Edit in JSFiddle

var multiply = function (number1, number2) {
    return number1 * number2;
};

//create a twice function at run time from multiply function, apply 2
var twice = function (number) {
    return multiply(2,number);
}; //or: var twice = multiply.bind(null, 2);

console.log(multiply(2, 3)); //logs 6
console.log(twice(3)); //logs 6