JS TypeOf

JavaScript typeof: Type Inspection & Operating Quirks

The typeof unary operator returns a string indicating the type of the unevaluated operand. While reliable for primitives, typeof has historical quirks when inspecting null, arrays, and functions.

typeof Return Values Matrix

flowchart TD
    TypeOf["typeof operand"] --> Prim["Primitives"]
    TypeOf --> Ref["Reference Objects"]
    
    Prim --> PStr["String -> 'string'"]
    Prim --> PNum["Number -> 'number'"]
    Prim --> PBool["Boolean -> 'boolean'"]
    Prim --> PBig["BigInt -> 'bigint'"]
    Prim --> PSym["Symbol -> 'symbol'"]
    Prim --> PUnd["Undefined -> 'undefined'"]
    Prim --> PNull["null -> 'object' (HISTORICAL QUIRK!)"]
    
    Ref --> RFunc["Function -> 'function'"]
    Ref --> RArr["Array / Date / Object -> 'object'"]

Complete typeof Evaluation Table

Practical Code Example

// Demonstrating typeof inspections, quirks, and reliable type check functions

// 1. Primitive and Function typeof checks
console.log(`typeof "Text": ${typeof "Text"}`);          // "string"
console.log(`typeof 42: ${typeof 42}`);                  // "number"
console.log(`typeof NaN: ${typeof NaN}`);                // "number" (Quirk!)
console.log(`typeof (() => {}): ${typeof (() => {})}`);  // "function"

// 2. The Null and Array Quirks
console.log(`typeof null: ${typeof null}`);              // "object" (Quirk!)
console.log(`typeof []: ${typeof []}`);                  // "object"

// 3. Reliable Robust Type Checker Function
function getExactType(value) {
  if (value === null) return "null";
  if (Array.isArray(value)) return "array";
  return typeof value;
}

console.log(`Exact Type null: ${getExactType(null)}`);   // "null"
console.log(`Exact Type []: ${getExactType([])}`);       // "array"
console.log(`Exact Type 10: ${getExactType(10)}`);       // "number"

Best Practices & Gotchas

• Do Not Use typeof to Check for null: typeof null === "object" evaluates to true. Always check for null using val === null.
• Do Not Use typeof for Arrays: typeof [] returns "object". Use Array.isArray(val).
• Undeclared Variable Check: typeof undeclaredVar returns "undefined" without throwing a ReferenceError, making it safe for environment checks.

Self-Check Challenge

Write a function isFunction(val) using typeof that returns true if val is a callable function.