Math.random()*5

var randomNum = Math.random()*5;
alert(randomNum); // 2.9045290905811183 
alert(parseInt(randomNum,10)); // 2
alert(Math.floor(randomNum)); // 2
alert(Math.ceil(randomNum)); // 3

// max - 期望的最大值
parseInt(Math.random()*max,10)+1;
Math.floor(Math.random()*max)+1;
Math.ceil(Math.random()*max);

// max - 期望的最大值
parseInt(Math.random()*(max+1),10);
Math.floor(Math.random()*(max+1));

// max - 期望的最大值
// min - 期望的最小值 
parseInt(Math.random()*(max-min+1)+min,10);
Math.floor(Math.random()*(max-min+1)+min);

Math.random(); //该方法产生一个0到1之间的浮点数。
Math.floor(Math.random()*10+1); //1-10
Math.floor(Math.random()*24);//0-23 

<script language="JavaScript"><!--
// The Central Randomizer 1.3 (C) 1997 by Paul Houle (houle@msc.cornell.edu)
// See: http://www.msc.cornell.edu/~houle/javascript/randomizer.html 
rnd.today=new Date();
rnd.seed=rnd.today.getTime();
function rnd() {
　　　　rnd.seed = (rnd.seed*9301+49297) % 233280;
　　　　return rnd.seed/(233280.0);
};
function rand(number) {
　　　　return Math.ceil(rnd()*number);
};
// end central randomizer. -->
</script>

<script language="javascript">  
var native_random = Math.random;
Math.random = function(min, max, exact) {
 if (arguments.length === 0) 
 {
  return native_random();
 } 
 else if (arguments.length === 1) 
 {
  max = min;
  min = 0;
 }
 var range = min + (native_random()*(max - min));
 return exact === void(0) ? Math.round(range) : range.toFixed(exact);
};
document.write(Math.random());
document.write('<br />');
document.write(Math.random(10));
document.write('<br />');
document.write(Math.random(3,10));
document.write('<br />');
document.write(Math.random(2,10,4));
</script>

<script type="text/javascript">
var random = (function(){
 var high = 1, low = 1 ^ 0x49616E42;
 var shuffle = function(seed){
  high = seed;
  low = seed ^ 0x49616E42;
 }
 
 return function(){
   var a = new Date()-0
    shuffle(a);
   high = (high << 16) + (high >> 16);
   high += low;
    low += high;
    return high;
  }
})();
 
alert( random() );
</script>