Loops (for / while)

A for loop repeats a block a set number of times — useful when you know how many iterations you need, like stepping through an array. A while loop repeats as long as a condition stays true — useful when you don't know the count in advance.

Example 1: for — a known number of steps

let result = "";
for (let i = 1; i <= 5; i++) {
  result += i + " ";
}
document.getElementById("output").textContent = "Counted: " + result;

The three parts inside the parentheses run in a fixed order: the starting value (let i = 1) runs once, the condition (i <= 5) is checked before every loop, and the update (i++) runs after every loop — this exact three-part shape is one of the most common patterns you'll write in any C-family language, not just JavaScript.

Example 2: while — an unknown number of steps

let total = 0;
let n = 1;
while (total < 20) {
  total += n;
  n++;
}
document.getElementById("output").textContent = "Total reached: " + total;

Here we don't know in advance how many additions it'll take to reach 20 — while is the right tool exactly when the stopping condition depends on something computed as you go, not a fixed count you already know.