how can I get the value from a checkbox form in html?

Question

I'm going to write a page the allow user click the checkbox of the item and then calculate the total amount of it

But I found that I can't get the element from checked check box. How do i return the value of that checkbox

Here is my code :

function calculate(){
if(document.forms["listForm"].getElementsByTagName("item1").checked==true)
    prize1 = document.forms["listForm"].getElementsByTagName("item1Q").value * document.forms["listForm"].getElementsByTagName("item1").value
total = prize1
alert("The total is "+total)
}

 <form action="" id="listForm" >
<table border="1" >
  <tr> 
    <td style="text-align: center; width: 500px"><b>Books</b></td>
    <td style="text-align: center; width: 50px"><b>Quantity</b></td>
  </tr>
  <tr> 
    <td><input type="checkbox" name="item1" value="19.95" />
      XHTML Basic, $19.95</td>
    <td><input type="text" name="item1Q" value="0" size="2" maxlength="2" /></td>

Answer 1

Your form's name attribute is missing.

define like this one

<form action="" id="listForm" name="listForm" >

define JS:

function calculate() {
        if (document.forms["listForm"]["item1"].checked == true)
            prize1 = document.forms["listForm"]["item1Q"].value * document.forms["listForm"]["item1"].value
        total = prize1
        alert("The total is " + total)
    }

Answer 2

function calculate(){
if(document.forms["listForm"]["item1"].checked=true)

var prize1 = Number(document.forms["listForm"]["item1Q"].value) *       Number(document.forms["listForm"] ["item1"].value)
var total = prize1;
alert("The total is "+total);
}


<form action="" id="listForm" >
<table border="1" >
  <tr> 
<td style="text-align: center; width: 500px"><b>Books</b></td>
<td style="text-align: center; width: 50px"><b>Quantity</b></td>
</tr>
<tr> 
<td><input type="checkbox" name="item1" value="19.95"  onchange="calculate()"/>
  XHTML Basic, $19.95</td>
<td><input type="text" name="item1Q" value="2" size="2" maxlength="2" />     </td>

Answer 3

Using jQuery instead of Javascript would make code more readable and manageable. If this is feasible for you please try below code:

https://jsfiddle.net/6q3o6ghn/

<script>
  $(document).ready(function(){
  $('input[name=item1]').on('change',function(){

   if($(this).is(':checked') == true) {
     calculate($(this).val());
    }
  });

  });
  function calculate(_value) {
        prize1 =  $('input[name=item1Q]').val() * _value;
        total = prize1
        alert("The total is " + total)
    }

</script>

 <form action="" name="listForm" id="listForm" >
    <table border="1" >
      <tr> 
        <td style="text-align: center; width: 500px"><b>Books</b></td>
        <td style="text-align: center; width: 50px"><b>Quantity</b></td>
      </tr>
      <tr> 
        <td><input type="checkbox" name="item1" value="19.95"/>
          XHTML Basic, $19.95</td>
        <td><input type="text" name="item1Q" value="0" size="2" maxlength="2" /></td></tr>