<input type="button" onclick="myFunction()" value="今天吃什麼">

<p id="aaa">

function myFunction()

{

   var maxNum = 3;  

   var minNum = 0;  

   var n = Math.floor(Math.random()*(maxNum - minNum + 1)) + minNum;  

  if(n==0){

  document.getElementById("aaa").innerHTML = "麥當勞";

}

  else if(n==1){

  document.getElementById("aaa").innerHTML = "便當";

}

  else if(n==2){

  document.getElementById("aaa").innerHTML = "八方雲集";

}

  else if(n==3){

  document.getElementById("aaa").innerHTML = "響蔬";

} 

}