Assert a value is not null or undefined
An assertion function that narrows away null and undefined and throws a clear error otherwise.
- TypeScript
- GreenSnippets
- MIT
- Executed with Node.js 22 (type stripping)
function assertDefined<T>(value: T, message = "Expected value to be defined"): asserts value is NonNullable<T> {
if (value === null || value === undefined) throw new Error(message);
}
function requireValue<T>(value: T, message?: string): NonNullable<T> {
assertDefined(value, message);
return value;
}Example / expected output
const user = requireValue(users.get(id), `No user ${id}`); // User, never undefinedHow it works
Non-null assertions (value!) just silence the compiler. An assertion function narrows the type and fails loudly with a message if the assumption is wrong. Use it at boundaries where the type system cannot know a value is present, such as Map.get or DOM queries.
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