var txt="Hello World!"
document.write(txt.length)

<script type="text/javascript">
var str="Hello world!" //w小寫
document.write(str.indexOf("H") + "<br />") //0
document.write(str.indexOf("World") + "<br />") //-1
document.write(str.indexOf("world")) //6
</script>

<script type="text/javascript">
var str="Hello world!"
document.write(str.match("world") + "<br />") //world
document.write(str.match("World") + "<br />") //null
document.write(str.match("worlld") + "<br />") //null
document.write(str.match("world!")) //world!
</script>

<script type="text/javascript">
var str="Visit cnblogs.com!"
document.write(str.replace(/cnblogs.com/,"Mamogu.com"))
</script>