事件处理程序 DOM事件处理程序 案例3: script> function checkUsername0( <fo rm me d p st action=http://www.example.org/register var elMsg document getElementByld(feedback,) if (this value length 5)( Label for="username">Create a elMsg. text Content="Username must be 5 username:</labe1> characters or more Input type=text" id="username"/> ]else <div id="feedback">/div> eMsg textcontent= Label for= password">Create a password: </labeI> <input ty p password var elUsername document getElementByld id="password"/> Username) Input type="submit" value="sign uSername addEventlistener Blur, checkUsername, false K/form> script事件处理程序 DOM事件处理程序 案例3: < f o r m m e t h o d = " p o s t " action="http://www.example.org/register"> <label for="u sername">Creat e a username: </label> <input type="text" id="username" /> <div id="feedback"></div> <label for="p assword">Creat e a password: </label> < i n p u t t y p e = " p a s s w o r d " id="password" /> <input type="submit" value="sign up" /> </form> <script> function checkUsername() { var elMsg = document.getElementById('feedback'); if (this.value.length < 5) { elMsg.textContent = 'Username must be 5 characters or more'; } else { elMsg.textContent = ''; } } var elUsername = document.getElementById ('username'); elUsername.addEventListener ('blur', checkUsername, false); </script>