Promise returning undefined

问题:

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");

回答1:

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();
}); 
}

var result = fileExists("url_to_file").then(foo());

Try this.

your function returns nothing . If you return the promise you can hold on to the data once its resolved.

回答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.

回答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);

回答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);
});