Why this chain of promises immediately resolves?

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());