Promise Returning Undefined

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

Solution 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.

Solution 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.

Solution 3:

You should return the promise, because you need to assign your result variable asynchronous.

function fileExists(url) {
  return 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(console.log);

Solution 4:

Your function must return a Promise.

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

Then just call the function and print the resolved text

fileExists("url_to_file").then(function (text) {
    console.log(text);
});