How can I generate random number except one in javascript

You can use a do-while loop that repeats the Random.Range call until the number isnt the one you don't want. The random number is stored in the 'result' variable.

 var unwantedNumber:int;
 var result:int;
 
 do{
   result=Random.Range(0,3);
 }while(result==unwantedNumber);

 var unwantedNum : int = 10;
 var currentNum : int;
 
 function Start() {
    GenerateNum();
 }
 
 function GenerateNum() {
      current = Random.Range(0,20);
      if(current == unwantedNum)
         GenerateNum();
      else
         NextStep();
 }
 
 function NextStep() {
    //some code
 }