extract-methods.js

const cache = new WeakMap();
const extract = (receiver, method) => {
  let cacheEntry = cache.get(receiver);
  if (!cacheEntry) {
    cacheEntry = new Map();
    cache.set(receiver, cacheEntry);
  }
  let boundMethod = cacheEntry.get(method);
  if (!boundMethod) {
    boundMethod = method.bind(receiver);
    cacheEntry.set(method, boundMethod);
  }
  return boundMethod;
};

// ==================================

class Person {
  constructor(name) {
    this.name = name;
  }
  describe() {
    return 'Person named ' + this.name;
  }
}
const jane = new Person('Jane');
const func = extract(jane, jane.describe);
assert.equal(func(), 'Person named Jane');

// Extract again: same value
assert.equal(extract(jane, jane.describe), func);