Promises

.08 May, 2025

Let's build Promise from scratch.

My current understanding

The use of promises is basically to add a function to javascript's task queue once a value has been obtained by a long-running process (that process is usually carried out in a different thread or other part of the os, like when we request some value over the network or from hard disk).

1let p = new Promise((resolve,reject) => {
2  try {
3    const value = //obtain some value from a long running task.
4    resolve(value)
5  } catch (e) {
6    reject(e)
7  }
8})

let p = new Promise((resolve,reject) => {
  try {
    const value = //obtain some value from a long running task.
    resolve(value)
  } catch (e) {
    reject(e)
  }
})

Now p is container for the value we actually want. Later in our code, when we want to do something with that value we can do that like so:

1p.then((val) => {
2  // do something with this value
3})

p.then((val) => {
  // do something with this value
})

what we've done above is basically we've added the function (given to .then as an argument) to javascript's task queue. The event loop will pick it up when it gets time and add it to the call stack to be executed.

Breaking it down

So a promise is container for a value. It's constructor takes in a function. That function gets two arguments - a resolve function and a reject function. A promise also has a state indicating whether it is pending, fulfilled (resolve has been called), or rejected (reject has been called).

Each time we call .then on the promise we are adding a function to a list. When the promise is fulfilled we need to execute all the functions in that list.

We'll implement the constructor first:

class MyPromise {
  constructor(func) {
    this.value = undefined
    this.state = "pending"
    this.fulfillmentTasks = []
    this.rejectionTasks = []
    this.resolve = this.resolve.bind(this)
    this.reject= this.reject.bind(this)
    if (func) {
      
      func(this.resolve,this.reject)
    }
  }
  resolve(value) {
      if (this.state !== "pending") return this 
      this.state = "fulfilled"
      this.value = value
      this.rejectionTasks = []
      this.fulfillmentTasks.map(t => setTimeout(t, 0)) 
      this.fullfillmentTasks = []
      return this
    }
  reject(value) {
      if (this.state !== "pending") return this 
      this.state = "rejected"
      this.value = value
      this.fullfillmentTasks = []
      this.rejectionTasks.map(t => setTimeout(t, 0)) 
      this.rejectionTasks = []
      return this
    }
}

Implementing .then

MyPromise.prototype.then = function(onFulfilled, onRejected) {
    switch (this.state) {
      case "pending": {
        this.fulfillmentTasks.push(() => onFulfilled(this.value))
        this.rejectionTasks.push(() => onRejected(this.value))  
        break
      }
      case "fulfilled": 
        setTimeout(() => onFulfilled(this.value), 0)
        break
      case "rejected":
        setTimeout(() => onRejected(this.value), 0)
        break
    }
}


const p = new MyPromise((res,rej) => {
  res(2)
})

p.then((val) => console.log(val))
p.then((val) => console.log(val*2))
p.then((val) => console.log(val*3))
p.then((val) => console.log(val*4))

Chaining `.then` callbacks:

1const p = new Promise((res,rej) => {
2  res(2)
3})
4
5p.then((val) => val*2).then((val) => val*2).then((val) => val*2).then((val) => console.log(val))
6
7// should print 16

const p = new Promise((res,rej) => {
  res(2)
})

p.then((val) => val*2).then((val) => val*2).then((val) => val*2).then((val) => console.log(val))

// should print 16

For this we need to return a promise from .then function

MyPromise.prototype.then = function(onFulfilled, onRejected) {
    const newPromise = new MyPromise()

    const fullfillmentTask = () => {
      const newValue = onFulfilled(this.value)
      newPromise.resolve(newValue)
    } 
    switch (this.state) {
      case "pending": {
        this.fulfillmentTasks.push(fullfillmentTask)
        this.rejectionTasks.push(() => onRejected(this.value))  
        break
      }
      case "fulfilled": 
        setTimeout(fullfillmentTask, 0)
        break
      case "rejected":
        setTimeout(() => onRejected(this.value), 0)
        break
    }
    return newPromise
}


const newp = new MyPromise((res,rej) => {
  res(2)
})

newp.then((val) => val*2).then((val) => val*2).then((val) => val*2).then((val) => console.log("val ->", val))

module.exports.MyPromise = MyPromise

Flattening the `.then`

1const p = new MyPromise((res) => {
2  const p1 = new MyPromise((r) => r(2))
3  res(p1)
4})
5
6p.then((val) => val.then(v =>  console.log(v)))
7//prints 2
8
9// But what we want is :
10p.then(val => console.log(val))
11//prints 2
12
13//so in effect p "becomes" p1

const p = new MyPromise((res) => {
  const p1 = new MyPromise((r) => r(2))
  res(p1)
})

p.then((val) => val.then(v =>  console.log(v)))
//prints 2

// But what we want is :
p.then(val => console.log(val))
//prints 2

//so in effect p "becomes" p1

To handle the above scenario:

p needs to "become" p1
p's value is p1's value
p's state is p1's state
we can't call p.resolve() once p1 resolves

const imp = require("./main.js")
const MyPromise = imp.MyPromise
function isThenable(val) {
  return typeof val === "object" && val !== null && typeof val.then === "function"
}

MyPromise.prototype.fulfill = function(val) {
  if (isThenable(val)) return
  this.value = val
  this.state = "fulfilled"
  this.rejectionTasks = []
  this.fulfillmentTasks.map(t => setTimeout(t, 0)) 
  this.fullfillmentTasks = []
}

MyPromise.prototype.resolve = function(value) {
  if (this.settled) return this
  this.settled = true

  if (isThenable(value)) {
    value.then((v) => this.fulfill(v))
  } else {
    this.fulfill(value)
  }
  return this
}


//testing the flattening pattern.

const p0 = new MyPromise((res) => {
  const p1 = new MyPromise((r) => r(2))
  res(p1)
})

p0.then(val => console.log("flattened val -> ", val))

My current understanding

Breaking it down

Chaining .then callbacks:

Flattening the .then

Chaining `.then` callbacks:

Flattening the `.then`