Basic Javascript If not working

I have made a basic javascript if statement but it does not seem to working.

Here's my code:

HTML:

<span id = 'link'>
    <a onclick = 'show()' href = 'javascript:void(0);'>Show</a>
</span>
<span id = 'box'>
    asdf
</span> 

Javascript:

function show(){
    var span1 = document.getElementById("box").innerHTML;

    if(span1 == 'asdf'){
        document.getElementById("box").innerHTML = 'test';
    }
}

Any suggestions?