Question

I’ve built a script to add quantities/units and generate a total that displays sales tax.

How can I get this calculator to recognise #discount and subtract it from the total before the GST (10% sales tax) is calculated and added?

Also, is it possible to generate the total when the page loads? Instead of a user having to press the ‘Generate total’ button?

HTML

<ul>
<li> Item 1 (<input type="text" name="others" size="4" value="5" readonly="readonly" class="readonly_field"/> units)</li>
<li> Item 2 (<input type="text" name="others" size="4" value="1" readonly="readonly" class="readonly_field"/> units)</li>
<li> Item 3 (<input type="text" name="others" size="4" value="3" readonly="readonly" class="readonly_field"/> units)</li>
</ul>


<input type="button" value="Generate Total" onclick="total()"  /><br><br>

Discount <input type="text" id="discount" name="discount" value="500"/><br><br>

Total Units: <input type="text" id="units_total" name="units_total" readonly="readonly" /><br>
Sub Total: <input type="text" id="sub_total" name="sub_total" readonly="readonly" /><br>
Includes GST: <input type="text" id="gst_total" name="gst_total" readonly="readonly" /><br>
Total: <input type="text" id="g_total" name="g_total" readonly="readonly" />

JS

function total(){
var total_value = 0;
var all_others = document.getElementsByTagName("input");

for(var i=0; i<all_others.length; i++){
if(all_others[i].type!="text" || all_others[i].name!="others"){
    continue;
}
total_value += parseFloat(all_others[i].value);
}

document.getElementById("units_total").value = (total_value).toFixed(1);

document.getElementById("sub_total").value = ((total_value) *100).toFixed(2);

document.getElementById("g_total").value = (((total_value * 10/100) + total_value) * 100).toFixed(2);

document.getElementById("gst_total").value = ((total_value * 10/100) * 100).toFixed(2);
}

Answer 1

Firstly, to get your function to execute on window load, wrap it in a load event:

window.onload = function() {
    total();
}

Secondly, to get it to figure in discount, you just need to modify your variable a few times, but then when adding them together, make sure you parse them with .parseFloat():

if (document.getElementById('discount').value != '') {
    var discount = document.getElementById('discount').value;
}
else {
    var discount = 0;
}
var sub_total = (((total_value) * 100).toFixed(2) - discount).toFixed(2);
var gst = ((total_value * 10 / 100) * 100).toFixed(2);

document.getElementById("sub_total").value = sub_total;

document.getElementById("gst_total").value = gst;

document.getElementById("g_total").value = (parseFloat(sub_total) + parseFloat(gst)).toFixed(2);

DEMO