Roadmap

    3 - Equality narrowing

    Equality narrowing

    Apart from the typeof operator we can also use other switch statements and equality checks like ===, !==, ==, and != to narrow types. For example:

    ts
    function example(x: string | number, y: string | null) {
      x.toUpperCase(); // Error
    Property 'toUpperCase' does not exist on type 'string | number'.
  Property 'toUpperCase' does not exist on type 'number'.2339Property 'toUpperCase' does not exist on type 'string | number'.
  Property 'toUpperCase' does not exist on type 'number'.
     
      y.toUpperCase(); // Error
    'y' is possibly 'null'.18047'y' is possibly 'null'.
     
      // But
     
      if (x === y) {
        // both are 'string' because that is the only type common to both
        // so we can use string methods without an error
     
        x.toUpperCase();
        y.toUpperCase(); // No Error
      }
    }

    Since string is the only type in common between the possible values of x and y, Typescript knows this and allows us to perform string operations.