Data Types & Type Coercion

20 min read

PrimitivesReference TypesType Coercion== vs ===Falsy ValuestypeofNaN

JavaScript's Type System

JavaScript is dynamically typed — variables don't have fixed types. There are 7 primitive types and 1 structural type.

Primitive vs Reference Types

Primitives (Immutable)	Reference (Mutable)
`string`	`Object`
`number`	`Array`
`bigint`	`Function`
`boolean`	`Map / Set`
`undefined`	`Date / RegExp`
`null`
`symbol`

The typeof Operator Quirks

typeof "hello"     // "string"
typeof 42          // "number"
typeof true        // "boolean"
typeof undefined   // "undefined"
typeof null        // "object"  ← FAMOUS BUG (since 1995!)
typeof []          // "object"  ← Arrays are objects
typeof function(){} // "function"

Type Coercion: == vs ===

== performs type coercion (converts types before comparing). === checks value AND type (strict equality).

Expression	==	===	Why
`0 == ""`	true	false	Both coerce to 0
`0 == "0"`	true	false	"0" → 0
`"" == "0"`	false	false	Different strings
`null == undefined`	true	false	Special rule in spec
`NaN == NaN`	false	false	NaN ≠ anything

Falsy Values (All 8)

// These are ALL the falsy values in JS:
false, 0, -0, 0n, "", null, undefined, NaN

// Everything else is truthy, including:
// "0", "false", [], {}, function(){}

Implicit Coercion Traps

console.log(1 + "2");    // "12" (number → string)
console.log("6" - 1);    // 5   (string → number)
console.log(true + true); // 2   (boolean → number)
console.log([] + []);     // ""  (arrays → strings → concat)
console.log({} + []);     // "[object Object]" or 0
console.log([] + {});     // "[object Object]"

Safe Type Checking

// Use these instead of typeof for edge cases:
Array.isArray([]);           // true
Number.isNaN(NaN);           // true (not global isNaN!)
Number.isFinite(42);         // true
Object.prototype.toString.call(null); // "[object Null]"