lazyMap.js

// Example: https://repl.it/repls/WhoppingPotableBoards

function lazyArray(data, func) {
  const out = [];

  for (let i = 0; i < data.length; i++) {
    function set(value) {
        delete out[i];

        return out[i] = value;
    }

    Object.defineProperty(out, i.toString(), {
      configurable: true,
      enumerable: true,
      set, get: () => set(func(data[i])), 
    });
  }

  return out;
}


/**
 * Will act like an array but when accessing an index for the first time the function will be evaluated.
 */
const a = lazyArray([1, 2, 3, 4], x => x * x);