Loops

map() and filter()

You can use map() to do something to each item in a collection and create a new collection containing the changed items:

function toUpper(string) {
  return string.toUpperCase();
}

const cats = ['Leopard', 'Serval', 'Jaguar', 'Tiger', 'Caracal', 'Lion'];

const upperCats = cats.map(toUpper);

console.log(upperCats);
// [ "LEOPARD", "SERVAL", "JAGUAR", "TIGER", "CARACAL", "LION" ]

You can use filter() to test each item in a collection, and create a new collection containing only items that match:

function lCat(cat) {
  return cat.startsWith('L');
}

const cats = ['Leopard', 'Serval', 'Jaguar', 'Tiger', 'Caracal', 'Lion'];

const filtered = cats.filter(lCat);

console.log(filtered);
// [ "Leopard", "Lion" ]

for loop

for loop

const cats = ['Leopard', 'Serval', 'Jaguar', 'Tiger', 'Caracal', 'Lion'];

for (let i = 0; i < cats.length; i++) {
	console.log(cats[i]);
}

for...of loop

const cats = ['Leopard', 'Serval', 'Jaguar', 'Tiger', 'Caracal', 'Lion'];

for (const cat of cats) {
	console.log(cat);
}

If you want to exit a loop before all the iterations have been completed, you can use the break statement. A break statement will immediately exit the loop and make the browser move on to any code that follows it.
The continue statement works similarly to break, but instead of breaking out of the loop entirely, it skips to the next iteration of the loop.

while and do...while

Syntax for while:

initializer
	while (condition) {
	// code to run
	
	final-expression
}

Syntax for do...while:

initializer
do {
	// code to run
	
	final-expression
} while (condition)