Function isUndefined

isUndefined(u: unknown): u is undefined
Type guard that checks if a value is undefined.

Type Narrowing Behavior:
- Narrows the input type to undefined when true
- Useful for explicit undefined checks
Parameters
- u: unknown
  The value to check
Returns u is undefined
true if u is undefined, false otherwise. When true, TypeScript narrows the type to undefined.
Example
```
const values: (number | undefined)[] = [1, undefined, 2];

const undefinedValues = values.filter(isUndefined);

assert.deepStrictEqual(undefinedValues, [undefined]);
```
- Defined in src/guard/is-type.mts:23