Types and Coercion

Primitives vs objects

JavaScript has two categories of values:

Primitives — immutable, stored by value:

string, number, bigint, boolean, undefined, null, symbol

Objects — mutable, stored by reference:

Plain objects {}, arrays [], functions, Map, Set, Date, etc.

// Primitives: each variable holds an independent copy
let a = 42;
let b = a;
b = 99;
console.log(a); // 42 — unchanged

// Objects: both variables point to the same object
const x = { count: 0 };
const y = x;
y.count = 5;
console.log(x.count); // 5 — same object

typeof

typeof returns a string describing the type of a value. A few results are surprising:

typeof 42            // 'number'
typeof 'hello'       // 'string'
typeof true          // 'boolean'
typeof undefined     // 'undefined'
typeof null          // 'object'  ← historical bug, not fixable
typeof {}            // 'object'
typeof []            // 'object'  ← arrays are objects
typeof function(){}  // 'function'
typeof Symbol()      // 'symbol'
typeof 9007n         // 'bigint'

The typeof null === 'object' quirk has been in the spec since JavaScript 1.0. To check for null specifically, use value === null.

== vs ===

== performs type coercion before comparing. === does not — it checks both value and type.

0 == false     // true  — false coerces to 0
0 === false    // false — number vs boolean

'' == false    // true  — both coerce to 0
'' === false   // false

null == undefined  // true  — special rule
null === undefined // false

1 == '1'   // true  — '1' coerces to 1
1 === '1'  // false

Truthy and falsy

Every value is either truthy or falsy when used in a boolean context.

Falsy values — only these six:

false, 0, '' (empty string), null, undefined, NaN

Everything else is truthy — including [], {}, '0', and 'false'.

if ([]) console.log('empty array is truthy');   // prints
if ({}) console.log('empty object is truthy');  // prints
if (0)  console.log('zero is truthy');          // does NOT print

Runnable coercion demo

// Spot the coercion surprises
console.log(0 == false);      // true
console.log(0 === false);     // false
console.log(null == undefined); // true
console.log(null === undefined); // false
console.log([] == false);     // true  — [] coerces to '' coerces to 0
console.log(Boolean([]));     // true  — [] is truthy!
console.log(typeof null);     // object

JavaScript

console.log('0 == false:', 0 == false);
console.log('0 === false:', 0 === false);
console.log('null == undefined:', null == undefined);
console.log('null === undefined:', null === undefined);
console.log('[] == false:', [] == false);
console.log('Boolean([]):', Boolean([]));
console.log('typeof null:', typeof null);