while loop not generating any result javascript [closed]

I am trying to print out 20 sets of random 4-digit number, I used basic While loop for this but nothing is printed out. Can anyone catch an error on my code..?

     // Function returns the four digit key codes between min-max 
    function generateKeyCode(min, max) {
       
        // Randomize the key code with Math.random() function
        var keyCode = Math.round((Math.random() * max) + min);  
    
        // Convert a number to a string with the toString() method.
        // Use the length property of the text string.
        var leadingZeroCount = 4 - keyCode.toString().length; 
        
        // Add leading zeros to keyCode, if needed (eg. 9 -> 0009)
        // with loop
        for (var j = 1; j <= leadingZeroCount; j++) { 
            keyCode = "0" + keyCode;  
    
        }  
        
        // Return four digit key code as a text
        return keyCode; 
    }
    
    // For hundred times
        // Call generateKeyCode() function 
        // Print key code to the document
        var output="";
        var j=0;
        var keyCodePrint = generateKeyCode(0,9999); 
    
        while(j<20) {
             output += keyCodePrint;
             j++;
        }
        document.getElementById("keycodes").innerHTML = output;

<div id="keycodes"/>