gistfile1.js

/**
 * Random number generator, never returns the same number
 * more than once, only returns integers. 
 * @param min minimum number, inclusive
 * @param max maximum number, inclusive
 * Both @params are optional
 */
var generateRandomNumber = (function(){
    
    var used = {};
    
    return function(min, max) {
        
        min = min || 0;
        max = max || 9999999;
        
        var i, attempts = 0;
        
        while ( used[i = Math.floor(Math.random() * (max - min + 1) + min)] ) {
            if ( attempts++ > max-min ) {
                return null;
            }
        }
        
        used[i] = true;
        return i;
        
    };
    
})();

// E.g.
generateRandomNumber(0, 2); // 0
generateRandomNumber(0, 2); // 2
generateRandomNumber(0, 2); // 1
generateRandomNumber(0, 2); // null


/////////////////////////////////////////////
// Another version, that can be instantiated:
/////////////////////////////////////////////


var RandomNumberGen = (function(){
    
    function RandomNumberGen(min, max) {
        this.used = {};
        this.min = min || 0;
        this.max = max || 9999999;
    }
    
    RandomNumberGen.prototype.gen = function() {
        
        var i, attempts = 0,
            max = this.max,
            min = this.min,
            used = this.used;
        
        while ( used[i = Math.floor(Math.random() * (max - min + 1) + min)] ) {
            if ( attempts++ > max-min ) {
                return null;
            }
        }
        
        used[i] = true;
        return i;
    
    };
    
    return RandomNumberGen;
    
})();

// E.g.

var myGen = new RandomNumberGen(0, 2);
myGen.gen(); // 0
myGen.gen(); // 2
myGen.gen(); // 1
myGen.gen(); // null