为什么然后返回一个新的Promise而不是返回onFulfilled返回的Promise

Question

I was wondering why, if then()'s onFulfilled handler returns a promise p2, how come then() returns a new promise p3 instead of just returning p2?

Example:

let p1 = new Promise(function(resolve, reject) {
    resolve(42);
});

let p2 = new Promise(function(resolve, reject) {
    resolve(43);
});

let p3 = p1.then(function(value) {
    // first fulfillment handler
    console.log(value);     // 42
    return p2;
});

p3.then(function(value) {
    // second fulfillment handler
    console.log(value);     // 43
});

Why doesn't p1.then() just return p2 instead of making a new promise p3? It would have the same result.

Answer 1

Because then() returns a value immediately , before your callback even runs.

It cannot possibly know what you're going to return.

Answer 2

Simply because p3 is created and returned before the onFulfilled callback will run - then callbacks are always asynchronous. It cannot know yet what value will be returned in the future, so it creates a promise for it.