Stdlib example

stdlib.js

// in your library
const ArrayImpl = {
  sum: els => els.reduce((p, c) => p + c),
};

const ObjectImpl = {
  map: (f, obj) => Object.keys(obj).reduce((p, c) => Object.assign(p, { [c]: f(obj[c]) }), {}),
};

module.export = {
  Array: Object.assign(ArrayImpl, Array), // probably need to do some more to assign symbols etc.
  Object: Object.assign(ObjectImpl, Object),
};

// in your runtime
const { Array } = require('stdlib.js'); // Array has all the methods of Array, plus your library methods

// ... use as you would with no danger of breaking a non-you library
// if tc39 ever add a conflicting API, you can instead change the name of the import
// and refactor behind the scenes without breaking your own userland