Generate random numbers that are in sorted into order without duplicates Javascript?

I am creating a lottery number generator and am having trouble randomly generating 6 numbers without duplicates. Here is the code so far for generating 6 numbers and adding them to the randomNumbers array and sorting them:

var randomNumbers = [];  
var number; 

function addNumsToArray() {  
    for(var i = 1; i <= 6; i++) {  
        number = Math.floor(Math.random() * 59 + 1);  
        randomNumbers.push(number);  
    }  
}  

function sortArray() {  
    randomNumbers.sort(function(a, b){return a - b});  
    console.log(randomNumbers);  
}  

addNumsToArray();  
sortArray();