How to iterate an array synchronously using lodash, underscore or bluebird

Question

I have an array that contains filenames at each index. I want to download those files one at a time (synchronously). I know about 'Async' module. But I want to know whether any functions in Lodash or Underscore or Bluebird library supports this functionality.

Answer 1

You can use bluebird's Promise.mapSeries:

var files = [
    'file1',
    'file2'
];

var result = Promise.mapSeries(files, function(file) {
    return downloadFile(file); // << the return must be a promise
});

Depending on what you use to download file, you may have to build a promise or not.

Update 1

An exemple of a downloadFile() function using only nodejs:

var http = require('http');
var path = require('path');
var fs = require('fs');

function downloadFile(file) {
    console.time('downloaded in');
    var name = path.basename(file);

    return new Promise(function (resolve, reject) {
        http.get(file, function (res) {
            res.on('data', function (chunk) {
                fs.appendFileSync(name, chunk);
            });

            res.on('end', function () {
                console.timeEnd('downloaded in');
                resolve(name);
            });
        });
    });
}

Update 2

As Gorgi Kosev suggested, building a chain of promises using a loop works too:

var p = Promise.resolve();
files.forEach(function(file) {
    p = p.then(downloadFile.bind(null, file));
});

p.then(_ => console.log('done'));

A chain of promise will get you only the result of the last promise in the chain while mapSeries() gives you an array with the result of each promise.