为什么这一系列承诺会立即解决？

Question

Can someone explain to me why the resulting promise ( d ) from the code below is resolved immediately?

//promises that are never resolved nor rejected
var a = new Promise(function(r,re){});
var b = new Promise(function(r,re){});
var c = new Promise(function(r,re){});

var d = [a, b, c].reduce(function (previousPromise, promise) {
    return previousPromise.then(promise);
  }, Promise.resolve());

I'm creating an array of promises that are pending forever, so the resulting promise should also be pending forever as it waits for all subsequent promises to finish ( as presented here ). I've been using promises for a while now, but I'm clearly missing something here.

Answer 1

then doesn't take a Promise as an input, it takes 2 functions, 1 for fulfillment and 1 for rejection.

The reason d is resolved is due to the fact that calling .then with a non-callable value (even a number literal - 1, or undefined ) causes the onFulfilled function to be replace by "Identity", which simply re-fulfills with whatever value was resolved in the previous step. See PerformPromiseThen

Try like this:

//promises that are never resolved nor rejected
var a = function() { return new Promise(function(r,re){}); };
var b = function() { return new Promise(function(r,re){}); };
var c = function() { return new Promise(function(r,re){}); };
// or simply, a = b = c after setting the function for c

var d = [a, b, c].reduce(function (previousPromise, fn) {
    return previousPromise.then(fn, /* not passing a rejection handler... */);
  }, Promise.resolve());

Or alternatively...

//promises that are never resolved nor rejected
var a = new Promise(function(r,re){});
var b = new Promise(function(r,re){});
var c = new Promise(function(r,re){});

var d = [a, b, c].reduce(function (previousPromise, promise) {
    return previousPromise.then(function() {return promise;});
  }, Promise.resolve());

And since you're using promises & ES6, you could be more concise:

let a = new Promise(() => {});
let b = new Promise(() => {});
let c = new Promise(() => {});

let d = [a, b, c].reduce((previousPromise, promise) =>
  previousPromise.then(() => promise),
  Promise.resolve());

Answer 2

var d = [a, b, c].reduce(function (previousPromise, promise) {
    return previousPromise.then(promise);
}, Promise.resolve());

Your initial state of d promise is resolved, you set is as Promise.resolve(). Check documentation of reduce https://developer.mozilla.org/ru/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce