How to turn a callback function into a promise

Question

I need to make this code return a promise. Idk how to do that and i wanna learn. If you can, i would also need some explications, thanks!

var url = "....";

function showMoviesList(callbackFunction){
  fetch(url + "/movies", {
    method: "GET",
    headers: {
        "Content-Type": "application/x-www-form-urlencoded"
    }
  }).then(function(response){
    return response.json();
  }).then(function(moviesArray){
    callbackFunction(moviesArray);
  });
}

`"Content-Type": "application/x-www-form-urlencoded"` is nonsense. You are making a GET request. There is no request body to describe the type of. — Quentin, Apr 17 '20 at 17:37

score 0 · Answer 1 · answered Apr 17 '20 at 17:37

0

fetch already returns a promise. So just return the return value of that, and get rid of everything related to the callbackFunction variable.

answered Apr 17 '20 at 17:37

Quentin

914,110
126
1,211
1,335

score 0 · Answer 2 · answered Apr 17 '20 at 17:37

0

This would be

var url = "....";
function showMoviesList(callbackFunction){
  return fetch(url + "/movies", {
    method: "GET",
    headers: {
       "Content-Type": "application/x-www-form-urlencoded"
  }
  }).then(function(response){
    return response.json();
  });
}

answered Apr 17 '20 at 17:37

Wiktor Zychla

47,367
6
74
106

Ok. I got it. Turns out to be very simple.Thank you so much! – Andrew Apr 17 '20 at 17:54

score 0 · Answer 3 · answered Apr 17 '20 at 17:40

So return the fetch, make then return the json and you can use .then() with your function call.

var url = "....";

function showMoviesList(callbackFunction) {
  return fetch(url + "/movies").then(function(response) {
    return response.json();
  });
}

showMoviesList.then(function(json) {
  console.log(json);
})

How to turn a callback function into a promise

3 Answers3