Promise returning undefined

Question

I am trying to use promise to send an ajax request to a php script which checks if a file exist on the server and returns a boolean value.

I have the below code but the fileExists function always return undefined.

How can I wrap the promise in a function and have the function return the promise value?

function fileExists(url) {
    var promise = new Promise(function(resolve, reject) {
        var xhr = new XMLHttpRequest();
        xhr.onload = function() {
            resolve(this.responseText);
        };
        xhr.onerror = reject;
        xhr.open('GET', url);
        xhr.send();
    }); 
    promise.then(function(e) {
        return e;
    });
}

var result = fileExists("url_to_file");

Answer 1

Try this:

function fileExists(url) {
  return promise = new Promise(function(resolve, reject) {
    var xhr = new XMLHttpRequest();
    xhr.onload = function() {
      resolve(this.responseText);
    };
    xhr.onerror = reject;
    xhr.open('GET', url);
    xhr.send();
  }); 
}

fileExists("url_to_file").then(text => console.log(text));

Your function returns nothing. If you return the promise you can hold on to the data once it's resolved.

Answer 2

This part of your code:

promise.then(function(e) {
    return e;
});

only returns e to the callback function. You have to handle the result within that callback.

promise.then(function() {
    // handle result;
});

Or might as well return the promise as shown by @Ole.