Typescript Nullable<T>

Typescript supports two ways of telling if a value is missing or uninitialized. Usually, it doesn't matter if its null or undefined since people have different approaches and might use one or both of them. A common way to check if a value is missing is:

if (value == null) {

The loose comparison with null checks for both null and undefined. Since we can operate on both types, one might want to get rid of errors like type null/undefined is not assignable to type | null/undefined... by introducing the following type:

export type Nullable<T> = T | null | undefined;

Now, instead of a long repetitive declaration:

function (data: string | null | undefined)

the short and clean version can be used:

function (data: Nullable<string>)


class Formatter {
  insertUndescores(data: Nullable<string>): string {
    return data?.split('').join('_') ?? ''; 

const formatter = new Formatter();
formatter.insertUndescores('An example');