A JavaScript multimethod library.

Multimethod.js

const clauses = new Symbol();
	
function Multimethod(){
	const self = !!new.target ? this : Object.create(Multimethod.prototype);
	self[clauses] = [];
	return new Proxy(self,{
		apply: function(target, thisArg, args){
			let curImpl = () => throw new Error("No Matching Implementation");
			let curLen = -1;
			for(const { thisPredicate, predicates, impl } of self[clauses]){
				if(predicates.length > curLen && (thisPredicate || () => true)(thisArg) && predicates.every((p, i) => {
					const v = args[i];
					try { return !p || v instanceof p || p(v); }
					catch(_){ return false; }
				})){
					curImpl = impl;
					curLen = predicates.length;
				}
			}
			curImpl.apply(thisArgs, args);
		}
	});
}

Multimethod.prototype.clause = function(thisPredicate, predicates, impl){
	this[clauses].push({ thisPredicate, predicates, impl });
};		

module.exports = Multimethod;