Cannot use “map” function within async module

问题:

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.

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.

I'm doing something like this:

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.

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

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

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

回答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.

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");
});

回答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:

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

回答3:

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

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");
});