constructor(executor) {
    if (typeof executor !== 'function') {
      throw new TypeError('executor must be a function.');
    }

    let resolvePromise;

    this.promise = new Promise(function promiseExecutor(resolve) {
      resolvePromise = resolve;
    });

    const token = this;

    // eslint-disable-next-line func-names
    this.promise.then(cancel => {
      if (!token._listeners) return;

      let i = token._listeners.length;

      while (i-- > 0) {
        token._listeners[i](cancel);
      }
      token._listeners = null;
    });

    // eslint-disable-next-line func-names
    this.promise.then = onfulfilled => {
      let _resolve;
      // eslint-disable-next-line func-names
      const promise = new Promise(resolve => {
        token.subscribe(resolve);
        _resolve = resolve;
      }).then(onfulfilled);

      promise.cancel = function reject() {
        token.unsubscribe(_resolve);
      };

      return promise;
    };

    executor(function cancel(message, config, request) {
      if (token.reason) {
        // Cancellation has already been requested
        return;
      }

      token.reason = new CanceledError(message, config, request);
      resolvePromise(token.reason);
    });
  }
A `CancelToken` is an object that can be used to request cancellation of an operation.

@param {Function} executor The executor function.

@returns {CancelToken}