JavaScript · Lesson 7 of 15
Conditionals and Loops
if/else, switch, for, while, for...of and truthy/falsy values.
- Beginner
- 12 min read
- 3 objectives
Before this lessonLesson 6: The DOM and Events
What you will learn
- Branch with if and switch
- Loop with for and for...of
- Explain truthy and falsy
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Control flow decides which code runs and how many times. JavaScript has the usual tools plus a few quirks around what counts as true.
if / else and switch
const score = 72;
if (score >= 90) console.log("A");
else if (score >= 70) console.log("B");
else console.log("C");
const day = "sat";
switch (day) {
case "sat":
case "sun":
console.log("weekend");
break;
default:
console.log("weekday");
}Output
B weekend
Truthy and falsy
Conditions do not need a boolean. These are falsy: false, 0, "", null, undefined, NaN. Everything else is truthy, including "0", [] and {}.
const name = "";
console.log(name || "anonymous"); // || picks the first truthy
console.log(0 ?? 10); // ?? only skips null/undefined
console.log(Boolean([]));Output
anonymous 0 true
Loops
for (let i = 0; i < 3; i++) console.log("i =", i);
const fruits = ["apple", "pear"];
for (const f of fruits) console.log(f); // values of an iterable
const user = { id: 1, name: "Ada" };
for (const key in user) console.log(key); // keys of an object
let n = 3;
while (n > 0) n--;
console.log(n);Output
i = 0 i = 1 i = 2 apple pear id name 0
Use break to leave a loop early and continue to skip to the next iteration.