for

In JavaScript, the for keyword starts a loop that runs an initializer once, then repeats its body while a condition stays true, running an update expression after each pass. All three parts of the head are optional. A missing condition counts as true, so for (;;) runs until a break or return ends it. A continue statement skips to the update.

A counter declared with let gets a fresh binding for each iteration, so a closure created in the body keeps that iteration’s value. A var counter is one binding shared by every iteration. A const counter throws a TypeError when the update tries to change it.

The same keyword starts three more loops. The for...of loop, added in ES2015, walks the values of an iterable and is covered under of. The for...in loop enumerates an object’s enumerable string keys, including inherited ones, and is covered under in. The for await...of loop, added in ES2018, consumes async iterables wherever await is allowed.

for ([<initialization>]; [<condition>]; [<afterthought>]) <statement>

for (<declaration> of <iterable>) <statement>
for (<declaration> in <object>) <statement>
for await (<declaration> of <asyncIterable>) <statement>

> const prices = [19.99, 5, 42.5];
undefined
> let total = 0;
undefined

> for (let i = 0; i < prices.length; i++) {
…   total += prices[i];
… }
67.49

> const readers = [];
undefined

> for (let i = 0; i < 3; i++) {
…   readers.push(() => i);
… }
3

> readers.map((read) => read())
[ 0, 1, 2 ]