Typescript making undefined properties optional

undefined-optional.ts

/**
 * Get the keys of the properties to which U can be assigned.
 */
type AssignableKeys<T, U> = {
  [K in keyof T]: U extends T[K] ? K : never
}[keyof T];

/**
 * Get the interface containing only properties to which undefined can be assigned.
 */
type UndefinableProperties<T> = {
  [K in AssignableKeys<T, undefined>]: T[K]
}

/**
 * Get all of the keys except those to which U can be assigned.
 */
type IncompatibleKeys<T, U> = {
  [K in keyof T]: U extends T[K] ? never : K
}[keyof T];

/**
 * Get the interface containing no properties to which undefined can be assigned.
 */
type OmitUndefinableProperties<T> = {
  [K in IncompatibleKeys<T, undefined>]: T[K]
}

/**
 * Get the interface where all properties are optional.
 */
type Optional<T> = {[K in keyof T]?: T[K]};

/**
 * Get the interface where properties that can be assigned undefined are
 * also optional.
 */
type UndefinedOptional<T> = OmitUndefinableProperties<T> & Optional<UndefinableProperties<T>>;