then() in JS Promise is never executed

Question

There is a funcion returning a promise:

req.postP = function () {

    var $promise = $.ajax({
        url: self._url(url),
        type: "POST",
        data: JSON.stringify(data),
        headers: headers,
        contentType: self.JSON_CONTENT_TYPE,
        dataType: "json"
    });

    return new Promise( (resolve, reject) => {
        return $promise.then(resolve, reject);
    } );

}

And another function that calls the first function:

req
    .postP(POST_REGISTER, userData)
    .then((data) => {
        // is never ecexuted
    }) 
    .catch((err) => {
        console.log(typeof err.then === "function"); // true
    });

The then() function is never executed.

It seems that the promise is inside the catch function as err since err.then is a function.

The req.postP() function is called by other function as well and works in those cases.

`return new Promise( (resolve, reject) => {...} );` ... do you ever `resolve` in whatever `...` is? — Jaromanda X, May 30 '18 at 06:11
If the `catch` block runs, then `reject` must be getting called instead of `resolve` — CertainPerformance, May 30 '18 at 06:12
or there's an error thrown in `...` - that will end in the same result as calling `reject` ... without seeing relevant code, it sure is a mystery — Jaromanda X, May 30 '18 at 06:13
well, clearly your jquerey ajax never succeeds - note, no need to do that by the way ... just `return Promise.resolve($.ajax({......}))` or even just `return $.ajax({....})` ... though, I don't trust jquerey promises :p — Jaromanda X, May 30 '18 at 06:45
You just need to `return $promise` because jquery ajax is already a promise — Tan Duong, May 30 '18 at 06:46
try ... `.catch(err => console.error)` - at least then you'll actually see what the error actually is ... and check the developer tools console to see if the request is succeeding — Jaromanda X, May 30 '18 at 06:47
@TanDuong - or `return Promise.resolve($promise)` (as I alluded to) because jquerey promises are not that good (especially in early versions - depends on the version of jquerey) — Jaromanda X, May 30 '18 at 06:48

score 0 · Answer 1 · answered May 30 '18 at 10:25

The casting to a standard promise, was not the problem.

The problem was the dataType. According to the jQuery docs:

The type of data that you're expecting back from the server.

Since the server does not return any data for this request on success - but jQuery expected data - the promise was always rejected.

Related: jQuery returning "parsererror" for ajax request

then() in JS Promise is never executed

1 Answers1