parallelExecute() {
        if (this.state === BatchStates.Error) {
            return;
        }
        if (this.completed >= this.operations.length) {
            this.emitter.emit("finish");
            return;
        }
        while (this.actives < this.concurrency) {
            const operation = this.nextOperation();
            if (operation) {
                operation();
            }
            else {
                return;
            }
        }
    }
Start execute operations. One one the most important difference between
this method with do() is that do() wraps as an sync method.