When Promise state become rejected or resolved

let promise = new Promise(function(resolve, reject) {

      setTimeout(() => reject(new Error("Whoops!")), 1000);

});



// reject runs the second function in .then

promise.then(

  result => alert(result), // doesn't run

  error => alert(error) // shows "Error: Whoops!" after 1 second

);

In the above code snippets even though I am calling reject but promise state is coming as resolved but when I am removing error => alert(error) from promise.then then I am getting promise state as rejected
If one is calling reject then promise state should be rejected not resolved am I correct?

edited Nov 14 '18 at 11:47

Hassan Imam

11.7k31330

asked Nov 14 '18 at 11:45

Alex_Angular

445

add a comment |

let promise = new Promise(function(resolve, reject) {

      setTimeout(() => reject(new Error("Whoops!")), 1000);

});



// reject runs the second function in .then

promise.then(

  result => alert(result), // doesn't run

  error => alert(error) // shows "Error: Whoops!" after 1 second

);

edited Nov 14 '18 at 11:47

Hassan Imam

11.7k31330

asked Nov 14 '18 at 11:45

Alex_Angular

445

add a comment |

let promise = new Promise(function(resolve, reject) {

      setTimeout(() => reject(new Error("Whoops!")), 1000);

});



// reject runs the second function in .then

promise.then(

  result => alert(result), // doesn't run

  error => alert(error) // shows "Error: Whoops!" after 1 second

);

edited Nov 14 '18 at 11:47

Hassan Imam

11.7k31330

asked Nov 14 '18 at 11:45

Alex_Angular

445

let promise = new Promise(function(resolve, reject) {

      setTimeout(() => reject(new Error("Whoops!")), 1000);

});



// reject runs the second function in .then

promise.then(

  result => alert(result), // doesn't run

  error => alert(error) // shows "Error: Whoops!" after 1 second

);

let promise = new Promise(function(resolve, reject) {

      setTimeout(() => reject(new Error("Whoops!")), 1000);

});



// reject runs the second function in .then

promise.then(

  result => alert(result), // doesn't run

  error => alert(error) // shows "Error: Whoops!" after 1 second

);

let promise = new Promise(function(resolve, reject) {

      setTimeout(() => reject(new Error("Whoops!")), 1000);

});



// reject runs the second function in .then

promise.then(

  result => alert(result), // doesn't run

  error => alert(error) // shows "Error: Whoops!" after 1 second

);

javascript ecmascript-6 promise es6-promise

edited Nov 14 '18 at 11:47

Hassan Imam

11.7k31330

asked Nov 14 '18 at 11:45

Alex_Angular

445

edited Nov 14 '18 at 11:47

Hassan Imam

11.7k31330

asked Nov 14 '18 at 11:45

Alex_Angular

445

edited Nov 14 '18 at 11:47

Hassan Imam

11.7k31330

edited Nov 14 '18 at 11:47

Hassan Imam

11.7k31330

edited Nov 14 '18 at 11:47

Hassan Imam

11.7k31330

asked Nov 14 '18 at 11:45

Alex_Angular

445

asked Nov 14 '18 at 11:45

Alex_Angular

445

asked Nov 14 '18 at 11:45

Alex_Angular

445

add a comment |

4 Answers
4

active

oldest

votes

The promise that is resolved is not the original promise, but the promise returned by then. The original promise is indeed rejected, and you can verify that by console.log(promise). But because you chained then it returns another promise ..

Return value [of then]

A Promise in the pending status. The handler function
(onFulfilled or onRejected) then gets called asynchronously (as soon
as the stack is empty). After the invocation of the handler function,
if the 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;

...

This second point is what applies to your case. You can verify that by observing that
Promise.reject().then(undefined, ()=>{})

returns a promise that gets resolved with an undefined value.

edited Nov 14 '18 at 12:48

answered Nov 14 '18 at 12:38

Husam Ibrahim

3,048415

add a comment |

You can also handle Promises in "then-catch" fashion like below

let promise = new Promise(function(resolve, reject) {

  setTimeout(() => reject('Error'));

});



promise

  .then(result => console.log('Result ', result)) 

  .catch(error => console.log('This is Error message -', error))

edited Nov 14 '18 at 12:07

answered Nov 14 '18 at 11:49

Nitish Narang

2,9401815

then accepts two arguments - one for success callback and the other for a rejection callback. Using it a valid way of dealing with Promise rejection.

– Szab
Nov 14 '18 at 12:03

1

Oh!! How did I miss that.. Damn!! Thank you @Szab..

– Nitish Narang
Nov 14 '18 at 12:06

Edited my answer.. Thank you for the knowledge @Szab .. I never realised this...

– Nitish Narang
Nov 14 '18 at 12:08

A promise should have exception handling mechanism. That is why we have .catch, the control goes to .catch when an exception is thrown from the promise body. if you call resolve the control goes to the first executor argument of .then and if you call reject, then it goes to the second argument of .then. So I think we should have both then and catch

– Parnab Sanyal
Nov 14 '18 at 12:56

add a comment |

The final status of the promise is also rejected in your case.
Try console.log(promise);
It has nothing to do with error => alert(error)

answered Nov 14 '18 at 11:56

Monica Acha

33219

add a comment |

The code you posted seems to work correctly both on Chrome and Firefox - the Promise was rejected as expected. reject function marks the Promise as rejected - you can then react upon that rejection either by passing a callback function into a second argument of then or by using the catch method. Both approaches are correct.

If you are encountering any unexpected behaviour, verify that you are not using some faulty polyfill that replaces the original Promise object.

Reference: Promise.prototype.then, Promise.prototype.catch

answered Nov 14 '18 at 12:02

Szab

1,053517

add a comment |

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4 Answers
4

active

oldest

votes

4 Answers
4

active

oldest

votes

Return value [of then]

A Promise in the pending status. The handler function
(onFulfilled or onRejected) then gets called asynchronously (as soon
as the stack is empty). After the invocation of the handler function,
if the 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;

...

This second point is what applies to your case. You can verify that by observing that
Promise.reject().then(undefined, ()=>{})

returns a promise that gets resolved with an undefined value.

edited Nov 14 '18 at 12:48

answered Nov 14 '18 at 12:38

Husam Ibrahim

3,048415

add a comment |

Return value [of then]

A Promise in the pending status. The handler function
(onFulfilled or onRejected) then gets called asynchronously (as soon
as the stack is empty). After the invocation of the handler function,
if the 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;

...

This second point is what applies to your case. You can verify that by observing that
Promise.reject().then(undefined, ()=>{})

returns a promise that gets resolved with an undefined value.

edited Nov 14 '18 at 12:48

answered Nov 14 '18 at 12:38

Husam Ibrahim

3,048415

add a comment |

Return value [of then]

A Promise in the pending status. The handler function
(onFulfilled or onRejected) then gets called asynchronously (as soon
as the stack is empty). After the invocation of the handler function,
if the 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;

...

This second point is what applies to your case. You can verify that by observing that
Promise.reject().then(undefined, ()=>{})

returns a promise that gets resolved with an undefined value.

edited Nov 14 '18 at 12:48

answered Nov 14 '18 at 12:38

Husam Ibrahim

3,048415

Return value [of then]

A Promise in the pending status. The handler function
(onFulfilled or onRejected) then gets called asynchronously (as soon
as the stack is empty). After the invocation of the handler function,
if the 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;

...

This second point is what applies to your case. You can verify that by observing that
Promise.reject().then(undefined, ()=>{})

returns a promise that gets resolved with an undefined value.

edited Nov 14 '18 at 12:48

answered Nov 14 '18 at 12:38

Husam Ibrahim

3,048415

edited Nov 14 '18 at 12:48

answered Nov 14 '18 at 12:38

Husam Ibrahim

3,048415

answered Nov 14 '18 at 12:38

Husam Ibrahim

3,048415

answered Nov 14 '18 at 12:38

Husam Ibrahim

3,048415

add a comment |

You can also handle Promises in "then-catch" fashion like below

let promise = new Promise(function(resolve, reject) {

  setTimeout(() => reject('Error'));

});



promise

  .then(result => console.log('Result ', result)) 

  .catch(error => console.log('This is Error message -', error))

edited Nov 14 '18 at 12:07

answered Nov 14 '18 at 11:49

Nitish Narang

2,9401815

then accepts two arguments - one for success callback and the other for a rejection callback. Using it a valid way of dealing with Promise rejection.

– Szab
Nov 14 '18 at 12:03

1

Oh!! How did I miss that.. Damn!! Thank you @Szab..

– Nitish Narang
Nov 14 '18 at 12:06

Edited my answer.. Thank you for the knowledge @Szab .. I never realised this...

– Nitish Narang
Nov 14 '18 at 12:08

A promise should have exception handling mechanism. That is why we have .catch, the control goes to .catch when an exception is thrown from the promise body. if you call resolve the control goes to the first executor argument of .then and if you call reject, then it goes to the second argument of .then. So I think we should have both then and catch

– Parnab Sanyal
Nov 14 '18 at 12:56

add a comment |

You can also handle Promises in "then-catch" fashion like below

let promise = new Promise(function(resolve, reject) {

  setTimeout(() => reject('Error'));

});



promise

  .then(result => console.log('Result ', result)) 

  .catch(error => console.log('This is Error message -', error))

edited Nov 14 '18 at 12:07

answered Nov 14 '18 at 11:49

Nitish Narang

2,9401815

then accepts two arguments - one for success callback and the other for a rejection callback. Using it a valid way of dealing with Promise rejection.

– Szab
Nov 14 '18 at 12:03

1

Oh!! How did I miss that.. Damn!! Thank you @Szab..

– Nitish Narang
Nov 14 '18 at 12:06

Edited my answer.. Thank you for the knowledge @Szab .. I never realised this...

– Nitish Narang
Nov 14 '18 at 12:08

A promise should have exception handling mechanism. That is why we have .catch, the control goes to .catch when an exception is thrown from the promise body. if you call resolve the control goes to the first executor argument of .then and if you call reject, then it goes to the second argument of .then. So I think we should have both then and catch

– Parnab Sanyal
Nov 14 '18 at 12:56

add a comment |

You can also handle Promises in "then-catch" fashion like below

let promise = new Promise(function(resolve, reject) {

  setTimeout(() => reject('Error'));

});



promise

  .then(result => console.log('Result ', result)) 

  .catch(error => console.log('This is Error message -', error))

edited Nov 14 '18 at 12:07

answered Nov 14 '18 at 11:49

Nitish Narang

2,9401815

You can also handle Promises in "then-catch" fashion like below

let promise = new Promise(function(resolve, reject) {

  setTimeout(() => reject('Error'));

});



promise

  .then(result => console.log('Result ', result)) 

  .catch(error => console.log('This is Error message -', error))

let promise = new Promise(function(resolve, reject) {

  setTimeout(() => reject('Error'));

});



promise

  .then(result => console.log('Result ', result)) 

  .catch(error => console.log('This is Error message -', error))

let promise = new Promise(function(resolve, reject) {

  setTimeout(() => reject('Error'));

});



promise

  .then(result => console.log('Result ', result)) 

  .catch(error => console.log('This is Error message -', error))

edited Nov 14 '18 at 12:07

answered Nov 14 '18 at 11:49

Nitish Narang

2,9401815

edited Nov 14 '18 at 12:07

answered Nov 14 '18 at 11:49

Nitish Narang

2,9401815

answered Nov 14 '18 at 11:49

Nitish Narang

2,9401815

answered Nov 14 '18 at 11:49

Nitish Narang

2,9401815

then accepts two arguments - one for success callback and the other for a rejection callback. Using it a valid way of dealing with Promise rejection.

– Szab
Nov 14 '18 at 12:03

1

Oh!! How did I miss that.. Damn!! Thank you @Szab..

– Nitish Narang
Nov 14 '18 at 12:06

Edited my answer.. Thank you for the knowledge @Szab .. I never realised this...

– Nitish Narang
Nov 14 '18 at 12:08

A promise should have exception handling mechanism. That is why we have .catch, the control goes to .catch when an exception is thrown from the promise body. if you call resolve the control goes to the first executor argument of .then and if you call reject, then it goes to the second argument of .then. So I think we should have both then and catch

– Parnab Sanyal
Nov 14 '18 at 12:56

add a comment |

then accepts two arguments - one for success callback and the other for a rejection callback. Using it a valid way of dealing with Promise rejection.

– Szab
Nov 14 '18 at 12:03

1

Oh!! How did I miss that.. Damn!! Thank you @Szab..

– Nitish Narang
Nov 14 '18 at 12:06

Edited my answer.. Thank you for the knowledge @Szab .. I never realised this...

– Nitish Narang
Nov 14 '18 at 12:08

A promise should have exception handling mechanism. That is why we have .catch, the control goes to .catch when an exception is thrown from the promise body. if you call resolve the control goes to the first executor argument of .then and if you call reject, then it goes to the second argument of .then. So I think we should have both then and catch

– Parnab Sanyal
Nov 14 '18 at 12:56

then accepts two arguments - one for success callback and the other for a rejection callback. Using it a valid way of dealing with Promise rejection.

– Szab
Nov 14 '18 at 12:03

Oh!! How did I miss that.. Damn!! Thank you @Szab..

– Nitish Narang
Nov 14 '18 at 12:06

Edited my answer.. Thank you for the knowledge @Szab .. I never realised this...

– Nitish Narang
Nov 14 '18 at 12:08

A promise should have exception handling mechanism. That is why we have .catch, the control goes to .catch when an exception is thrown from the promise body. if you call resolve the control goes to the first executor argument of .then and if you call reject, then it goes to the second argument of .then. So I think we should have both then and catch

– Parnab Sanyal
Nov 14 '18 at 12:56

add a comment |

The final status of the promise is also rejected in your case.
Try console.log(promise);
It has nothing to do with error => alert(error)

answered Nov 14 '18 at 11:56

Monica Acha

33219

add a comment |

The final status of the promise is also rejected in your case.
Try console.log(promise);
It has nothing to do with error => alert(error)

answered Nov 14 '18 at 11:56

Monica Acha

33219

add a comment |

The final status of the promise is also rejected in your case.
Try console.log(promise);
It has nothing to do with error => alert(error)

answered Nov 14 '18 at 11:56

Monica Acha

33219

The final status of the promise is also rejected in your case.
Try console.log(promise);
It has nothing to do with error => alert(error)

answered Nov 14 '18 at 11:56

Monica Acha

33219

answered Nov 14 '18 at 11:56

Monica Acha

33219

answered Nov 14 '18 at 11:56

Monica Acha

33219

answered Nov 14 '18 at 11:56

Monica Acha

33219

add a comment |

If you are encountering any unexpected behaviour, verify that you are not using some faulty polyfill that replaces the original Promise object.

Reference: Promise.prototype.then, Promise.prototype.catch

answered Nov 14 '18 at 12:02

Szab

1,053517

add a comment |

If you are encountering any unexpected behaviour, verify that you are not using some faulty polyfill that replaces the original Promise object.

Reference: Promise.prototype.then, Promise.prototype.catch

answered Nov 14 '18 at 12:02

Szab

1,053517

add a comment |

If you are encountering any unexpected behaviour, verify that you are not using some faulty polyfill that replaces the original Promise object.

Reference: Promise.prototype.then, Promise.prototype.catch

answered Nov 14 '18 at 12:02

Szab

1,053517

If you are encountering any unexpected behaviour, verify that you are not using some faulty polyfill that replaces the original Promise object.

Reference: Promise.prototype.then, Promise.prototype.catch

answered Nov 14 '18 at 12:02

Szab

1,053517

answered Nov 14 '18 at 12:02

Szab

1,053517

answered Nov 14 '18 at 12:02

Szab

1,053517

answered Nov 14 '18 at 12:02

Szab

1,053517

add a comment |

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