无法在异步模块中使用“map”功能

I am using node.js "async" module and need to use the "map" method.
Basically I have an array that contains other arrays. The inner arrays contains 2 elements, a type and an image filename.

我正在使用node.js“async”模块，需要使用“map”方法。基本上我有一个包含其他数组的数组。内部数组包含2个元素，一个类型和一个图像文件名。

var arr0 = [];
var arr1 = ["type1", "image1.jpg"];
jsonArr.push(obj1);
var arr2 = ["type2", "image2.jpg"];
jsonArr.push(obj2);

For each inner array, I want to get the base64 encoding of the image identified by the filename and add this encoding string as the third element of the array.

对于每个内部数组，我想获取由文件名标识的图像的base64编码，并将此编码字符串添加为数组的第三个元素。

I'm doing something like this:

我正在做这样的事情：

var fs = require("fs");
var async = require("async");

function getImageEncoding(arr, callback){
    console.log("getEncoding:" + arr + "\n");

    // Get image filename
    image = arr[1];

    // Read file and get base64 encoding
    fs.readFile(image, function(err, original_data){
    var base64Image = original_data.toString('base64');
    console.log("test:" + base64Image + "\n");

        // Modify current arr by appendingthe base64 encoding of the image
        callback(null, arr.push(base64Image));
    });
}

async.map(arr0, getImageEncoding, function(err, results){
console.log("in async.map: " + results + "\n");
});

I know the arr.push(base64Image) stuff is the thing that is not correct, but I cannot figure out how to return the modified element.

我知道arr.push（base64Image）的东西是不正确的东西，但我无法弄清楚如何返回修改后的元素。

In map(arr, iterator, callback) documentation, it is specified:

在map（arr，iterator，callback）文档中，指定了：

"The iterator is called with an item from the array and a callback for when it has finished processing."

“使用数组中的项目调用迭代器，并在完成处理时调用它。”

The thing is I cannot figure out how to feed the callback with the new arrays.

问题是我无法弄清楚如何用新数组提供回调。

3 个解决方案

#1

This doesn't work because callback in getImageEncoding is called with the return value of arr.push (which is 1), not arr after arr.push, which is what you want.

这不起作用，因为getImageEncoding中的回调是使用arr.push的返回值（为1）调用的，而不是arr.push之后的arr，这是你想要的。

function getImageEncoding(arr, callback){
    console.log("getEncoding:" + arr + "\n");

    // Get image filename
    image = arr[1];

    // Read file and get base64 encoding
    fs.readFile(image, function(err, original_data){
    var base64Image = original_data.toString('base64');
    console.log("test:" + base64Image + "\n");

        // Modify current arr by appendingthe base64 encoding of the image
        arr.push(base64Image);
        callback(err, arr);
    });
}

async.map(arr0, getImageEncoding, function(err, results){
console.log("in async.map: " + results + "\n");
});

#2

The problem is that you execute your callback with the return value of arr.push as your result, where you really want an array as the result. Just use concat instead:

问题是你用arr.push的返回值作为结果执行你的回调，你真的想要一个数组作为结果。只需使用concat：

callback(null, arr.concat(base64Image));

#3

You map it properly. You need to use the callback in the iterator, Try the documentation of after

你正确地映射它。你需要在迭代器中使用回调，试试after的文档

var fs = require("fs");
var after = require("after");

function getImageEncoding(tuple, callback){
    fs.readFile(arr[1], function(err, data){
        if (err) return callback(err)
        callback(null, tuple.concat(data.toString('base64'))
    });
}

after.map(arr0, getImageEncoding, function(err, results){
    console.log("in after.map: " + results + "\n");
});

#1