Method beforeOp

src/KeyPromiseQueue.js:22–30

(key)

  }

  beforeOp(key) {
    let tuple = this.queue[key];
    if (!tuple) {
      tuple = [0, Promise.resolve()];
      this.queue[key] = tuple;
    }
    tuple[0]++;
    return tuple;
  }

  afterOp(key) {
    const tuple = this.queue[key];