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.