Returning if condintion in for loop

function guid (l) {
    if(l === undefined){
        l = 32;
    }
    var ascii = '';
    for(var x=0; x<l; x++){
        var rnd = Math.floor(Math.random()*(122 - 48))+48;
        var convert = String.fromCharCode(rnd);
        if(!(convert.match(/[a-zA-Z0-9]/g))){
        // how to return 'rnd' until the lenght of the parameter is complete 
        }else{
            ascii += convert;
        }
    }
    return ascii;

}
console.log(guid());

This is my code. Can u give me advice how to execute my if statement ? My output now is based on how many times rnd passes the if statement.