Variables & Data Types

📦Variables Beginner

Prerequisites:

Variables are containers for storing data values. JavaScript has three ways to declare variables.

Variable Declarations

let - Block-scoped, reassignable

let message = "Hello";
message = "World"; // OK - can be reassigned

const - Block-scoped, not reassignable

const PI = 3.14159;
// PI = 3; // Error! Cannot reassign const

var - Function-scoped (legacy)

var oldStyle = "Avoid using var";

Best Practice

Use const by default. Use let only when you need to reassign. Avoid var.

Data Types

Primitive Types

// String
const name = "JavaScript";

// Number
const age = 25;
const price = 99.99;

// Boolean
const isActive = true;

// Undefined
let notAssigned;

// Null
const empty = null;

// Symbol (ES6)
const id = Symbol("id");

// BigInt (ES2020)
const bigNumber = 9007199254740991n;

Checking Types

typeof "hello"     // "string"
typeof 42          // "number"
typeof true        // "boolean"
typeof undefined   // "undefined"
typeof null        // "object" (historical bug)
typeof Symbol()    // "symbol"
typeof {}          // "object"
typeof []          // "object"

Type Coercion

JavaScript automatically converts types in certain situations:

// String concatenation
"5" + 3        // "53" (number becomes string)

// Numeric operations
"5" - 3        // 2 (string becomes number)
"5" * "2"      // 10

// Boolean context
Boolean("")    // false
Boolean("hi")  // true
Boolean(0)     // false
Boolean(1)     // true

Scope

// Global scope
const global = "I'm everywhere";

function example() {
  // Function scope
  const local = "I'm local";

  if (true) {
    // Block scope
    let blockScoped = "Only here";
    const alsoBlock = "Also only here";
  }
  // blockScoped is not accessible here
}

Next Steps

With variables mastered, learn about Functions to write reusable code.