Simple Promise implementation

rastas-promise.js

const PENDING = 1;
const RESOLVED = 2;
const REJECTED = 3;

const callLater = (fn) => setTimeout(fn, 0);

class Promise {
  constructor(initPromiseFn) {
    this._state = PENDING;
    this._value = undefined;
    this._callbacks = [];
    this._resolve = this._resolve.bind(this);
    this._reject = this._reject.bind(this);
    initPromiseFn(this._resolve, this._reject);
  }

  /* Helper methods */

  _resolve(value) {
    if (this._state === PENDING) {
      this._state = RESOLVED;
      this._value = value;
      while (this._callbacks.length > 0) {
        this._handleCallback(this._callbacks.pop());
      }
    }
  }

  _reject(value) {
    if (this._state === PENDING) {
      this._state = REJECTED;
      this._value = value;
      while (this._callbacks.length > 0) {
        this._handleCallback(this._callbacks.pop());
      }
    }
  }

  _handleCallback(callback) {
    if (this._state === RESOLVED) {
      callback.onResolved && callLater(() => callback.onResolved(this._value));
    } else if (this._state === REJECTED) {
      callback.onRejected && callLater(() => callback.onRejected(this._value));
    } else {
      this._callbacks.push(callback);
    }
  }

  /* Public methods */

  then(onResolved, onRejected) {
    return new Promise((resolve, reject) => {
      const callback = {
        onResolved: (value) => {
          let nextValue = value;
          if (onResolved) {
            try {
              nextValue = onResolved(value);
              if (nextValue && nextValue.then) {
                return nextValue.then(resolve, reject);
              }
            } catch (err) {
              return reject(err);
            }
          }
          resolve(nextValue);
        },
        onRejected: (value) => {
          let nextValue = value;
          if (onRejected) {
            try {
              nextValue = onRejected(value);
              if (nextValue && nextValue.then) {
                return nextValue.then(resolve, reject);
              }
            } catch (err) {
              return reject(err);
            }
          }
          reject(nextValue);
        },
      };
      this._handleCallback(callback);
    });
  }

  done(onResolved) {
    return this.then(onResolved);
  }

  catch(onRejected) {
    return this.then(undefined, onRejected);
  }

  /* Public static tools */

  static resolve(value) {
    return new Promise((resolve) => resolve(value));
  }

  static reject(value) {
    return new Promise((resolve, reject) => reject(value));
  }

  static delay(ms, value) {
    return new Promise((resolve) => setTimeout(() => resolve(value), ms));
  }

  static fromNode(fn) {
    return new Promise((resolve, reject) => {
      const resolveNode = (err, res) => {
        if (err) return reject(err);
        resolve(res);
      };
      fn(resolveNode);
    });
  }

  static promisify(nodeFn) {
    return (...args) => Promise.fromNode((resolveFn) => nodeFn(...args, resolveFn));
  }

  static promisifyAll(module) {
    Object.keys(module)
      .filter((key) => typeof module[key] === 'function' && !key.endsWith('Sync'))
      .forEach((key) => (module[`${key}Async`] = Promise.promisify(module[key])));
    return module;
  }

  static all(promises) {
    return new Promise((resolve, reject) => {
      const values = new Array(promises.length);
      let counter = 0;
      const tryResolve = (i) => (value) => {
        values[i] = value;
        counter++;
        if (counter === promises.length) {
          resolve(values);
        }
      };
      for (let i = 0; i < promises.length; i++) {
        const promise = promises[i];
        promise.then(tryResolve(i), reject);
      }
    });
  }
}

This was written during my presentation DIY Promise at Wunderdog.

Simple and brilliant!
Could you explain the use case of Promise.fromNode?

@qetr1ck-op thanks!

Promise.fromNode is a great utility to "promisify" Node's callback -style function on the fly. For example, if I'd like to read some file using fs.readFile:

const fs = require('fs')

Promise.fromNode(callback => fs.readFile('some-text', 'utf8', callback))
.then(text => console.log(text))
.catch(err => console.error(err))

You have mistake in this block

onRejected: (value) => {
let nextValue = value
if (onRejected) {
try {
nextValue = onRejected(value)
if (nextValue && nextValue.then) {
return nextValue.then(resolve, reject)
}
} catch (err) {
return reject(err)
}
}
resolve(nextValue) // you should use reject(nextValue) here
}
}
this._handleThenner(thenner)
})
}

Thanks @Jarosic, fixed that 👍