getElementById
<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>获取常规元素</title>
</head>
<body><div id="box"></div><script>console.log(document.getElementById("box"))var obox = document.getElementById("box")obox.innerHTML = "222222"</script>
</body>
</html>
getElementsByClassName
<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>获取常规元素</title>
</head>
<body><ul><li class="newsitem">111111111</li><li class="newsitem">111111111</li><li class="newsitem">111111111</li></ul><ul><li class="adsitem">111111111</li><li class="adsitem">111111111</li><li class="adsitem">111111111</li></ul><script>var items = document.getElementsByClassName("newsitem")console.log(items)// items[0].innerHTML = "news-111111111"for(var i=0;i<items.length;i++){items[i].innerHTML = "news-"+i}var items = document.getElementsByTagName("li")console.log(items)</script>
</body>
</html>
getElementsByTagName
<!DOCTYPE html>
<html lang="en">
<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>获取元素</title>
</head>
<body><input type="text" name="username"><input type="password" name="password"><script> var items = document.getElementsByName("username")console.log(items[0])var items1 = document.getElementsByName("password")console.log(items1[0])// 设置值items[0].value = "xiaoming"items1[0].value = "6666666666"</script>
</body>
</html>