Adding to an array with an asynchronous function

Question

assume I have an array of files, file_urls, I want to parse through and add the results to a result array:

var requestImageSize = require('request-image-size');

function parse_file(file) {
    return new Promise(function(resolve, reject) {
        /*this is an asynchronous function from an external library,
         it returns attributes about the file such as its size and dimensions
        */
        requestImageSize(file).then(function(result) { 
            resolve(result);            
        }
        .catch(function(err) {
            reject(err);
        });   
    })
}
var results = [];
var promise;

//assume file_urls is already populated with strings of urls to files
file_urls.each(function(index, elem) {
    //if it's the last element then return the promise so we can return the final results at the end
    if (index == file_urls.length-1) {
        promise = parse_file(this);
    }
    //otherwise process the promise
    else {
        parse_file(this).then(function(result) {
            results.push(result);
        }
    }
});

//add the final element and return the final results
promise.then(function(result) { 
    results.push(result);
    return result;
});

Since parse_file returns a promise and I'm iterating through many promises, how do I ensure that the results array has the correct number (and possibly order) of elements?

so far in my project it is returning an erratic number of elements, what should I do differently?

The problem here is that it's possible that the last promise can return before the first promise. As stated below, you may use Promise.AllUsing this has causes me compatibility issues before. As well as using promises in general. Another option would be to use callbacks. If the callback has been called x number of times (x representing the array length), then return the necessary values. — S. Walker
– S. Walker, Commented Sep 21, 2017 at 0:31

georg · Accepted Answer · 2017-09-21 00:27:06Z

2

The idiom you're looking for is Promise.all combined with .map:

Promise.all(file_urls.map(parse_file)).then(results => ...)

Promise.all ensures you get all results in correct order, it doesn't enforce any particular order of execution though.

answered Sep 21, 2017 at 0:27

georg

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