Lock tab key with javascript?

$(document).keydown(function(objEvent) {
    if (objEvent.keyCode == 9) {  //tab pressed
        objEvent.preventDefault(); // stops its action
    }
})

Expanding on Naftali aka Neal's answer, here's how you'd do it with vanilla JS and both start and stop Tab behavior buttons:

let stopTabFunction = function(e) {
  if (e.keyCode == 9) {
    e.preventDefault();
  }
};
document.getElementById('stopTabButton').onclick = function() {
  document.addEventListener('keydown', stopTabFunction);
};
document.getElementById('resumeTabButton').onclick = function() {
  document.removeEventListener('keydown', stopTabFunction);
};

<input type="text"/>
<input type="text"/>
<input type="text"/>
<input type="text"/>
<input type="text"/>
<input type="text"/>
<br/><br/>
<input type="button" id="stopTabButton" value="Stop Tab!"/>
<input type="button" id="resumeTabButton" value="Resume Tab!"/>

Note that this also works for Shift + Tab (reverse direction).

JSFiddle

However, in my case, I wanted slightly different behavior: I wanted to basically lock down Tab focus to a single div. To do this, I placed a div before and after it, gave them both tabindex="0" (document-defined tab order on the div's themselves), to make the outer edges of the div focusable, like so:

<div id="beforeMyDiv"></div>
<div id="myDiv">
  <!-- Only want Tab indexing to occur in here! -->
</div>
<div id="afterMyDiv"></div>

Then, I changed the function from earlier to this:

//Get the div's into variables etc.
//...

let forceTabFocusFunction = function (e) {
    if (e.keyCode == 9) {
        //Force focus onto the div.
        if (!myDiv.contains(document.activeElement)) {
            if (e.shiftKey) {
                afterMyDiv.focus();
            } else {
                beforeMyDiv.focus();
            }
        }
    }
};

That did the trick nicely.

You can do it like this:

$(":input, a").attr("tabindex", "-1");

That will disable getting focus with tab in all links and form elements.

Hope this helps