TS function to alter value in each items and can loop if it's array.

replaceNullStringWithUndefined.ts

/**
 * This function recursively replaces all 'null' string values with undefined in an object or array of objects.
 *
 * @author Nimit Suwannagate <ethaizone@hotmail.com>
 *
 * @param {T} obj - The object or array of objects to process. The object's values can be of type string, number, or undefined.
 * @returns {T} - The processed object or array of objects with all 'null' string values replaced with undefined.
 *
 * @template V - The type of the values in the object. Defaults to string | number | undefined.
 * @template R - The type of the object. Defaults to Record<string, V>.
 * @template T - The type of the input and output. Defaults to R[] | R.
 */
function replaceNullStringWithUndefined<V=string | number | undefined, R = Record<string, V>, T = R[] | R>(obj: T): T {
  // If the input object is an array, recursively apply the replaceNullStringWithUndefined function to each item in the array.
  if (Array.isArray(obj)) {
    return obj.map((item: R) => replaceNullStringWithUndefined(item)) as T
  }

  // The function will replace all 'null' string values with undefined in each item.
  // The result is cast to T, which is the type of the input and output of the function.
  return Object.entries(obj as { [s: string]: V }).reduce((acc, [key, value]) => {
    let newValue = value
    if (value === 'null') {
      newValue = undefined as V
    }
    return {
      ...acc,
      [key]: newValue
    };
  }, {} as T);
}