text: function changeImg() {
  	// let opt= document.getElementById("demo").src;
  	const TIGER= "images/tiger.gif";
  	const BIRD= "images/bird.gif";
  	if (opt == TIGER) {
  		opt= BIRD;
  	} else {
  		opt= TIGER;
  	}
  }
  
  function checkLogin() {
  	id= document.getElementById("user_id").value;
  	pw= document.getElementById("user_pw").value;
  	// console.log(id+","+pw);
  	if (id == "java") {
  		if (pw == "1234") {
  			console.log("PASS");
  		} else {
  			console.log("Incorrect PW");
  		}
  	} else {
  		console.log("Incorrect ID");
  	}
  }
  
  
  var p_tag= document.createElement("span");
  const CODE= createRandomCode();
  p_tag.innerText= CODE;
  var praent_tag= document.getElementById("code");
  praent_tag.appendChild(p_tag);
  
  function createRandomCode() {
  	// 1. i=10; j=26; chars= [[0,2,3...],[a,b,c...],[A,B,C..]];
  	// 2. chars= '0'~'9'; chars= 'A'~'Z; chars= 'a'~'z;
  	chars= "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  	const MAX= 36; let code= "";
  	for (i = 0; i<5; i++) 
  	{
  		j= Math.floor(Math.random()*36);
  		code+= chars.charAt(j);	
  	}
  	// 2. chars.substring(i,i+1);
  	return code;
  }
  
  function checkCode() {
  	const INPUT= document.getElementById("code_input").value;
  	if (CODE == INPUT) { alert("PASS"); }
  	else { alert("NOT"); }
  }
  
  let id_input= document.getElementById("ID02").value;
  let id_check_input= document.getElementById("email").value;
  let pw_input= document.getElementById("PW02").value;
  let pw_check_input= document.getElementById("check_pw").value;
  if (id_input=="" || id_check_input=="" || pw_input=="" || pw_check_input==""){
  	alert("Please enter blink.")
  }
  
  function join () {
  	if (pw_input == pw_check_input) {console.log("PASS");}
  	else { alert("Please check PW"); }
  }
  