Any vs Unknown vs Never

Any

• It can hold any value

• Skip the type checking , which may cause runtime error

Unknown

• Any value can be assigned to a variable of type unknown. So use unknown when a value might have any type

• However, when you try to access something from it, typescript will give you an error.

let test:unknown;
test.length;
test.join(",");
test.toFixed(2);
test.props

• In order to solve the issue, it is needed to add type checking to narrow down the type

let test: unknown;
if (test instanceof String) test.length;
if (Array.isArray(test)) test.join(',');
if (test instanceof Number) test.toFixed(2);
if (test instanceof Object) (test as { props: string }).props;

Never

• never type represents a value that never occurs

• Functions That Throw Errors

• If a function always throws an error and never returns a value, its return type can be annotated as never.

  function throwError(message: string): never {
    throw new Error(message);
  }

• Functions with Infinite Loops

• If a function contains an infinite loop that never terminates, it can be typed as never.

  function infiniteLoop(): never {
    while (true) {
      // Do something without ever returning
    }
  }