生成随机数

 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);  
      };  
      p(Math.random())  
      p(Math.random(10))  
      p(Math.random(3,10))  
      p(Math.random(2,10,4)) 

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