function writeDocument(variable) {
document.write(variable + "<br/>");
}
var sy="I am a student";
var syy=sy.replace(/\s*/g,"");
var sum=0;
for(var i of sy){
sum++;
}
writeDocument("字符串长度为:"+sum);
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
<script>
</script>
</head>
<body>
</body>
</html>