JavaScript Fundamentals

JavaScript Data Types

Beginner Lesson 3 of 10

JavaScript Data Types

JavaScript has dynamic typing - variables can hold any type of value.

Primitive Types

Strings

const single = 'Hello';
const double = "World";
const template = `Hello, ${name}!`;

// String methods
"hello".toUpperCase();     // "HELLO"
"hello".length;            // 5
"hello".charAt(0);         // "h"
"hello".includes("ell");   // true

Numbers

const integer = 42;
const float = 3.14;
const negative = -10;
const infinity = Infinity;
const notANumber = NaN;

// Number methods
Number.isInteger(42);      // true
Number.parseFloat("3.14"); // 3.14
(3.14159).toFixed(2);      // "3.14"

Booleans

const isTrue = true;
const isFalse = false;

// Truthy values: true, 1, "string", [], {}
// Falsy values: false, 0, "", null, undefined, NaN

Reference Types

Arrays

const arr = [1, 2, 3, 4, 5];

arr.push(6);           // Add to end
arr.pop();             // Remove from end
arr.map(x => x * 2);   // [2, 4, 6, 8, 10]
arr.filter(x => x > 2); // [3, 4, 5]
arr.reduce((a, b) => a + b, 0); // 15

Objects

const person = {
  name: "Alice",
  age: 25,
  greet() {
    return `Hi, I'm ${this.name}`;
  }
};

person.name;           // "Alice"
person["age"];         // 25
Object.keys(person);   // ["name", "age", "greet"]

Type Checking

typeof "hello"    // "string"
typeof 42         // "number"
typeof true       // "boolean"
typeof undefined  // "undefined"
typeof null       // "object" (quirk!)
typeof []         // "object"
typeof {}         // "object"
Array.isArray([]) // true

Next Steps

Continue to Functions →