A technique for constructing a JS map object with a default return value

catchall map example.js

// example inspired by https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy

const defaultKey = Symbol("default");

// a benefit of keying with a Symbol is that it is not enumerable in a for...in iteration
const map = {
  key1: "foo",
  key2: "bar",
  
  [defaultKey]: "baz",
};

const trap = {
  get(target, prop, receiver) {
    if (prop in target) return Reflect.get(...arguments);
    
    return target[defaultKey]
  },
};

const catchAllMap = new Proxy(map, trap);

// first set of examples:
console.log(catchAllMap.key1) // "foo"
console.log(catchAllMap.key2) // "bar"
console.log(catchAllMap.key3) // "baz"
console.log(catchAllMap.anything_at_all) // "baz"

// imo, the syntax above feels a bit clunky

// wrapping it into a helper function could be quite handy
// AND it removes the need to add a Symbol key to the object
function createCatchAllMap(map, defaultValue) {
  const trap = {
    get(target, prop, receiver) {
      if (prop in target) return Reflect.get(...arguments);

      return defaultValue;
    },
  };

  return new Proxy(map, trap);
}