Javascript add value if checkbox is checked -

<p>     <label for="hours">learn javascript hardy way’ - complete e-book (printed)</label>     <input type="number" min="0" id="chap7" name="hours" value="0"> </p>  <input type="checkbox" id="printed"/>please tick if printed version<br />  <p class="post">postage : <strong>£<output id="post">0</output></strong><p> 

i piece of javascript upon checkbox being checked, makes postage value 3.50. however, 3.50 per copy of complete ebook. have check value of number box , times value inside.

solution changes based on checkbox state:

var chap7 = document.getelementbyid('chap7'),      post = document.getelementbyid('post'),      printed = document.getelementbyid('printed');    printed.addeventlistener('change', function() {    var quantity = parseint(chap7.value, 10);        post.value = printed.checked ? quantity * 3.5 : quantity;  }, false);    chap7.addeventlistener('change', function() {    post.value = parseint(this.value, 10);  });

<p>      <label for="hours">learn javascript hardy way’ - complete e-book (printed)</label>      <input type="number" min="0" id="chap7" name="hours" value="0">  </p>    <input type="checkbox" id="printed"/>please tick if printed version<br />    <p class="post">postage : <strong>£<output id="post">0</output></strong><p>

---

solution changes based on text input state:

var chap7 = document.getelementbyid('chap7'),      post = document.getelementbyid('post');    chap7.addeventlistener('change', function(e) {    var quantity = parseint(chap7.value, 10);        post.value = quantity * 3.5;  }, false);

<p>      <label for="hours">learn javascript hardy way’ - complete e-book (printed)</label>      <input type="number" min="0" id="chap7" name="hours" value="0">  </p>  <p class="post">postage : <strong>£<output id="post">0</output></strong><p>