Equality

TypeScript also uses switch statements and equality checks like ===, !==, ==, and != to narrow types. For example:

function example(x: string | number, y: string | boolean) {
  if (x === y) {
    // We can now call any 'string' method on 'x' or 'y'.
    x.toUpperCase();
    y.toLowerCase();
  } else {
    console.log(x);
    console.log(y);
  }
}

When we checked that x and y are both equal in the above example, TypeScript knew their types also had to be equal. Since string is the only common type that both x and y could take on, TypeScript knows that x and y must be a string in the first branch.

Learn more from the following links:

• Equality Narrowing