将XMLhttpRequest转换为函数失败：异步还是其他？ [英] Turn XMLhttpRequest into a function fails: asynchronity or other?

问题描述

我尝试将XMLHttpRequest转换为函数，例如

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.

我成功加载文件，并转换为base64。然而，我坚持认为将结果作为输出：

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)

参见 在这里摆弄 。

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：我的最终实施是此处可见，并且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）不能返回常规函数之类的值。编写异步函数时使用的常见模式是：

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：

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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