Function isNullish

isNullish(u: unknown): u is null | undefined
Type guard that checks if a value is null or undefined (nullish).

This function uses the loose equality operator (==) to check for both null and undefined in a single comparison, which is the standard JavaScript idiom for nullish checks.

Type Narrowing Behavior:
- Narrows the input type to null | undefined when true
- Useful for checking if a value is "nullish" (either null or undefined)
Parameters
- u: unknown
  The value to check
Returns u is null | undefined
true if u is null or undefined, false otherwise. When true, TypeScript narrows the type to null | undefined.
Example
```
const values: (string | null | undefined)[] = ['Ada', null, undefined];

const nullishValues = values.filter(isNullish);

assert.deepStrictEqual(nullishValues, [null, undefined]);
```
- Defined in src/guard/is-type.mts:383