JavaScript Variables and Data Types
After Completing This Topic
You will be able to declare variables using let and const, and you will be able to distinguish the 6 basic data types in JavaScript.
What is a Variable?
A variable is a named container for values. You can store a value and then retrieve it later.
let userName = "Kim Developer";
let age = 28;
console.log(userName); // "Kim Developer"
console.log(age); // 28
// Variables declared with `let` can have their values changed.
age = 29;
console.log(age); // 29There are three keywords for declaring variables in JavaScript:
let count = 0; // Variable whose value can change.
const PI = 3.14; // Constant whose value cannot change.
var old = "Old"; // Legacy method — not used anymore.Just remember let and const. var has scope issues (the range in which a variable is valid), so it is not used in modern JavaScript. Use const for values that don't need to be changed, and let for values that need to be changed.
6 Data Types
Values in JavaScript have a type. Numbers and strings are treated differently.
// 1. String — text enclosed in quotes.
const name = "Hong Gildong";
const greeting = 'Hello';
// 2. Number — no distinction between integers and decimals.
const score = 95;
const ratio = 0.85;
// 3. Boolean — true or false.
const isLoggedIn = true;
const hasError = false;
// 4. null — used to intentionally represent "no value".
const selected = null;
// 5. undefined — a state where a value has not yet been assigned.
let result;
console.log(result); // undefined
// 6. Object — used to store multiple values together.
const user = {
name: "Kim Developer",
age: 28,
active: true
};
console.log(user.name); // "Kim Developer"Checking the Type with typeof
If you don't know what type a value has, you can use typeof. This is useful for debugging when the type in AI-generated code is different from what you expected.
console.log(typeof "hello"); // "string"
console.log(typeof 42); // "number"
console.log(typeof true); // "boolean"
console.log(typeof undefined); // "undefined"
console.log(typeof null); // "object" ← A famous bug in JavaScript.
// Number + string = concatenated as a string (be careful!).
console.log(10 + "5"); // "105" (not the number 15!)
console.log(10 - "5"); // 5 (subtraction converts to a number)The fact that typeof null returns "object" is a bug that has existed since the early days of JavaScript. It has not been fixed yet. Also, 10 + "5" results in "105" — this type coercion is one of the most common causes of errors in JavaScript.
→ Apply to bio: DevBench — Python Fundamentals