Spread operator.

spread.js

function sum (...args) {
  return args.reduce((a, b) => a + b, 0); 
}

var nums = [1,2,3];

// ES5
console.log('sum.apply(null, nums) =', sum.apply(null, nums));
// => sum.apply(null, nums) = 6

console.log('sum(...nums) =', sum(...nums));
// => sum(...nums) = 6