Steven Frostwick

We can tell TypeScript that a value is not null with the post-fix operator !. By adding the ! at the end of a function, we are saying "this function does not return null here", even if it normally could at other times.

For example:

// No post-fix !
const countryDropdown = document.querySelector('select#country');
// countryDropdown is Element | null

// With post-fix !
const yearDropdown = document.querySelector('select#year')!;
// yearDropdown is Element

In the example above, if we know for certain that <select id="year"> exists in the DOM, then by using the non-null assertion operator ! TypeScript knows that yearDropdown is an Element. This means we can avoid having to do null type checking before interacting with the yearDropdown variable.