Promise callbacks returning promises

Question

With regard to these great two sources: NZakas - Returning Promises in Promise Chains and MDN Promises, I would like to ask the following:

Each time that we return a

Answer 1

Basically p3 is return-ing an another promise : p2. Which means the result of p2 will be passed as a parameter to the next then callback, in this case it resolves to 43.

Whenever you are using the keyword return you are passing the result as a parameter to next then's callback.

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

Your code :

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

Is equal to:

p1.then(function(resultOfP1) {
    // resultOfP1 === 42
    return p2; // // Returning a promise ( that might resolve to 43 or fail )
})
.then(function(resultOfP2) {
    console.log(resultOfP2) // '43'
});

Btw, I've noticed that you are using ES6 syntax, you can have a lighter syntax by using fat arrow syntax :

p1.then(resultOfP1 => p2) // the `return` is implied since it's a one-liner
.then(resultOfP2 => console.log(resultOfP2));