned by then gets rejected with the thrown error as its value.
  • returns an already fulfilled promise, the promise returned by then gets fulfilled with that promise's value as its value.
  • returns an already rejected promise, the promise returned by then gets rejected with that promise's value as its value.
  • returns another pending promise object, the resolution/rejection of the promise returned by then will be subsequent to the resolution/rejection of the promise returned by the handler. Also, the resolved value of the promise returned by then will be the same as the resolved value of the promise returned by the handler.

Following, an example to demonstrate the asynchronicity of the then method. 

// using a resolved promise, the 'then' block will be triggered instantly,
// but its handlers will be triggered asynchronously as demonstrated by the console.logs
const resolvedProm = Promise.resolve(33);

let thenProm = resolvedProm.then(value => {
    console.log("this gets called after the end of the main stack. the value received and returned is: " + value);
    return value;
});
// instantly logging the value of thenProm
console.log(thenProm);

// using setTimeout we can postpone the execution of a function to the moment the stack is empty
setTimeout(() => {
    console.log(thenProm);
});

// logs, in order:
// Promise {[[PromiseStatus]]: "pending", [[PromiseValue]]: undefined}
// "this gets called after the end of the main stack. the value received and returned is: 33"
// Promise {[[PromiseStatus]]: "resolved", [[PromiseValue]]: 33}

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((resolve, reject) => {
  resolve('Success!');
  // or
  // reject(new Error("Error!"));
});

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

Chaining

The then method returns a Promise which allows for method chaining.

If the function passed as handler to then returns a Promise, an equivalent Promise will be exposed to the subsequent then in the method chain. The below snippet simulates asynchronous code with the setTimeout function. 

Promise.resolve('foo')
  // 1. Receive "foo", concatenate "bar" to it, and resolve that to the next then
  .then(function(string) {
    return new Promise(function(resolve, reject) {
      setTimeout(function() {
        string += 'bar';
        resolve(string);
      }, 1);
    });
  })
  // 2. receive "foobar", register a callback function to work on that string
  // and print it to the console, but not before returning the unworked on
  // string to the next then
  .then(function(string) {
    setTimeout(function() {
      string += 'baz';
      console.log(string); // foobarbaz
    }, 1)
    return string;
  })
  // 3. print helpful messages about how the code in this section will be run
  // before the string is actually processed by the mocked asynchronous code in the
  // previous then block.
  .then(function(string) {
    console.log("Last Then:  oops... didn't bother to instantiate and return " +
                "a promise in the prior then so the sequence may be a bit " +
                "surprising");

    // Note that `string` will not have the 'baz' bit of it at this point. This
    // is because we mocked that to happen asynchronously with a setTimeout function
    console.log(string); // foobar
  });

// logs, in order:
// Last Then: oops... didn't bother to instantiate and return a promise in the prior then so the sequence may be a bit surprising
// foobar
// foobarbaz

When a value is returned from within a then handler, it will effectively return Promise.resolve(<value returned by whichever handler was called>). 

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 + ' - A synchronous value works'); // 2 - A synchronous value works
});

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

A then call will return a rejected promise if the function throws an error or returns a rejected Promise. 

Promise.resolve()
  .then(() => {
    // Makes .then() return a rejected promise
    throw new Error('Oh no!');
  })
  .then(() => {
    console.log('Not called.');
  }, error => {
    console.error('onRejected function called: ' + error.message);
  });

In all other cases, a resolving Promise is returned. In the following example, the first then() will return 42 wrapped in a resolving Promise even though the previous Promise in the chain was rejected. 

Promise.reject()
  .then(() => 99, () => 42) // onRejected returns 42 which is wrapped in a resolving Promise
  .then(solution => console.log('Resolved with ' + solution)); // Resolved with 42

In practice, it is often desirable to catch rejected promises rather than use then's two case syntax, as demonstrated below. 

Promise.resolve()
  .then(() => {
    // Makes .then() return a rejected promise
    throw new Error('Oh no!');
  })
  .catch(error => {
    console.error('onRejected function called: ' + error.message);
  })
  .then(() => {
    console.log("I am always called even if the prior then's promise rejects");
  });

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(new Error('Error'));
  }, 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.error('rejected', e);
});

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

window.setImmediate style promise-based polyfill

Using a Function.prototype.bind() Reflect.apply (Reflect.apply()) method to create a (non-cancellable) window.setImmediate-style function. 

const nextTick = (() => {
  const noop = () => {}; // literally
  const nextTickPromise = () => Promise.resolve().then(noop);

  const rfab = Reflect.apply.bind; // (thisArg, fn, thisArg, [...args])
  const nextTick = (fn, ...args) => (
    fn !== undefined
    ? Promise.resolve(args).then(rfab(null, fn, null))
    : nextTickPromise(),
    undefined
  );
  nextTick.ntp = nextTickPromise;

  return nextTick;
})();

Specifications

Specification
ECMAScript Language Specification
# sec-promise.prototype.then

Browser compatibility

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See also