承诺返回未定义 [英] Promise returning undefined

问题描述

我正在尝试使用 promise 向 php 脚本发送 ajax 请求，该脚本检查服务器上是否存在文件并返回布尔值.

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.

我有以下代码，但 fileExists 函数总是返回 undefined.

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

如何将 promise 包装在函数中并让函数返回 promise 值?

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");

推荐答案

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());

试试这个.

你的函数什么都不返回.如果您返回承诺，您可以在数据解决后保留数据.

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

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