A chained sum function in JavaScript

chained-sum.js

//+ Jonas Raoni Soares Silva
//@ http://raoni.org

// Iteration 3: manually minified xD
const sum = (...args) => ((v, r = sum.bind(this, v)) => (r[Symbol.toPrimitive] = () => v, r))(args.reduce((a, b) => a + b, 0));

// Iteration 2: supports sum(1,2,3)
const sum = (...args) => {
	const operation = (...args) => args.reduce((a, b) => a + b, 0);
	let current = operation(...args);
	const chain = (...args) => (current = operation(current, ...args), chain);
	chain[Symbol.toPrimitive] = () => current;
	return chain;
};

// Iteration 1
const sum = (current) => {
	const chain = (...v) => {
		current = [].reduce.call(v, (a, b) => a + b, current);
		return chain;
	};
	chain[Symbol.toPrimitive] = () => current;
	return chain;
};

// Alternative implementation with a Proxy
const sum = (...args) => {
    const proxy = new Proxy((...args) => args.reduce((a, b) => a + b), {
        value: 0,
        get () {
            return () => this.value;
        },
        apply (target, _, args) {
            this.value = target(this.value, ...args);
            return proxy;
        }
    });
    return proxy(...args);
};

sum(1,2)(3)(4)(5,6);