isUndefined
function isUndefined(value: unknown): value is undefined;
The test is typeof value === "undefined", which is safe for a variable that was never declared, unlike a bare comparison.
isX and nonX answer a question and never interrupt control flow; requireX and requireNonX hand the value back or throw. The scheme is explained in Validation conventions.
Parameters
Parameter | Type | Description |
|---|---|---|
|
| The value to check. |
Returns
boolean — true when the value is undefined; otherwise false.
In TypeScript the guard narrows value to undefined inside the branch.
Examples
What passes and what does not
isUndefined(undefined); // => true
isUndefined(null); // => false
isUndefined(0); // => false
isUndefined(""); // => false
See also
Source
23 September 2026
This documentation was generated with AI (Claude) from the library's source code.