​x
 
<!DOCTYPE html>
<html>
<body>
​
<p>函数可以访问函数外部定义的变量：</p>
​
<button type="button" onclick="myFunction()">单击这里！</button>
​
<p id="demo"></p>
​
<script>
var a = 4;
function myFunction() {
  document.getElementById("demo").innerHTML = a * a;
} 
</script>
​
</body>
</html>
​