Let's write native JavaScript Array methods from scratch! Part 2 (Challenge)

Introduction

Hey, welcome back! Here is the sequel (Part 2) to the previous blog post. Are you ready for some more fun? 🔥

So here is the challenge!

Can you write the JavaScript Array methods listed below from scratch?

 

The Rules

• Don't use prototype. Instead, take the array as the first argument/parameter.
• Don't use other native JavaScript methods to solve the problems
• Feel free to use the latest ECMAScript standards such as the rest operator, spread operator, default values, etc...
• Pay attention to understand if a method modifies the original array or returns a new array! Or if anything is returned at all.
• Don't look up the solutions until you really tried!

 

Array methods

1. forEach ⬇
2. map ⬇
3. filter ⬇
4. reduce ⬇
5. find ⬇
6. findIndex ⬇
7. some ⬇
8. every ⬇

 

The Solutions

 

forEach

function forEach(array, callback) {
  for(let i=0; i<array.length; i++) {
    callback(array[i], i, array)
  }
}

map

function map(array, callback) {
  const result = []

  for(let i=0; i<array.length; i++) {
    result.push(callback(array[i], i, array))
  }

  return result
}

filter

function filter(array, callback) {
  const result = []

  for(let i=0; i<array.length; i++) {
    if(callback(array[i], i, array)) {
      result.push(array[i])
    }
  }

  return result
}

reduce

// here is my attempt, can you do better?
function reduce(array, callback, initialValue) {
  let accumulator = initialValue || array[0]

  if(initialValue === 0) {
    accumulator = 0
  }

  for(let i=0; i<array.length; i++) {
    if(initialValue === undefined && i === 0) {
      continue
    }

    accumulator = callback(accumulator, array[i], i, array)
  }

  return accumulator
}

find

function find(array, callback) {
  for(let i=0; i<array.length; i++) {
    if(callback(array[i], i, array)) {
      return array[i]
    }
  }
}

findIndex

function findIndex(array, callback) {
  for(let i=0; i<array.length; i++) {
    if(callback(array[i], i, array)) {
      return i
    }
  }

  return -1
}

some

function some(array, callback) {
  let flag = false

  for(let i=0; i<array.length; i++) {
    if(callback(array[i], i, array)) {
      flag = true
    }
  }

  return flag
}

every

function every(array, callback) {
  let flag = true

  for(let i=0; i<array.length; i++) {
    if(!callback(array[i], i, array)) {
      flag = false
    }
  }

  return flag
}

 

Parting words 👋

I hope you enjoyed the challenge as much as I did! 🎉. Feel free to provide any feedback including 🐛's. I would also love to see your codes, feel free to post them as comments. Thank you and adios.