What is the difference between Promise.resolve(thenable) and new Promise(thenable.then)?

Question

The question is the title. I'm trying to get a deeper understanding of promises and I think I've figured out how Promise.resolve(thenable) works, or at least mostly how it works, looking at the examples on MDN. I'm wondering if there's a difference between the two. I whipped up this example to show them behaving the same, in way that I would think would show differences in behaviour if there was going to be any. But obviously this test alone isn't good enough to conclude there's nothing different about them, so I've come here.

let thenable = {
    then(resolve) {
        setTimeout(()=>{
            resolve((()=>{
                console.log('test');
                return thenable;
            })());
        }, 1000);
    },
};

let p1 = Promise.resolve(thenable);
let p2 = new Promise(thenable.then);

Answer 1

In your example, the two work mostly the same. You're right, passing resolve and reject to the then method invocation of a thenable is what basically happens when you are resolving a promise with the thenable object.

However, there are a few differences:

• the then method of a thenable is called as a method on (with the this context as) the thenable object, unlike your new Promise(thenable.then) does - a .bind(thenable) would fix that.
• Promise.resolve (obviously) has to check first whether the passed object is a thenable at all, it also handles obscure cases where accessing (not calling) .then throws

• a subtle discrepance in timing: Promise.resolve schedules even the then call asynchronously:

  Promise.resolve({
    get then() {
      console.log("property access");
      return function(res, rej) {
        console.log("method call");
        res("fulfillment");
      };
    }
  }).then(console.log);
  console.log("sync execution end");