createMultiMap.js

function createMultiMap(items, { getKeys } = {}) {
  if (typeof getKeys !== "function") {
    throw new Error(
      "The `getKeys` function passed into `createMultiMap` must be a function."
    );
  }

  function set(m, keys, value) {
    if (keys.length === 0) {
      return;
    }
    const [first, ...rest] = keys;

    let innerMap = m.has(first) ? m.get(first) : new Map();
    m.set(first, innerMap);
    for (let [index, key] of rest.entries()) {
      if (index === rest.length - 1) {
        innerMap.set(key, value);
      } else if (!innerMap.has(key)) {
        innerMap.set(key, new Map());
      }
      innerMap = innerMap.get(key);
    }
  }

  const map = items.reduce((m, item) => {
    const keys = getKeys(item);

    set(m, keys, item);

    return m;
  }, new Map());

  return {
    has(keys) {
      if (keys.length === 0) {
        return false;
      }
      let has = false;
      let innerMap = map;
      for (let key of keys) {
        if (!innerMap.has(key)) {
          has = false;
          break;
        }
        innerMap = innerMap.get(key);
        has = true;
      }
      return has;
    },
    get(keys) {
      if (keys.length === 0) {
        return;
      }
      let innerMap = map;
      for (let key of keys) {
        if (!innerMap.has(key)) {
          return;
        }
        innerMap = innerMap.get(key);
      }
      return innerMap;
    },
    set(keys, value) {
      if (keys.length === 0) {
        return;
      }
      set(map, keys, value);
    },
  };
}

I never unit tested the above so I'm sure it might be subtly broken in some way.

Also, I feel it would be a good idea to see whether something like this could be used to create a trie data structure (e.g. following this great article by Chidi Williams). Or, whether it's best to build this as a normal graph.