Below function returns undefined if parameter is null otherwise returns the parameter’s value as-is.

function nullToUndefined<BasicType>(targetValue: BasicType | null): BasicType | undefined {

  // `void 0` because `undefined` literal could be shadowed
  // See https://eslint.org/docs/latest/rules/no-undefined
  return targetValue === null ? void 0 : targetValue;

}

Although this function could be replaced with nullish coalescing in most cases, sometimes it could avoid trouble:

class User {


  private _name?: string;

  public set name(value: string | null) {
    this._name = nullToUndefined(value);

    this._name = value ?? undefined;    // ESLint: Unexpected use of undefined.(no-undefined)
    this._name = value ?? void 0;       // ESLint: Expected 'undefined' and instead saw 'void'.(no-void)

  }

}

Albeit inside nullToUndefined ESLint will report no-void, for the library it could be fine.

It works from the viewpoint of JavaScript, but in below case I got TS2322 error:

function appendUnits(target: number | null): string | undefined {
  return typeof target === "number" ? `${ target }kb` : nullToUndefined(target);
}

Type 'string | null | undefined' is not assignable to type 'string | undefined'.
  Type 'null' is not assignable to type 'string | undefined'.(2322)

Below code does not work:

function nullToUndefined<BasicType extends Exclude<unknown, null>>(targetValue: BasicType | null): BasicType | undefined {
  return targetValue === null ? void 0 : targetValue;
}

The only working solution is overloading:

export default function nullToUndefined(targetValue: null): undefined;

export default function nullToUndefined<BasicType extends Exclude<unknown, null>>(targetValue: null): BasicType | undefined;

export default function nullToUndefined<BasicType>(targetValue: BasicType | null): BasicType | undefined {
  return targetValue === null ? undefined : targetValue;
}

Are there ways to simplify the code?