isDefined.ts

import * as _ from 'lodash';

/**
 * Checks to see if a value is defined (not null/undefined) on a given object.
 * Is a type-guard function, so the argument will be cast to a version of itself
 *  where that field is no longer nullable.
 * @example
 * type Foo = { x?: string }
 * const foo: Foo = ...
 * if (isDefined(foo, 'x')) {
 *  console.log(foo.x.length)
 * }
 */
export const isDefined = <T extends object, F extends keyof T>(
  t: T,
  field: F
): t is T & Required<Pick<T, F>> => {
  return !!_.isNil(t[field]);
};