forever.js

import noop from 'lodash/noop';

import onlyOnce from './internal/onlyOnce';
import ensureAsync from './ensureAsync';

/**
 * Calls the asynchronous function `fn` with a callback parameter that allows it
 * to call itself again, in series, indefinitely.

 * If an error is passed to the
 * callback then `errback` is called with the error, and execution stops,
 * otherwise it will never be called.
 *
 * @name forever
 * @static
 * @memberOf module:ControlFlow
 * @method
 * @category Control Flow
 * @param {Function} fn - a function to call repeatedly. Invoked with (next).
 * @param {Function} [errback] - when `fn` passes an error to it's callback,
 * this function will be called, and execution stops. Invoked with (err).
 * @example
 *
 * async.forever(
 *     function(next) {
 *         // next is suitable for passing to things that need a callback(err [, whatever]);
 *         // it will result in this function being called again.
 *     },
 *     function(err) {
 *         // if next is called with a value in its first parameter, it will appear
 *         // in here as 'err', and execution will stop.
 *     }
 * );
 */
export default function forever(fn, errback) {
    var done = onlyOnce(errback || noop);
    var task = ensureAsync(fn);

    function next(err) {
        if (err) return done(err);
        task(next);
    }
    next();
}