Variables & Types

# Variables & Types

const vs let vs var

> Rule of thumb: start with const. Switch to let only when you actually reassign.

const PI = 3.14159;
let count = 0;
count++;            // ok
// PI = 3;          // TypeError — Assignment to constant

const makes the binding constant, not the value. The object can still mutate:

const user = { name: "alice" };
user.name = "bob";        // ✅ allowed — mutating contents
// user = { name: "x" };  // ❌ reassigning the binding

The 8 types

Primitives (7)

Reference (1)

typeof — the runtime type check

typeof "hi"        // "string"
typeof 42          // "number"
typeof true        // "boolean"
typeof undefined   // "undefined"
typeof Symbol()    // "symbol"
typeof 1n          // "bigint"
typeof {}          // "object"
typeof []          // "object"   ← arrays are objects!
typeof null        // "object"   ← legendary historical bug, can't be fixed
typeof function(){}// "function" ← functions are objects, but typeof says "function"

To test for an array, use Array.isArray(x). For null, use x === null.

Number quirks (the famous ones)

0.1 + 0.2          // 0.30000000000000004   (IEEE-754 floats)
1 / 0              // Infinity
-1 / 0             // -Infinity
"abc" * 2          // NaN
NaN === NaN        // false       ← NaN is the only value not equal to itself
Number.isNaN(NaN)  // true         ← the safe way to check
Number.MAX_SAFE_INTEGER  // 9007199254740991  (use BigInt past this)

Strings — three quote styles

const a = 'single';
const b = "double";
const name = "alice";
const msg = hi ${name}, you have ${unread} messages;     // template literal
const block = `line 1
line 2
line 3`;     // multi-line for free

null vs undefined — when to use which

• undefined — assigned by JavaScript when nothing has been assigned yet (uninitialised variable, missing object property, function with no return).
• null — assigned by you to mean "intentionally empty / no value".

let x;             // x === undefined  (auto)
let y = null;      // explicit "no value"
({}).foo           // undefined — missing property

Conversion (when to do it explicitly)

Number("42")       // 42
Number("42px")     // NaN
parseInt("42px")   // 42      (stops at first non-digit)
parseFloat("3.14") // 3.14
String(42)         // "42"
(42).toString()    // "42"
Boolean(0)         // false
Boolean("")        // false
Boolean(null)      // false
Boolean(undefined) // false
Boolean(NaN)       // false
// every other value → true   (these 6 are the "falsy" values)