Basic javascript loops

There are now four ways to open a for loop in javascript

• For of
• For await of
• For in
• Classic For

While they differ a bit on speed and the variables they declare implicitly, the actual body of the for loop doesn't change much between the different methods.

For of

The easiest way to loop through an array is with the for…of loop

const fruits = ['apple', 'banana', 'cherry']
 
for (const fruit of fruits) {
  console.log(fruit)
}
 
// apple
// banana
// cherry

We're not limited to arrays though, since all iterators work the same in javascript. That means we can just as easily loop through a string

for (const letter of 'javascript') {
  console.log(letter)
}
 
// j
// a
// v
// a
// s
// c
// r
// i
// p
// t

Many emojis are stored as multiple codepoints, and some emojis are created by joining multiple other emojis.

For…of will iterate over them one by return, returning every 👨 in a 👨‍👩‍👧‍👦

for (const person of '👨‍👩‍👧‍👦') {
  console.log(person)
}
 
// 👨
// 👩
// 👧
// 👦

We can use the return keyword to break out of a function early. In a loop, we can use the continue keyword to break out of the current iteration and start the next iteration immediately.

const sequence = [0, 1, 2, 3, 4, 5]
 
for (const number of sequence) {
  if (isOdd(number)) continue
 
  console.log(number)
}
 
// 0
// 2
// 4

Loops also have the break keyword, which will cancel not only the current iteration but also the rest of the loop

const sequence = [0, 1, 2, 3, 4, 5]
 
for (const number of sequence) {
  if (number >= 3) break
 
  console.log(number)
}
 
// 0
// 1
// 2

At the end of every iteration, the current scope is discarded and a new one is opened, so it's ok to use const or let at the start

for (const item of items)
// or
for (let item of items)

For await of

If you try to use await inside a loop, the execution will pause until the promise resolves and then it'll proceed as usual. To allow the loops to proceed concurrently, you can either await the promises before starting the loop or using for await of to do the same thing

for (const x of await Promise.all(arrayOfPromises))
// or
for await (const x of arrayOfPromises)

For in

In javascript, objects are not strictly iterable. If you want to loop through the keys of an object, you can either use for in or convert the keys to an array

const hashMap = {
  abcd: { … },
  abce: { … },
  abcf: { … },
  abcg: { … },
  abch: { … }
}

for (const key of Object.keys(hashMap)) {
  const value = hashMap[key]
}
// or
for (const key in hashMap) {
  const value = hashMap[key]
}

If the only thing we need the key for is to access the value, we can skip a step and loop through the values directly

for (const value of Object.values(hashMap) { … }

If we need both key and value, my preferred method is to use Object.entries, which returns a [key, value] pair, and destructure them right in the head the loop

for (const [key, value] of Object.entries(hashMap)) { … }

Since arrays are just objects with numeric keys, we can use that same syntax to get the index of our array elements inside our loop

for (const [i, element] of Object.entries(array)) { … }

To run a loop a specific number of times, we can create an array with that many elements and then loop through it.

Iterators skip over empty array slots, so we need to fill it with at least undefined first, and then optionally map its index

const array = Array(25).fill() // [ empty, empty, empty, …]
// or
const array = Array(25).fill().map((_, i) => i) // [ 0, 1, 2, … ]
 
for (const i of array) { … }

Classic for

The other option is to use the classic for loop syntax. This gives you the most control at the cost of readability.

for (let i; i < 25; i++) { … }