Simple ES6 partial function

partial.js

const partial = (fn, ...args) => partialApply(fn, args);

const partialApply = (fn, args) => fn.bind(null, ...args);

// example
const add = (a, b) => a + b;
const sum = (...args) => args.reduce(add, 0);
const add10 = partial(sum, 4, 6);
const add40 = partialApply(sum, [5, 15, 20]);

sum(1,2,3)      // 6
add10(10)       // 20
add40(add10())  // 50

Sandbox: http://jsbin.com/tiyezok/edit?js,console,output