api-chain.js
function(cb) {
        cb = this.wrap(cb);
        this._callbacks.push(cb);
        if (!this._isQueueRunning) {
            if (this.start) {
                this.start();
            } else this.next();
            // this.start();
        }
        return this;
    }
function(cb) {
        cb = this.wrap(cb);
        this._callbacks.push(cb);
        if (!this._isQueueRunning) {
            if (this.start) {
                this.start();
            } else this.next();
            // this.start();
        }
        return this;
    }
function ( cb ) { cb = this . wrap ( cb ) ; this . _callbacks . push ( cb ) ; if ( ! this . _isQueueRunning ) { if ( this . start ) { this . start ( ) ; } else this . next ( ) ; // this.start(); } return this ; }
add a callback to the execution chain
add a callback to the execution chain