JavaScript 实现按钮颜色动态切换与参数函数优化

<!DOCTYPE html> 
<html lang="en"> 
<head> 
 <meta charset="UTF-8"> 
 <title>Document</title> 
</head> 
<body> 
 <button id="btn1" onclick="btn(1)">按钮1</button> 
 <button id="btn2" onclick="btn(2)">按钮2</button> 
 <button id="btn3" onclick="btn(3)">按钮3</button> 
 
 <script> 
  //设置背景颜色 
  //如果设置参数函数会节省函数数量吧 
  //设置flag+参数函数 
  flag = "btn1"; 
  function btn1(){ 
   document.getElementById("btn2").style.color = "black"; 
   document.getElementById("btn3").style.color = "black"; 
   document.getElementById("btn1").style.color = "red"; 
  } 
  function btn2(){ 
   document.getElementById("btn1").style.color = "black"; 
   document.getElementById("btn2").style.color = "red"; 
   document.getElementById("btn3").style.color = "black"; 
  } 
  function btn3(){ 
   document.getElementById("btn1").style.color = "black"; 
   document.getElementById("btn2").style.color = "black"; 
   document.getElementById("btn3").style.color = "red"; 
  } 
  function btn(num){ 
   if(num == 1){ 
    document.getElementById(flag).style.color = "black"; 
    document.getElementById(flag).style.backgroundColor = "white"; 
    document.getElementById("btn1").style.color = "red"; 
    document.getElementById("btn1").style.backgroundColor = "blue"; 
    flag = "btn1"; 
   } 
   if(num == 2){ 
    document.getElementById(flag).style.color = "black"; 
    document.getElementById(flag).style.backgroundColor = "white"; 
    document.getElementById("btn2").style.color = "red"; 
    document.getElementById("btn2").style.backgroundColor = "blue"; 
    flag = "btn2"; 
   } 
   if(num == 3){ 
    document.getElementById(flag).style.color = "black"; 
    document.getElementById(flag).style.backgroundColor = "white"; 
    document.getElementById("btn3").style.color = "red"; 
    document.getElementById("btn3").style.backgroundColor = "blue"; 
    flag = "btn3"; 
   } 
  } 
 
 </script> 
</body> 
</html>