Lazy lookup

lazy-lookup.js

const getIdMap = memoizeOne((array) => {
  return array.reduce((previous, current) => {
   previous[current.id] = array[current];
   return previous;
  }, {});
});

const foo = { id: 'foo' };
const bar = { id: 'bar' };

// our lovely ordered structure
const ordered = [ foo, bar ];

// lazily computed map for fast lookups
const map1 = getMap(ordered);

map1['foo'] === foo; // true
map1['bar'] === bar; // true
map1['baz'] === undefined; // true

const map2 = getMap(ordered);
// returned the same map as before - no recomputation required
const map1 === map2;

Interesting, just a caveat: you declare getIdMap, but does call to getMap.

Also:

- return array.reduce((previous, current) => {
-   previous[current.id] = array[current];
+ return array.reduce((previous, current, index) => {
+   previous[current.id] = array[index];