Type Guards (x is T)

Implement five type guard functions. In TypeScript, a type guard is a function whose return type is x is T — a type predicate that narrows the type of a variable in the surrounding scope:

function isString(x: unknown): x is string {
  return typeof x === 'string';
}

const value: unknown = 'hello';
if (isString(value)) {
  value.toUpperCase(); // TypeScript knows value is string here ✅
}

The five guards

TypeScript rules

• isNumber must return false for NaN (it is typeof 'number' but not a valid number)
• isObject must return false for arrays (they are objects but not plain objects)
• isObject must return false for null (typeof null === 'object' is JS's famous bug)