Loops: for, while, for...of

The classic for loop

Loops are why programs beat hand work: the same three lines process 5 items or 5 million. JavaScript keeps Python's while almost unchanged and adds two for forms you need to tell apart.

Python's for i in range(5) becomes a three-part header in JavaScript:

for (let i = 0; i < 5; i++) {
  console.log(i);
}

Read the header left to right:

1. let i = 0 runs once before the loop starts.
2. i < 5 is checked before every lap. False means stop.
3. i++ runs after every lap. It is shorthand for i = i + 1.

So this prints 0, 1, 2, 3, 4, exactly like range(5). Want range(1, 11)? Start at let i = 1 and run while i <= 10.

for...of walks the items themselves

When you do not need a counter, for...of visits each item directly, like Python's for ch in text:

for (const ch of "abc") {
  console.log(ch);
}
// a  b  c, one per line

It works on anything iterable — any value whose items can be visited one at a time: strings now, arrays in unit 4. Note the item variable is const because each lap gets a fresh one, you never reassign it yourself.

break exits a loop early and continue skips to the next lap, both exactly as in Python.