Javascript开关大于/小于运算符不起作用

Question

The following code gets a screen resolution and works - the variable ScreenWidth contains the correct values yet the case greater than ">" and less than "<" do not work and the code always displays the default image. Changing the first case to "case (ScreenWidth = 800):" does indeed work. I have also tried using "switch (TRUE)" which does absolutely nothing at all, not even the default case.

So my question is "Why are the greater than / less than operators not working?"

<script type="text/javascript">
var ScreenWidth;
ScreenWidth = (screen.width);

switch (ScreenWidth)
{
    case (ScreenWidth > 799): 
        ShowImage = ('Big.jpg');
    break;

    case (ScreenWidth < 800):
        ShowImage = ('Small.jpg');
    break;

    default :
        ShowImage = ('Default.jpg');
}

</script>

Answer 1

Why are the greater than / less than operators not working?

Because you are comparing a number ( ScreenWidth ) against a boolean ( ScreenWidth > 799 ). switch compares the values using strict comparison for equality testing, so comparing different data types will always result in false .

Changing the first case to case (ScreenWidth = 800): does indeed work.

That's because = is an assignment and the result of ScreenWidth = 800 is 800 , so you are comparing against a number, which is fine.

I have also tried using "switch (TRUE)" which does absolutely nothing at all, not even the default case.

Well, TRUE does not exist in JavaScript. JS is case-sensitive . switch(true) should work fine.