Basic javascript loops
There are now four ways to open a for loop in javascript
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 forβ¦of loop
1const fruits = ['apple', 'banana', 'cherry']23for (const fruit of fruits) {4 console.log(fruit)5}67// apple8// banana9// cherry10
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
1for (const letter of 'javascript') {2 console.log(letter)3}45// j6// a7// v8// a9// s10// c11// r12// i13// p14// t15
Many emojis are stored as multiple codepoints, and some emojis are created by joining multiple other emojis.
Forβ¦of will iterate over them one by return, returning every π¨ in a π¨βπ©βπ§βπ¦
1for (const person of 'π¨βπ©βπ§βπ¦') {2 console.log(person)3}45// π¨6// π©7// π§8// π¦9
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.
1const sequence = [0, 1, 2, 3, 4, 5]23for (const number of sequence) {4 if (isOdd(number)) continue56 console.log(number)7}89// 010// 211// 412
Loops also have the break
keyword, which will cancel not only the current iteration but also the rest of the loop
1const sequence = [0, 1, 2, 3, 4, 5]23for (const number of sequence) {4 if (number >= 3) break56 console.log(number)7}89// 010// 111// 212
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
1for (const item of items)2// or3for (let item of items)4
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
1for (const x of await Promise.all(arrayOfPromises))2// or3for await (const x of arrayOfPromises)4
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
1const hashMap = {2 abcd: { β¦ },3 abce: { β¦ },4 abcf: { β¦ },5 abcg: { β¦ },6 abch: { β¦ }7}8
1for (const key of Object.keys(hashMap)) {2 const value = hashMap[key]3}4// or5for (const key in hashMap) {6 const value = hashMap[key]7}8
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
1for (const value of Object.values(hashMap) { β¦ }2
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
1for (const [key, value] of Object.entries(hashMap)) { β¦ }2
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
1for (const [i, element] of Object.entries(array)) { β¦ }2
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
1const array = Array(25).fill() // [ empty, empty, empty, β¦]2// or3const array = Array(25).fill().map((_, i) => i) // [ 0, 1, 2, β¦ ]45for (const i of array) { β¦ }6
Classic for
The other option is to use the classic for loop syntax. This gives you the most control at the cost of readability.
1for (let i; i < 25; i++) { β¦ }2