during.js

import noop from 'lodash/noop';
import onlyOnce from './internal/onlyOnce';

/**
 * Like [`whilst`]{@link module:ControlFlow.whilst}, except the `test` is an asynchronous function that
 * is passed a callback in the form of `function (err, truth)`. If error is
 * passed to `test` or `fn`, the main callback is immediately called with the
 * value of the error.
 *
 * @name during
 * @static
 * @memberOf module:ControlFlow
 * @method
 * @see [async.whilst]{@link module:ControlFlow.whilst}
 * @category Control Flow
 * @param {Function} test - asynchronous truth test to perform before each
 * execution of `fn`. Invoked with (callback).
 * @param {Function} fn - A function which is called each time `test` passes.
 * The function is passed a `callback(err)`, which must be called once it has
 * completed with an optional `err` argument. Invoked with (callback).
 * @param {Function} [callback] - A callback which is called after the test
 * function has failed and repeated execution of `fn` has stopped. `callback`
 * will be passed an error, if one occured, otherwise `null`.
 * @example
 *
 * var count = 0;
 *
 * async.during(
 *     function (callback) {
 *         return callback(null, count < 5);
 *     },
 *     function (callback) {
 *         count++;
 *         setTimeout(callback, 1000);
 *     },
 *     function (err) {
 *         // 5 seconds have passed
 *     }
 * );
 */
export default function during(test, fn, callback) {
    callback = onlyOnce(callback || noop);

    function next(err) {
        if (err) return callback(err);
        test(check);
    }

    function check(err, truth) {
        if (err) return callback(err);
        if (!truth) return callback(null);
        fn(next);
    }

    test(check);
}