Promise.prototype.then()

El método then() retorna un Promise. Recibe dos argumentos: funciones callback  para los casos de éxito y fallo del Promise.

Nota: Si ambos argumentos son omitidos, o se proveen no-funciones, un nuevo Promise es creado sin handlers adicionales, simplemente adoptan el estado final del Promise que entonces es llamado. Si el primer argumento es omitido o se provee una no-función, el nuevo Promise que es creado simplemente adopta el  estado fulfillment del Promise que entonces es llamado (si se convierte en fulfilled). Si el segundo argument es omitido o se provee una no-función, el nuevo Promise que es creado simplemente adopta  el estado de rechazo del Promise que entonces es llamado (si se convierte en rechazado).

Sintaxis

p.then(alCumplir[, enRechazo]);

p.then(function(value) {
   // cumplimiento
  }, function(reason) {
  // rechazo
});

Parámetros

Retorna un Promise el cual es determinado por las funciones input:

• Si alCumplir o enRechazo arroja un error, o retorna un Promise rechazado, then retorna un Promise rechazado.
• Si alCumplir o enRechazo retorna un Promise que resuelve, o retorna cualquier otro valor, then retorna un Promise resuelto.

alCumplir

Una Funcion llamada cuando Promise se cumple. Esta función tiene un argumento, el valor cumplimiento.

enRechazo Optional

Una Funcion llamada cuando Promise es rechaza. Esta función tiene un argumento, la razón de rechazo.

Valor de retorno

Un Promise.

Descripción

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

Ejemplos

Usando el metodo then

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

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

You can pass a lambda to then and if it 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 setTimout 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 return the unworked on
  // string to the next then
  .then(function(string) {
    setTimeout(function() {
      string += 'baz';
      console.log(string);
    }, 1)
    return string;
  })
  // 3. print helpful messages about how the code in this section will be run
  // before string is actually processed by the mocked asynchronous code in the
  // prior 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);
});

When a value is simply returned from within a then lambda, 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 + '- This synchronous usage is virtually pointless'); // 2- This synchronous usage is virtually pointless
});

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 'Oh no!';
  })
  .then( () => { 
    console.log( 'Not called.' );
  }, reason => {
    console.error( 'onRejected function called: ', reason );
});

In all other cases, a resolving Promise is returned. In the following example, the first then() will return 42 wrapped 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 'Oh no!';
  })
  .catch( reason => {
    console.error( 'onRejected function called: ', reason );
  })
  .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(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

Browser compatibility

To contribute to this compatibility data, please write a pull request against this file: https://github.com/mdn/browser-compat-data/blob/master/javascript/promise.json.

No compatibility data found. Please contribute data for "javascript/promise" to the MDN compatibility data repository.

See also

• Promise
• Promise.prototype.catch()