PromiseKeeper

promise-keeper.js

class PromiseKeeper {
  constructor(executor) {
    this.state = 'pending',
    this.callbacks = [];
    const resolve = value => {
      if (this.state !== 'pending') return;
      this.state = 'fulfilled';
      this.value = value;
      for (const cb of this.callbacks) {
        cb.onFullfilled(value);
       }
    };

    const reject = value => {
      if (this.state !== 'pending') return;
      this.state = 'rejected';
      this.value = value;
      for (const cb of this.callbacks) {
        cb.onRejected(value);
      }
    }

    try {
      executor(resolve, reject);
    } catch(e) {
      reject(e);
    }
  }

  then(onFulfilled, onRejected) {
    return new PromiseKeeper((resolve, reject) => {
      const handleCallback = () => {
        try {
          if (this.state == 'fulfilled') {
            const result = onFulfilled ? onFulfilled(this.value): this.value
            resolve(result);
          } else if (this.state == 'rejected') {
            const result = onRejected ? onRejected(this.value) : reject(this.value);
            resolve(result);
          }
        } catch(e) {
          reject(e);
        }
      }

      if (this.state == 'pending') {
        this.callbacks.push({
          onFullfilled: () => handleCallback(),
          onRejected: () => handleCallback()
        })
      } else {
        handleCallback();
      }
    });
  }
  catch(onRejected) {
    return this.then(null, onRejected);
  }
  finally(onFinally) {
    return this.then(
      (value) => {
        onFinally && onFinally();
        return value;
      },
      (reason) => {
        onFinally && onFinally();
        throw reason; 
      }
    );
  }
};