JavaScript countdown

Why wont it countdown?

<script language="JavaScript" TYPE="text/javascript">
var container = document.getElementById('dl');
var seconds = 10;
var timer;
function countdown() {
seconds--;
if(seconds > 0) {
    container.innerHTML = 'Please wait <b>'+seconds+'</b> seconds..';
} else {
    container.innerHTML = '<a 开发者_开发技巧href="download.php">Download</a>';
    clearInterval(timer);
}
}
timer = setInterval(countdown, 1000);
</script>

<div id="dl"></div>
<input type="button" onclick="setInterval(countdown, 1000);" id="dl" value="Download" />

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Try to move the <script> block after the <div id="dl"></div>.

This way, when document.getElementById('dl'); is executed, the corresponding element in the page will already exist.

With what you posted, when document.getElementById('dl'); is executed, the corresponding <div> is not there yet -- and, so, cannot be found.

---

Because you are trying to reach the element before it exists, as the code runs before the element is loaded.

Move the line that locates the element inside the function:

<script type="text/javascript">

var seconds = 10;
var timer;

function countdown() {
  var container = document.getElementById('dl');
  seconds--;
  if(seconds > 0) {
    container.innerHTML = 'Please wait <b>'+seconds+'</b> seconds..';
  } else {
    container.innerHTML = '<a href="download.php">Download</a>';
    clearInterval(timer);
  }
}

timer = setInterval(countdown, 1000);

</script>

<div id="dl"></div>
<input type="button" onclick="setInterval(countdown, 1000);" id="dl" value="Download" />

---

The setInterval takes a string to execute, not a function pointer.

timer = setInterval("countdown()", 1000);