isNonNullable utility, allows you to simply write .filter(isNonNullable) on an array to exclude null/undefined and get the correct array type as a result.

README.md

isNonNullable

The isNonNullable function is a type guard that filters out null and undefined values from an array.

I've had to do this type assertion inline so many times that I just made it into a reusable utility in most of my projects.

You can use it like this to filter out null and undefined values from an array:

const values = [1, null, 2, undefined, 3];
const nonNullableValues = values.filter(isNonNullable);
// nonNullableValues is now [1, 2, 3]

Why not just use Boolean like a lot of JavaScript examples do?

Those do not provide a proper type guard, so TypeScript cannot infer that the resulting array contains only non-nullable values. Of course it also removes other falsy values like 0 and ''.

const values = [1, null, 2, undefined, 3];

const nonNullableValues = values.filter(Boolean);
nonNullableValues.map(value => value.toFixed(2)); // Error: 'value' is possibly 'null' or 'undefined'.

const values = [1, null, 2, undefined, 3];

const nonNullableValues = values.filter(isNonNullable);
nonNullableValues.map(value => value.toFixed(2)); // No error, TypeScript knows values are non-nullable.

Why not just use .filter(v => v != null) inline?

Well shucks... that does seem to work 😥. It used to not work which forced me to write .filter((v): v is NonNullable<typeof v> => v != null) over and over again, which is why I created the isNonNullable utility. But it seems that in TypeScript 5.5 they made improvements to type inference that now the inline v != null to work as a proper type guard.

isNonNullable.ts

/**
 * Type guard to check if a value is non-nullable.
 *
 * This is useful for filtering out null and undefined values from arrays.
 * Unlike Boolean this will only filter out null and undefined values and the type of the value will be narrowed accordingly.
 *
 * @param value The value to check.
 * @returns True if the value is not null or undefined, false otherwise.
 *
 * @example If condition
 * ```ts
 * if (isNonNullable(value)) {
 *   // Value is now narrowed to exclude null and undefined
 * }
 * ```
 *
 * @example Filtering an array
 * ```ts
 * const values = [1, null, 2, undefined, 3];
 * const nonNullableValues = values.filter(isNonNullable);
 * // nonNullableValues is now [1, 2, 3]
 * ```
 */
export function isNonNullable<T>(value: T): value is NonNullable<T> {
	return value != null;
}