type_guards.ts

function isDate(input: unknown) { 
  if (input instanceof Date) 
    return; 
  else 
    throw new Error('Input must be a Date!'); 
} 

function getDayOfWeek(input: unknown) : number { 
  isDate(input); 
  return input.getFullYear() // TypeScript will fail compilation here!
} 


console.log(getDayOfWeek(new Date('2019-01-01'))); 
console.log(getDayOfWeek('2019-01-01'));