A map that lazily gets and caches values on access

lazy-container.js

function makeLazy(name, fn, obj = {}) {
  const nullSym = Symbol('nil'); //could change this to be more es5 friendly
  let val = nullSym; //I'm not sure if holding the value in this closure or right on the object is better

  Object.defineProperty(obj, name, {
    enumerable: true,
    get() {
      if(val === nullSym) {
        val = fn();
      }
      return val;
    },
    set(newVal) {
      val = newVal;
    }
  });

  return obj;
}

function makeLazyContainer(map, targetObj = {}) {
  for(let name in map) {
    makeLazy(name, map[name], targetObj);
  }
  return targetObj;
}

/* Implement it... */

//DOM query isn't made until needed, and then it's cached
const lazyDomEls = makeLazyContainer({
  body: () => document.querySelector('body'),
  divs: () => document.querySelectorAll('div')
});

// Usage:
lazyDomEls.body; //the query is made on first access
lazyDomEls.body; //subsequent accesses just return the cached value