JS Practice — Built-in Objects

Type Guard — isOneOf

📖 Usage Example

function isOneOf<T extends readonly unknown[]>(vals: T, x: unknown): x is T[number] {
  return (vals as readonly unknown[]).includes(x)
}
isOneOf(['red', 'blue'] as const, 'red')   // → true

isOneOf creates a type guard that narrows to a literal union from a fixed list of values.

Type Guards: isOneOf

Challenge: Implement isOneOf<T extends readonly unknown[]>(x: unknown, values: T): x is T[number] — checking if x is one of a fixed set of literal values.

isOneOf('red', ['red', 'green', 'blue'] as const)  // → true
isOneOf('pink', ['red', 'green', 'blue'] as const) // → false

💡 Hints (1)

• Array.includes uses strict equality — perfect for value checking.