Async While

This is a small utility for generating while loops with ES6 Promises.

Install

Download a UMD bundle from the releases page. The variable asyncWhile will be attached to window.

<script type="text/javascript" src="async-while.js"></script>

If using Browserify or Node.js, you can install via NPM.

$ npm install async-while

Usage

First, load async while. By default, the global Promise object is used, but you can create a while generator with a custom Promise class, too.

// simple
var asyncWhile = require("async-while");
 
// while with Bluebird promises
var Promise = require("bluebird");
var asyncWhile = require("async-while").generate(Promise);

Now, generate a while function with asyncWhile. You'll need two functions: a synchronous condition and an action.

var myWhile = asyncWhile(function(data) {
    // synchronous conditional
    return true;
}, function(data) {
    // loop content goes here
    return doSomethingAsync(data);
});

This is just a method that can be called directly or used in .then() statements.

myWhile(somedata).then(function(result) {
    console.log(result);
});
 
// or
someAsyncAction().then(myWhile).then(function(result) {
    console.log(result);
});

Here is a common use case of the while loop, processing list items in a series:

function eachSeries(items, onEach, ctx) {
    var index = -1;
 
    return asyncWhile(function() {
        // bump index before every loop
        index++;
 
        // synchronously checks if there are more
        return index < items.length;
    }, function() {
        return onEach.call(ctx, items[index], index, items);
    })();
}