ensureAsync.js

import setImmediate from './internal/setImmediate';
import initialParams from './internal/initialParams';

/**
 * Wrap an async function and ensure it calls its callback on a later tick of
 * the event loop.  If the function already calls its callback on a next tick,
 * no extra deferral is added. This is useful for preventing stack overflows
 * (`RangeError: Maximum call stack size exceeded`) and generally keeping
 * [Zalgo](http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony)
 * contained.
 *
 * @name ensureAsync
 * @static
 * @memberOf module:Utils
 * @method
 * @category Util
 * @param {Function} fn - an async function, one that expects a node-style
 * callback as its last argument.
 * @returns {Function} Returns a wrapped function with the exact same call
 * signature as the function passed in.
 * @example
 *
 * function sometimesAsync(arg, callback) {
 *     if (cache[arg]) {
 *         return callback(null, cache[arg]); // this would be synchronous!!
 *     } else {
 *         doSomeIO(arg, callback); // this IO would be asynchronous
 *     }
 * }
 *
 * // this has a risk of stack overflows if many results are cached in a row
 * async.mapSeries(args, sometimesAsync, done);
 *
 * // this will defer sometimesAsync's callback if necessary,
 * // preventing stack overflows
 * async.mapSeries(args, async.ensureAsync(sometimesAsync), done);
 */
export default function ensureAsync(fn) {
    return initialParams(function (args, callback) {
        var sync = true;
        args.push(function () {
            var innerArgs = arguments;
            if (sync) {
                setImmediate(function () {
                    callback.apply(null, innerArgs);
                });
            } else {
                callback.apply(null, innerArgs);
            }
        });
        fn.apply(this, args);
        sync = false;
    });
}