doDuring.js

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

/**
 * The post-check version of [`during`]{@link module:ControlFlow.during}. To reflect the difference in
 * the order of operations, the arguments `test` and `fn` are switched.
 *
 * Also a version of [`doWhilst`]{@link module:ControlFlow.doWhilst} with asynchronous `test` function.
 * @name doDuring
 * @static
 * @memberOf module:ControlFlow
 * @method
 * @see [async.during]{@link module:ControlFlow.during}
 * @category Control Flow
 * @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} test - asynchronous truth test to perform before each
 * execution of `fn`. Invoked with (...args, callback), where `...args` are the
 * non-error args from the previous callback of `fn`.
 * @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`.
 */
export default function doDuring(fn, test, callback) {
    callback = onlyOnce(callback || noop);

    var next = rest(function(err, args) {
        if (err)  return callback(err);
        args.push(check);
        test.apply(this, args);
    });

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

    check(null, true);

}