NodeJS promise resolution

Question

const a = [1, 2, 3, 4, 5];

const f = () => new Promise((resolve, reject) => resolve(4));

const g = () => {
  Promise.all(a.map((member) => f().then((res) => res)))
    .then((result) => {
      console.log(result)
    });
}

g();

Why do I not need another then attached to {return res;} here?

I read that when you have a return (something) inside a then, another then must be attached, but its not the case here. Help?

Answer 1

Promise.all expects an array of promises. .then returns a promise. Therefore your mapping logic converts an array of numbers to an array of promises, exactly what you need.

.then((res) => {return res;}) is completely unnecessary btw, return f(); would suffice. You can even simplify your current code further to:

Promise.all(a.map(f)).then(result => console.log(result));

I read that when you have a return (something) inside a then , another then must be attached

This has nothing to do with .then . .then simply returns a promise. To access the result of a promise you need to attach a handler via .then .

You don't need to do this here because you are passing the promises to Promise.all . You are accessing that result via .then((result)=>{console.log(result)}) .

Answer 2

Why do I not need another then attached to {return res;} here?

I read that when you have a return (something) inside a then, another then must be attached, but its not the case here. Help?

There is another .then() attached to Promise.all() . Did you mean .catch() should be attached to avoid Uncaught (in promise) ?

Note also, Promise.all() is not return ed from g() call, to further chain the Promise .

.then() and .catch() chained within .map() callback can be utilized to either handle error or rejected Promise and return a resolved Promise to .then() chained to Promise.all() ; or to throw current or new Error() to Promise.all() call.

The pattern can also be used to return all promises passed to .map() , whether resolved or rejected to .then() , without immediately calling .catch() chained to Promise.all() .

 function f (index) { return new Promise(function (resolve, reject) { if (index !== 4) resolve(4); else reject("err at index " + index) }) } var a =[1, 2, 3, 4, 5]; function g () { return Promise.all(a.map((member, index)=>{ return f(index).then((res) => {return res;}) .catch(e => {console.log(e); throw new Error(e)}) })) .then((result)=>{console.log(result); return result}) .catch(e => { // handle error here, return resolved `Promise`, // or `throw new Error(e)` to propagate error to // `.catch()` chained to `g()` call console.log("handle error within g", e); return "error " + e.message + " handled"}); } g() .then(data => {console.log(data) /* `error err at 4 handled` */ }) .catch(e => console.log("done", e));