Improved sum()

// ES 2015
function sum(...args){
    return args.reduce((a, b)=>a+b, 0);
}

/* Langauge features used:
 *
 * spread operator    - ...name
 * reduce             - Array.prototype.reduce(callback(a,b) [, initial_value])
 * lambda expressions - (argument-list) => (return-value)
 */

// ES5
function sum(){
    var total = 0;
    var i;

    for(i = 0; i < arguments.length; i++)
        total += arguments[i];

    return total;
}