Looping in JavaScript

For

The most popular way of iterating in JavaScript is using the for statement.

for (let i = 0;  i < 10; i++) {
  console.log(i)
} // 0,1,2,3,4,5,6,7,8,9 => undefined

For In

The for...in statement is used to iterate in an arbitrary order over enumerable properties of an object keyed by strings only (therefore not by symbols).

const o = { a: 1, b: 2 }

for (const k in o) {
  console.log(k)
  console.log(o[k])
} // "a", 2, "b", 2 => undefined

For Of

The for...in iterator iterates over property names but the for..of iterator iterates over property values..

const o = { a: 1, b: 2 }

for (const v of o) {
  console.log(v)
} // 1, 2 => undefined

While

The while loop runs as long as a specified condition is of a truthy value.

let n = 0

while (n < 10) {
  console.log(n)
  n++
} // 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 => 9

Do While

The do...while statement repeats until a specified condition is falsy.

let n = 0

do {
  console.log(n)
  n++
} while (n < 10) // 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 => 9

The difference of the do...while loop to the while loop is that the code block will run at least once with do...while even when the provided condition is falsy.

Break

The break statement can be used with for iterators, loops and, as already noted, with switch. Using the break statement stops the relevant iteration or loop.

Continue

While the break statement stops a given iteration or a loop as a whole the continue statement omits only a given part of the iteration process and continues with the following parts.

Labeled Loops

JavaScript allows for labeled loops that identify them and make them referable in other parts of the code. The break and continue statements can be used with labeled loops to target them by their name. This can be used for example in nested loops.

let n = 0

myLoop: while (n < 3) {
  console.log(n)
  n++
}

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