JS Practice — Built-in Objects

Type Guard — isNumber

📖 Usage Example

function isNumber(x: unknown): x is number {
  return typeof x === 'number'
}
isNumber(42)    // → true
isNumber(NaN)   // → true  (NaN has type number!)

typeof x === 'number' is true even for NaN, so NaN passes the isNumber type guard.

Type Guards: isNumber

Challenge: Implement isNumber(x: unknown): x is number.
Note: NaN is of type number in JavaScript — it should return true.

isNumber(42)      // → true
isNumber(NaN)     // → true  (NaN is type number!)
isNumber('42')    // → false

💡 Hints (1)

• typeof NaN === "number" is true in JavaScript.