Js & Jquery Can't Detect HTML Elements, and Say's They Are Undefined

JS & jQuery can't detect html elements, and say's they are undefined

Because your script tag is above the HTML defining the elements that it acts on, it can't find them because they don't exist as of when that code runs. Here's the order in which things are happening in your page:

1. The html and head elements are created
2. The meta element is created and its content noted by the parser
3. The script element for jQuery is created
4. The parser stops and waits for the jQuery file to load
5. Once loaded, the jQuery file is executed
6. Parsing continues
7. The script element for your code is created
8. The parser stops and waits for your file to load
9. Once loaded, your script code is run — and doesn't find any elements, because there are no div elements yet
10. Parsing continues
11. The browser finishes parsing and building the page, which includes creating the elements you're trying to access in your script

Ways to correct it:

1. Move the script elements to the very end, just before the closing </body> tag, so all of the elements exist before your code runs. Barring a good reason not to do this, this is usually the best solution.

2. Use jQuery's ready feature.

3. Use the defer attribute on the script element, but beware that not all browsers support it yet.

But again, if you control where the script elements go, #1 is usually your best bet.

JS & jQuery can't detect html elements, and say's they are undefined

Because your script tag is above the HTML defining the elements that it acts on, it can't find them because they don't exist as of when that code runs. Here's the order in which things are happening in your page:

1. The html and head elements are created
2. The meta element is created and its content noted by the parser
3. The script element for jQuery is created
4. The parser stops and waits for the jQuery file to load
5. Once loaded, the jQuery file is executed
6. Parsing continues
7. The script element for your code is created
8. The parser stops and waits for your file to load
9. Once loaded, your script code is run — and doesn't find any elements, because there are no div elements yet
10. Parsing continues
11. The browser finishes parsing and building the page, which includes creating the elements you're trying to access in your script

Ways to correct it:

1. Move the script elements to the very end, just before the closing </body> tag, so all of the elements exist before your code runs. Barring a good reason not to do this, this is usually the best solution.

2. Use jQuery's ready feature.

3. Use the defer attribute on the script element, but beware that not all browsers support it yet.

But again, if you control where the script elements go, #1 is usually your best bet.

Why is html element undefined?

The issue is because i will contain the final value set by the for loop when the click event occurs. To fix this you can use a closure.

However, a better approach is to avoid the issue by removing the need for the for loop completely. You can do this by using a single common class on all the elements instead of an incremental one:

$('.anchor').click(e => {
  e.preventDefault();
  let target = $(e.target).attr('href');
  
  $('html, body').animate({
    scrollTop: $(target).offset().top - 10
  }, 1000);
});

.links {
  position: fixed;
  top: 10px;
  left: 10px;
}

.content { 
  margin: 0 0 1000px 100px; 
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="links">
  <a href="#foo" class="anchor">Go to #foo</a><br />
  <a href="#bar" class="anchor">Go to #bar</a>
</div>

<div class="content" id="foo">Foo</div>
<div class="content" id="bar">Bar</div>

why javascript cant find element?

Your script tag appears before the div id="box". Therefore, it is run before that div is created, and returns null.
To fix this, try moving the script tag under the div.

jQuery keyUp never fires

You need to run the keyup function after the document finishes loading

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script><html>
<head>  <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
  <script type="text/javascript" src="http://code.jquery.com/jquery-     latest.min.js"></script>  <script type="text/javascript">    $(document).ready(function() {      $("#target").keyup(function() {        alert("Handler for .keyup() called.");      });    });  </script></head>
<body>  <form>    <input id="target" type="text">  </form></body>
</html>

JQuery - $ is not defined

That error can only be caused by one of three things:

1. Your JavaScript file is not being properly loaded into your page
2. You have a botched version of jQuery. This could happen because someone edited the core file, or a plugin may have overwritten the $ variable.
3. You have JavaScript running before the page is fully loaded, and as such, before jQuery is fully loaded.

First of all, ensure, what script is call properly, it should looks like

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js" type="text/javascript"></script>

and shouldn't have attributes async or defer.

Then you should check the Firebug net panel to see if the file is actually being loaded properly. If not, it will be highlighted red and will say "404" beside it. If the file is loading properly, that means that the issue is number 2.

Make sure all jQuery javascript code is being run inside a code block such as:

$(document).ready(function () {
  //your code here
});

This will ensure that your code is being loaded after jQuery has been initialized.

One final thing to check is to make sure that you are not loading any plugins before you load jQuery. Plugins extend the "$" object, so if you load a plugin before loading jQuery core, then you'll get the error you described.

Note: If you're loading code which does not require jQuery to run it does not need to be placed inside the jQuery ready handler. That code may be separated using document.readyState.

Why does jQuery or a DOM method such as getElementById not find the element?

The element you were trying to find wasn’t in the DOM when your script ran.

The position of your DOM-reliant script can have a profound effect on its behavior. Browsers parse HTML documents from top to bottom. Elements are added to the DOM and scripts are (generally) executed as they're encountered. This means that order matters. Typically, scripts can't find elements that appear later in the markup because those elements have yet to be added to the DOM.

Consider the following markup; script #1 fails to find the <div> while script #2 succeeds:

<script>
  console.log("script #1:", document.getElementById("test")); // null
</script>
<div id="test">test div</div>
<script>
  console.log("script #2:", document.getElementById("test")); // <div id="test" ...
</script>

JQuery code that hides and shows divs doesn't work when integrated with my live site

It seems like you might be attaching the handler before the object exists in the DOM. If I inject your script into the page after it loads (i.e., copy and paste it into the JavaScript console), then it seems to work.

Three things you could try:

You could try attaching the listener to document first, which is always there, and then use the #filecheck selector:

$(document).on('change', '#filecheck', function () {
     // Your code here
});

This might not be the best performance-wise, but could do the trick.

You could also try wrapping the code in a ready() handler:

$(document).ready(function() {
    // Your event handler code
});

You could also try moving your file handling script to the end of the document body and see if that works.

Cannot access element id's in bootstrap modal using jquery, getting undefined error

Attach your event to a container instead to the element directly. Also use on('click', ... instead of click directly.

$("#tileModal").on('click', '#saveChange', function () {
    var midVal = $('#test1').val();
    console.log(midVal);
});

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