已执行但未抛出承诺（未处理的承诺拒绝）

Question

I'm brand new submitting questions. I've had a good look at all the other questions that have been submitted and struggling to find a similar issue to what I have. If there is a similar question, my apologies.

I've having issues with a very basic Promise I created for this question, where I need to reject a array loop where one iteration is the same:

var array = [1, 2, 3, 4, 5];
var promise = array.forEach(n => {
  new Promise((resolve, reject) => {
    if (n == 5) return reject();
    return resolve();
  });
});

Promise.all(promise).then(() => {
  console.log('true');
}).catch(() => {
  console.log('false');
});

When console logging each iteration, the rejection gets called but doesn't catch in the promise and I can't fathom why. I've spent almost 3 hours finding out the reasoning behind this.

Answer 1

Array.prototype.forEach does not return anything, thus promise is undefined . What you should use instead is Array.prototype.map . Also note that you actually have to return the newly created promises:

var array = [1, 2, 3, 4, 5];
var promise = array.map(n => {
  return new Promise((resolve, reject) => {
    if (n == 5) return reject();
    return resolve();
  });
});

Promise.all(promise).then(() => {
  console.log('true');
}).catch(() => {
  console.log('false');
});

You could even write it like this:

var promise = array.map(n => new Promise((resolve, reject) => (n == 5) ? reject() : resolve()));

Answer 2

Whenever you catch an error you should properly log it, and not just log false or something like that.

So if you write console.error(err); instead of console.log('false') , then you would get the error that's something like TypeError: "can't convert undefined to object" or TypeError: Cannot read property 'Symbol(Symbol.iterator)' of undefined for the line with the Promise.all(promise) . And that should give you the hint that promise does not hold an array with Promises but undefined .

The promise is undefined because forEach does not return anything. What you are looking for is map .

The curly bracket around your new Promise((resolve, reject) => {}) prevent that the new Promise is returned, so either add a return in front of the new Promise or remove those curly bracket.

And you should always reject with an actual error, otherwise you won't be able to get a proper stack trace for the error.

 var array = [1, 2, 3, 4, 5]; var promise = array.map(n => new Promise((resolve, reject) => { if (n == 5) return reject(new Error('rejected for n==5')); return resolve(); }) ) Promise.all(promise).then(() => { console.log('true'); }).catch(err => { console.error(err); });