JS Practice — Built-in Objects

Type Guard — isInstance

📖 Usage Example

function isInstance<T>(x: unknown, ctor: new (...a: any[]) => T): x is T {
  return x instanceof ctor
}
isInstance(new Date(), Date)   // → true

A generic isInstance guard uses instanceof with a constructor type to narrow any class.

Type Guards: isInstance

Challenge: Implement isInstance<T>(x: unknown, ctor: new (...args: any[]) => T): x is T — a generic instanceof check.

isInstance(new Date(), Date)       // → true
isInstance('hello', Date)          // → false
isInstance(new Error('e'), Error)  // → true

💡 Hints (1)

• x instanceof ctor is the built-in mechanism for checking prototype chain membership.