cacheMap

cache-map.js

// cache an immutable list
// Ie: if an item changes, it's cacheKey *must* change as well
const cache = {};
function cacheMap(objectKey, mapFn) {
  const objectCache = cache[objectKey] || {};
  const nextObjectCache = cache[objectKey] = {};

  return this.map(item =>
    nextObjectCache[item.cacheKey] =
      objectCache[item.cacheKey] || mapFn(item)
  );
}

// example:
const list = [{ val: 'foo', cacheKey: 'foo'}, { val: 'bar', cacheKey: 'bar'}];
const doIt = () => list::cacheMap('list', ({val, cacheKey}) => {
  console.log('evaluate ', cacheKey);
  return `(${val})`;
}).join(',');
 
console.log(doIt());
//=> evaluate foo
//=> evaluate bar
//=> (foo),(bar)

list.push({ val: 'ack', cacheKey: 'ack' });
console.log(doIt());
//=> evaluate ack
//=> (foo),(bar),(ack)