Function isNonNullish

isNonNullish<T>(u: T): u is NonNullable<T>
Type guard that checks if a value is not null or undefined (non-nullish).

This function uses the loose inequality operator (!=) to check that a value is neither null nor undefined in a single comparison. This is equivalent to TypeScript's NonNullable<T> utility type.

Type Narrowing Behavior:
- Excludes both null and undefined from the input type when true
- Equivalent to applying TypeScript's NonNullable<T> utility type
- Commonly used to filter out nullish values from arrays
Type Parameters
- T
  The type of the input value
Parameters
- u: T
  The value to check
Returns u is NonNullable<T>
true if u is not null and not undefined, false otherwise. When true, TypeScript narrows the type to NonNullable<T>.
Example
```
const values: (string | null | undefined)[] = ['Ada', null, undefined];

const definedValues = values.filter(isNonNullish);

assert.deepStrictEqual(definedValues, ['Ada']);
```
- Defined in src/guard/is-type.mts:413