将 XMLhttpRequest 变成函数失败:异步还是其他?

问题描述

我尝试将 XMLHttpRequest 变成这样的函数

I try to turn an XMLHttpRequest into a function such

var getImageBase64 = function (url) { // code function
    var xhr = new XMLHttpRequest(url); 
    ... // code to load file 
    ... // code to convert data to base64
    return wanted_result; // return result of conversion
}
var newData = getImageBase64('http://fiddle.jshell.net/img/logo.png'); // function call
doSomethingWithData($("#hook"), newData); // reinjecting newData in wanted place.

我已成功加载文件并转换为 base64.然而，我一直未能将结果作为输出:

I'am successful to load the file, and to convert to base64. I'am however consistenly failling to get the result as an output :

var getImageBase64 = function (url) {
    // 1. Loading file from url:
    var xhr = new XMLHttpRequest(url);
    xhr.open('GET', url, true); // url is the url of a PNG image.
    xhr.responseType = 'arraybuffer';
    xhr.onload = function(e) { 
        if (this.status == 200) { // 2. When loaded, do:
            console.log("1:Response?> " + this.response); // print-check xhr response 
            var imgBase64 = converterEngine(this.response); // converter
        }
    }
    xhr.send();
    return xhr.onload(); // <fails> to get imgBase64 value as the function's result.
}

console.log("4>>> " + getImageBase64('http://fiddle.jshell.net/img/logo.png') ) // THIS SHOULD PRINT THE BASE64 CODE (returned resukt of the function  getImageBase64)

请参阅小提琴.

See Fiddle here.

如何使其工作，以便将新数据作为输出返回?

解决方案:我的最终实现是在此处可见，然后继续JS:如何加载位图图像并获取其base64代码?.

Solution: my final implementation is visible here, and on JS: how to load a bitmap image and get its base64 code?.

推荐答案

JavaScript 中的异步调用(如 xhr)无法像常规函数那样返回值.编写异步函数时常用的模式是这样的:

Asynchronous calls in JavaScript (like xhr) can't return values like regular functions. The common pattern used when writing asynchronous functions is this:

function asyncFunc(param1, param2, callback) {
  var result = doSomething();
  callback(result);
}

asyncFunc('foo', 'bar', function(result) {
  // result is what you want
});

所以您的翻译示例如下所示:

So your example translated looks like this:

var getImageBase64 = function (url, callback) {
    var xhr = new XMLHttpRequest(url); 
    ... // code to load file 
    ... // code to convert data to base64
    callback(wanted_result);
}
getImageBase64('http://fiddle.jshell.net/img/logo.png', function(newData) {
  doSomethingWithData($("#hook"), newData);
});

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