disable submit input in html

Question

When I tried this code the button input stays disabled even if there is a value in the input, why?

<form action ="/buy" method = "POST">
    <input id="symbol" type='text' name= 'symbol'/>
    <input id="shares" type= 'text' name = 'shares'/>
    <input id="submit" type= 'submit' disabled/>
</form>
<script>
    document.querySelector('#shares').onkeyup = function(){
        if (document.querySelector('#shares').value === '') {
            document.querSelector('#submit').disabled = true;
        } else {
           document.querSelector('#submit').disabled = false;  
        }
    }
</script>

Answer 1

Everything is right apart from this statement in else, you have an error with syntax

<form action ="/buy" method = "POST">
    <input id="symbol" type='text' name= 'symbol'/>
    <input id="shares" type= 'text' name = 'shares'/>
    <input id="submit" type= 'submit' disabled/>
</form>
<script>
    document.querySelector('#shares').onkeyup = function(){
        if (document.querySelector('#shares').value === '') {
            document.querySelector('#submit').disabled = true;
        } else {
           document.querySelector('#submit').disabled = false;  
        }
    }
</script>

Working fiddle: https://jsfiddle.net/akcvtf2r/