helpers.d.ts

// The following helpers were found at
// https://github.com/Microsoft/TypeScript/issues/12215#issuecomment-319495340

/**
 * From https://github.com/Microsoft/TypeScript/issues/12215#issuecomment-307871458
 * The Diff type is a subtraction operator for string literal types. It relies on:
 *  - T | never = T
 *  - T & never = never
 *  - An object with a string index signature can be indexed with any string.
 */
declare type StringDiff<T extends string, U extends string> = ({[K in T]: K} &
  {[K in U]: never} & {[K: string]: never})[T];

/**
 * From https://github.com/Microsoft/TypeScript/issues/12215#issuecomment-311923766
 * Omits keys in K from object type T
 */
declare type ObjectOmit<T extends object, K extends keyof T> = Pick<T, StringDiff<keyof T, K>>;

/**
 * Returns a version of type T where all properties which are also in U are optionalized.
 * Useful for makding props with defaults optional in React components.
 * Compare to flow's $Diff<> type: https://flow.org/en/docs/types/utilities/#toc-diff
 */
declare type ObjectDiff<T extends object, U extends object> = ObjectOmit<T, keyof U & keyof T> &
  {[K in (keyof U & keyof T)]?: T[K]};

/**
 * A more relaxed version of ObjectOmit, that allows you to kill any keys from an object,
 * not just the onces that exist on it
 */
declare type ObjectOmitAny<T, K extends string> = Pick<T, StringDiff<keyof T, K>>;