What am I missing? Calculation won’t show in the field

I am trying to make an area calculator and I cannot figure out what am I missing.

the area field is not updating when entering numbers in each field.

Any advice please? I wonder if I’m missing something very important.

Many thanks in advance

Here is my html:

                <tr>
                    <th style="text-align: left;"><label>Slab Width (m): </label></th>
                    <th><div class="input-box">
                        <input type="text" name="slab_width" onkeypress="return isNumber(event)" onkeyup="calculateArea()" required="" class="form-control">
                    </div></th>
                </tr>
                <tr>
                    <th style="text-align: left;"><label>Slab Length (m): </label></th>
                    <th><div class="input-box">
                        <input type="text" name="slab_length" onkeypress="return isNumber(event)" onkeyup="calculateArea()" required="" class="form-control">
                    </div></th>
                </tr>
                <tr>
                    <th style="text-align: left;"><label>Total Area (m2):</label></th>
                    <th><div class="input-box">
                       <input type="text" name="slabsArea" disabled="" class="form-control">
                    </div></th>
                </tr>
            
            </table>```


             And here is the script for it:

    function calculateArea() {
    var form = document.getElementById("calc");
    var sLength = parseFloat(form.elements["slab_length"].value);
    var sHeight = parseFloat(form.elements["slab_width"].value);

    var slabsArea = slabWidth * slabLength;
    
    //slabsArea = Math.round(slabsArea);
    //document.getElementById("slabsArea").value = slabsArea;
    form.elements["slabsArea"].value = slabsArea;
return slabsArea;

        }