Typescript checks null/undefined

I wrote a function that returned string value and possibly null value, but Typescript only recognized the string value and ignored the null value.

Let’s examine the code below. I have a function called getFullname that accepts a string argument (username) and returns a string value (fullname), and it also can return null if there is no username.

const getFullname = (username?: string) => {
  switch (username) {
    case 'nick':
      return 'Nicky Hilton';
    case 'cadc':
      return 'Candy Crush';
    case 'tom':
      return 'Tommy Five';
    default:
      return null;
  }
};

Link playground

Obviously, Typescript should indicate the possible return values are Nick Hilton, Candy Crush, Tommy Five and null. However, if strictNullChecks option is set to false or ignored (in tsconfig.json), Typescript will remove the null value out of the return value list.

I don’t know why Typescript has that option and when/where it should apply for, but it is mentioned the Typescript’s document here. So, just give it a look and don’t let’s Typescript surprises us.