Null to Undefined

null-to-undefined.ts

/**
 * Converts an optional type that accepts null to an optional type
 * that accepts undefined
 */
type NullToUndefined<T> = T extends null ? undefined : T

/**
 * Converts an object type with optional properties that allow null to a
 * type with properties that allow undefined instead. Non-Optional
 * properties will remain unchanged.
 */
type WithoutNullValues<T> = { [key in keyof T]: NullToUndefined<T[key]> }

/** Replaces null values with undefined */
function withoutNullValues<T>(object: T): WithoutNullValues<T> {
  const objectKeys = Object.keys(object)

  const patchedObject = objectKeys.reduce((accumulator, key) => {
    accumulator[key] = object[key] !== null ? object[key] : undefined
    return accumulator
  }, {})

  return patchedObject as WithoutNullValues<T>
}

// examples
const sourceObject = {
  someOptional: null,
  someNonOptional: 'test',
  someChileObject: {
    someOptional: null,
    someNonOptional: 'test',
  }
}

const fixedObject = withoutNullValues(object)
fixedObject.someChildObject.someOptional // undefined and not null