lib/forever.js
function forever(fn, errback) {
    var done = onlyOnce(errback);
    var task = wrapAsync(ensureAsync(fn));

    function next(err) {
        if (err) return done(err);
        if (err === false) return;
        task(next);
    }
    return next();
}
function forever(fn, errback) {
    var done = onlyOnce(errback);
    var task = wrapAsync(ensureAsync(fn));

    function next(err) {
        if (err) return done(err);
        if (err === false) return;
        task(next);
    }
    return next();
}
function forever ( fn , errback ) { var done = onlyOnce ( errback ) ; var task = wrapAsync ( ensureAsync ( fn ) ) ; function next ( err ) { if ( err ) return done ( err ) ; if ( err === false ) return ; task ( next ) ; } return next ( ) ; }
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 {AsyncFunction} fn - an async 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).
@returns {Promise} a promise that rejects if an error occurs and an errback
is not passed
@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.
}
);
Calls the asynchronous function fn with a callback parameter that allows it to call itself again in series indefinitely .