String literal type in TypeScript

With the help of the type union operator, you can combine not only built-in types, but also strings. For example, let's make it so that a variable can only take one of two string values:

typescriptlet str: 'success' | 'error';

Let's check. Let's write a variable one of the lines:

typescriptstr = 'success';

Let's write the second one:

typescriptstr = 'error';

But trying to write a different line will result in an error:

typescriptstr = 'eee'; //