Define a custom hash() method for use with ES6 maps

Question

To illustrate the problem, consider the following simple object

function Key( val ) {
  this._val = val;
}

Now I create a ES6 Map

Answer 1

I've created a class called CanonMap in my library big-m to encapsulate mapping by hash instead of reference.

By default, it works with tuples, Dates, and simple objects:

const { CanonMap } = "big-m";

const myMap = new CanonMap();
myMap.set(
  ["Farooq", "867-5309"],
  36.59
);

myMap.get(
  ["Farooq", "867-5309"]
) === 36.59;

myMap.set(
  {name: "Farooq", number: "867-5309"},
  36.59
);

myMap.get(
  {number: "867-5309", name: "Farooq"} // Insensitive to key ordering
) === 36.59;

myMap.set(new Date(2012, 6, 5), "Tuesday");
myMap.get(new Date(2012, 6, 5)) === "Tuesday";

It can also be extended with a custom "canonizer" function that determines how to hash values:

import {naiveCanonize, jsonCanonize, JsonCanonMap, CanonMap} from "big-m";

// Same as default canonizer, but with greater recursion depth (default is 2)
new CanonMap([], 6);

// Canonize by ID with fallback to naive
const customCanonMap = new CanonMap([
  [{id: "TEST1", x: 7}, 77],
  [{ x: 7 }, 88]
], lookup => lookup.id || naiveCanonize(lookup));

customCanonMap.get({id: "TEST1", x: 8}) === 77; // Ignores other values, uses ID
customCanonMap.get({x: 8}) === undefined; // Uses all fields, so lookup fails

// Default canonizer with JSON.stringify
new CanonMap([], jsonCanonize);
// equivalent to
new CanonMap([], lookup => JSON.stringify(lookup));
// also equivalent to
new JsonCanonMap(); 

Finally, to implement a CanonMap that makes use of a prototype hash function on the object itself, as you described, you could do something like this:

const selfHashingCanonMap = new CanonMap([], lookup => {
  if ("hash" in lookup) {
    return lookup.hash();
  } else {
    return naiveCanonize(lookup);
  }
});