Control flow: if, else, switch, and loops in JavaScript

Directing Execution Flow

Control flow determines which code runs and how many times. Every meaningful program branches and repeats. JavaScript gives you conditional statements and several loop constructs to handle both.

Conditionals

const score = 72;

if (score >= 90) {
  console.log("A");
} else if (score >= 70) {
  console.log("B");
} else {
  console.log("C or below");
}

switch (day) {
  case "Mon": console.log("Start of week"); break;
  case "Fri": console.log("End of week");   break;
  default:    console.log("Midweek");
}

Loops

// classic for — when you need the index
for (let i = 0; i < 5; i++) {
  console.log(i);
}

// for-of — iterating values of an iterable
const colors = ["red", "green", "blue"];
for (const color of colors) {
  console.log(color);
}

// while — when stop condition is not count-based
let n = 1;
while (n < 100) {
  n *= 2;
}
console.log(n); // 128

Truthy and Falsy

Six values are falsy in JavaScript: false, 0, "", null, undefined, and NaN. Everything else is truthy. This is what if evaluates — not just strict booleans. Use break to exit a loop early and continue to skip to the next iteration when a condition is met inside the loop body.