为什么我已经定义的全局变量被IE8中的另一个var语句覆盖？

Question

I have two script tags: one to set up my environment, and another to read that setup and act upon it. I've reduced the bug to the following:

<script>
    window.myVar = 'hello world';
</script>
<script>
    var myVar;
    console.log(window.myVar); // Should be 'hello world'
</script>

In IE9+, Chrome, etc. 'hello world' gets logged. In IE8, however, undefined gets logged instead. What gives?

Answer 1

This seems to be an obscure IE8 bug.

By simply putting all of the JS into a single script tag, you avoid the problem entirely. Not sure of a great workaround if you need them to be in separate script tags, but I'd love to hear one.

<!-- This triggers the bug in IE8. -->
<script>
    window.myVar = 'hello world';
</script>
<script>
    var myVar;
    console.log(window.myVar); // Should be 'hello world'
</script>


<!-- This does not trigger the bug in IE8. -->
<script>
    window.myOtherVar = 'hello world again';
    var myOtherVar;
    console.log(window.myOtherVar); // Should be 'hello world again'
</script>

Here it is in a JSFiddle . If you pull it up in IE8 with the F12 developer tools open, you'll see undefined and then 'hello world again' logged.

Answer 2

If these are both your scripts, why not define the variable (if you must define it twice) the same way both times?

<script>
 window.myVar = 'hello world';
</script>
<script>
 window.myVar = window.myVar || undefined;
 console.log(window.myVar); // Should be 'hello world'
</script>