TypeScript: isNullish, nonNullish and assertNonNullish

Baptiste Buisson

There are a few gems we have implemented in NNS-dapp that makes our dev's life easier on a daily basis. Among those, the following three little TypeScript functions have proven to be really useful.

​isNullish

​How often have you code if…else​ statement that checks if an object is undefined​ or ​null​?

// Pseudo code (assuming optional chaining does not exist 😉)
const test = (obj: MyObject | undefined | null) => {
    if (obj === undefined || obj === null) {
        return;
    }

    obj.fn();
}

Thanks to generics and ​type predicates, we have developed a helper that makes us avoid code duplication while preserving type safety.

export const isNullish = <T>(argument: T | undefined | null): argument is undefined | null =>
   argument === null || argument === undefined;

The use of a generic T​ will scope the usage of the function to the types we have declared in our project.

As for the type guard, it narrows TypeScript to understand that the variable is indeed from the specific expected type. In other words, this function makes TypeScript understands that the parameter is — if it matches the checks of the function — indeed ​undefined​ or ​null.

​We can use the helper as following:

const test = (obj: MyObject | undefined | null) => {
    // 1. Avoid code duplication
    if (isNullish(obj)) {
        return;
    }

    // 2. TypeScript checks it is defined
    obj.fn();
}

nonNullish

Sometimes we need the opposite, we need to know if something is defined. While we can negate previous shorthand function to get to know if it is the case, we also want to preserve the type safety.

This can be achieved by using the utility NonNullable to construct a type by excluding ​undefined​ and ​null​.

export const nonNullish = <T>(argument: T | undefined | null): argument is NonNullable<T> =>
   !isNullish(argument);

That way, TypeScript will understand that indeed, an object is defined.

const test = (obj: MyObject | undefined | null) => {
    //1. Avoid code duplication
    if…