在异步循环中等待诺言

Question

Why does this code snippet doesn't work as expected, so that console.debug(images) is called before the object is filled with data? I want the two loops to run parallel, but await Promise.all should wait for the loops to finish. Both the first loop and the second loop should run simultaneously.

const images = {
     taskImages: [],
     solutionImages: []
};

await Promise.all(Object.keys(files).map((key) => {
    files[key].map(async (file) => {
        const fileId = getFileId(file.path);
        const result = await storeImage(fileId, file.path);
        if (result) {
            images[key].push(fileId);
            console.debug("Pushed " + key);
        }
    });
}));

console.debug(images);

Answer 1

The function in the outer .map() call doesn't return anything, so Promise.all doesn't get any promises to wait for.

The solution is just to wrap the inner map() in Promise.all(...) and return that:

await Promise.all(Object.keys(files).map((key) => {
    return Promise.all(files[key].map(async (file) => {...}));
}));

Answer 2

Simple solution 1:

Solution 2:

you also can use

npm async library

Link

async.parallel({
    one: function(callback) {
        setTimeout(function() {
            callback(null, 1);
        }, 200);
    },
    two: function(callback) {
        setTimeout(function() {
            callback(null, 2);
        }, 100);
    }
}, function(err, results) {
    // results is now equals to: {one: 1, two: 2}
});