Assertions

The as keyword

The as keyword tells the compiler to treat a value as a narrower type. It never converts data at runtime, it only changes the static view. Reach for it when you know more than the checker can prove.

const input: unknown = "hi"; const text: string = input as string;

Non-null !

The exclamation mark removes null and undefined from the type in one character. Use it only when logic above guarantees a real value. A wrong mark crashes at runtime with no compiler warning to save you.

function head(items: string[]): string { const first: string | undefined = items[0]; return first!; }

const assertions

Appending as const freezes a literal into its narrowest form. Arrays become readonly tuples and strings keep exact text types. Exact literals power safer option unions and config objects.

const verbs = ["get", "post"] as const; type Verb = (typeof verbs)[number];

Assertions can lie

Assertions bypass checks, so a wrong one compiles cleanly and fails later. Prefer guards and generics that prove the type instead of claiming it. When an assertion survives review, add a comment that states the reason.

const n: unknown = 42; if (typeof n === "string") { console.log(n.toUpperCase()); }

Next: Classes

Article author: Arthur Isaev