Extends native JS `Map` class to allow for default values when a `key` does not exist on the map

MapWithDefaultValues.js

class MapWithDefaultValues extends Map {
	constructor(...args) {
		super(...args);
		this._defaultGetValue = undefined;
	}

	setDefault(value) {
		this._defaultGetValue = value;
		return this;
	}

	clearDefault() {
		return this.setDefault(undefined);
	}

	get(key, ...rest) {
		if (super.has(key)) {
			return super.get(key);
		}

		let [defaultValue] = rest;
		// Allow for `get(key, undefined)` to override `this._defaultGetValue`
		if (rest.length === 0) {
			defaultValue = this._defaultGetValue;
		}

		return typeof defaultValue === 'function' ? defaultValue(key, this) : defaultValue;
	}
}

const assert = require('assert');

let mapDefault = new MapWithDefaultValues();
mapDefault.setDefault('default');
mapDefault.set('a', 1);
assert.strictEqual(mapDefault.get('a'), 1);
assert.strictEqual(mapDefault.get('a', 'otherDefault'), 1);
assert.strictEqual(mapDefault.get('none'), 'default');
assert.strictEqual(mapDefault.get('alsoNone', 'override'), 'override');

// When a function is passed as the default, it is called with `(key, map)` as its 1st and 2nd arguments,
// and that function's return value is used as the default.
let mapDefaultFunc = new MapWithDefaultValues();
mapDefaultFunc.setDefault(key => {
	return typeof key === 'number' ? key * 2 : 'default';
});

assert.strictEqual(mapDefaultFunc.get('b'), 'default');
assert.strictEqual(mapDefaultFunc.get(5), 10); // 5 * 2

assert.strictEqual(
	mapDefaultFunc.get('none', key => {
		const keyLength = key?.length ?? 0;
		return keyLength * 10;
	}),
	40
); // 'none'.length * 10

mapDefaultFunc.set('x', -1);
mapDefaultFunc.set('y', -2);

assert.strictEqual(
	mapDefaultFunc.get('alsoNone', (key, map) => {
		return map.size;
	}),
	2
);

// If you want a defaultValue that is itself a function,
// you'll need to pass a function *that returns* a function,
// since function defaultValues are evaluated first.
let funcsMap = new MapWithDefaultValues();
funcsMap.setDefault(() => {
	return function identity(val) {
		return val;
	};
});
funcsMap.set('double', v => v * 2);
funcsMap.set('triple', v => v * 3);

let fn = funcsMap.get('double');
assert.strictEqual(fn(3), 6);

fn = funcsMap.get('quad');
assert.strictEqual(fn(3), 3); // Uses default `identity` since 'quad' does not exist

fn = funcsMap.get('negate', () => {
	return function negate(v) {
		return -v;
	};
});

assert.strictEqual(fn(3), -3);