Nix Override Design Pattern (https://nixos.org/nixos/nix-pills/override-design-pattern.html) in JavaScript.

makeOverridable.js

const makeOverridable = obj => ({
  ...obj,
  override: overrides => makeOverridable({
    ...obj,
    ...overrides,
  }),
})

makeOverridable for functions that have type ({ ...args }) => ({ ...obj }) should actually be:

const makeOverridableMaker = f => args => ({
  ...f(args),
  override: argOverrides => makeOverridableMaker(f)({
    ...args,
    ...argOverrides,
  }),
})

Example usage:

const makeDouble = obj => Object.entries(obj).map(([k, v]) => [k, v * 2]).reduce((obj, entry) => (obj[entry[0]] = entry[1], obj), {})
const makeDoubleAndOverridable = makeOverridableMaker(makeDouble)
const obj = makeDoubleAndOverridable({ a: 1, b: 2 }) // => { a: 2, b: 4, override: ƒ }
const obj2 = obj.override({ c: 3 }) // => { a: 2, b: 4, c: 6, override: ƒ }
const obj3 = obj2.override({ b: 21 }) // => { a: 2, b: 42, c: 6, override: ƒ }