Promise

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Promise

A Pen by Mingkai Cao on CodePen.

License.

script.js

function Promise(exec) {
  this.state = 'pending';
  this.value = null;
  this.error = null;
  this.consumersResolve = [];
  this.consumersReject = [];
  this.consumersFinally = [];
  exec(this.resolve.bind(this), this.reject.bind(this));
}

Promise.prototype.resolve = function(val) {
  if (this.state !== 'pending') {
    return;
  }
  this.value = val;
  this.state = 'resolved';
  this.broadcast();
}

Promise.prototype.reject = function(err) {
  if (this.state !== 'pending') {
    return;
  }
  this.error = err;
  this.state = 'rejected';
  this.broadcast();
}

Promise.prototype.broadcast = function() {
  if (this.state === 'pending') {
    return;
  } else if (this.state === 'resolved') {
    this.consumersResolve.forEach( fun => {
      fun && fun.call(null, this.value);
    })
  } else {
    this.consumersReject.forEach( fun => {
      fun && fun.call(null, this.error);
    })
  }
  this.consumersFinally.forEach( fun => {
      fun && fun();
  })
}

Promise.prototype.then = function(onResolve, onReject) {
  if (onResolve) this.consumersResolve.push(onResolve);
  if (onReject) this.consumersReject.push(onReject);
  this.broadcast();
  return this;
}

Promise.prototype.catch = function(onReject) {
  if (onReject) this.consumersReject.push(onReject);
  return this;
}

Promise.prototype.finally = function(onFinally) {
  if (onFinally) this.consumersFinally.push(onFinally);
}


// TEST
let p = new Promise(function(resolve, reject) {
  setTimeout(function() {
    resolve(5);
  }, 1000)
});

p.then(function(val) {
  console.log({val});
}).finally(function() {
  console.log("Finally executed");
})

easy version Promise