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Jquery can't trigger keyup event

I'm using jquery in a web app. I have a page which is listening for keypresses like this:
document.addEventListener('keyup', function (event) {
event.preventDefault();
var key = event.keyCode;
console.log("pressed key "+key);
});
That works in terms of me pressing keys - it outputs the key code to the console.
I now want to simulate a key press with a different function, which i'm doing like this:
var keycode = 27;
$(document).trigger(
jQuery.Event( 'keyup', { keyCode: keycode, which: keycode } )
);
But, nothing happens - specifically, i don't see the console log from the first function.
Can anyone see my mistake?
EDIT: it works if I change the first function to use jQuery .on() rather than native js .addEventListener, but i'd still like to understand why. Is document a different entity to $(document)?

To address the edit, when you select an element using the jQuery $() notation it selects it as a jQuery object with lots of jQuery methods available. Using the vanilla JavaScript document selector will create a DOM element, but not a jQuery object. This page has more info on alternative ways to make jQuery objects when needed: http://jqfundamentals.com/chapter/jquery-basics

Jquery have a function that you can bind at your eventlistener .keyup().
You can read the full documentation here: https://api.jquery.com/keyup/

KeyboardEvent object, determine if it's keyUp or down

Having a single KeyboardEvent object, is it possible to know if it corresponds to a KeyDown or KeyUp event?
(I know I can keep somewhere the event KeyUp or Down that triggers it but I am looking for a way to assess that independently of the trigger)

Look at the type property on the object you've got:
let b = document.getElementById('b');
b.onkeydown = b.onkeyup = function(e){ console.log(e.type); };
<button id="b">Button</button>

Event 'e' in Javascript Function's Parameter [duplicate]

I am new to JavaScript/jQuery and I've been learning how to make functions. A lot of functions have cropped up with (e) in brackets. Let me show you what I mean:
$(this).click(function(e) {
// does something
});
It always appears that the function doesn't even use the value of (e), so why is it there so often?

e is the short var reference for event object which will be passed to event handlers.
The event object essentially has lot of interesting methods and properties that can be used in the event handlers.
In the example you have posted is a click handler which is a MouseEvent
$(<element selector>).click(function(e) {
// does something
alert(e.type); //will return you click
}
DEMO - Mouse Events DEMO uses e.which and e.type
Some useful references:
http://api.jquery.com/category/events/
http://www.quirksmode.org/js/events_properties.html
http://www.javascriptkit.com/jsref/event.shtml
http://www.quirksmode.org/dom/events/index.html
http://www.w3.org/TR/DOM-Level-3-Events/#event-types-list

DISCLAIMER: This is a very late response to this particular post but as I've been reading through various responses to this question, it struck me that most of the answers use terminology that can only be understood by experienced coders. This answer is an attempt to address the original question with a novice audience in mind.
Intro
The little '(e)' thing is actually part of broader scope of something in Javascript called an event handling function. Every event handling function receives an event object. For the purpose of this discussion, think of an object as a "thing" that holds a bunch of properties (variables) and methods (functions), much like objects in other languages. The handle, the 'e' inside the little (e) thing, is like a variable that allows you to interact with the object (and I use the term variable VERY loosely).
Consider the following jQuery examples:
$("#someLink").on("click", function(e){ // My preferred method
e.preventDefault();
});
$("#someLink").click(function(e){ // Some use this method too
e.preventDefault();
});
Explanation
"#someLink" is your element selector (which HTML tag will trigger this).
"click" is an event (when the selected element is clicked).
"function(e)" is the event handling function (on event, object is created).
"e" is the object handler (object is made accessible).
"preventDefault()" is a method (function) provided by the object.
What's happening?
When a user clicks on the element with the id "#someLink" (probably an anchor tag), call an anonymous function, "function(e)", and assign the resulting object to a handler, "e". Now take that handler and call one of its methods, "e.preventDefault()", which should prevent the browser from performing the default action for that element.
Note: The handle can pretty much be named anything you want (i.e. 'function(billybob)'). The 'e' stands for 'event', which seems to be pretty standard for this type of function.
Although 'e.preventDefault()' is probably the most common use of the event handler, the object itself contains many properties and methods that can be accessed via the event handler.
Some really good information on this topic can be found at jQuery's learning site, http://learn.jquery.com. Pay special attention to the Using jQuery Core and Events sections.

e doesn't have any special meaning. It's just a convention to use e as function parameter name when the parameter is event.
It can be
$(this).click(function(loremipsumdolorsitamet) {
// does something
}
as well.

In that example, e is just a parameter for that function, but it's the event object that gets passed in through it.

The e argument is short for the event object. For example, you might want to create code for anchors that cancels the default action. To do this you would write something like:
$('a').click(function(e) {
e.preventDefault();
}
This means when an <a> tag is clicked, prevent the default action of the click event.
While you may see it often, it's not something you have to use within the function even though you have specified it as an argument.

In jQuery e short for event, the current event object. It's usually passed as a parameter for the event function to be fired.
Demo: jQuery Events
In the demo I used e
$("img").on("click dblclick mouseover mouseout",function(e){
$("h1").html("Event: " + e.type);
});
I may as well have used event
$("img").on("click dblclick mouseover mouseout",function(event){
$("h1").html("Event: " + event.type);
});
Same thing!
Programmers are lazy we use a lot of shorthand, partly it decreases our work, partly is helps with readability. Understanding that will help you understand the mentality of writing code.

Today I just wrote a post about "Why do we use the letters like “e” in e.preventDefault()?" and I think my answer will make some sense...
At first,let us see the syntax of addEventListener
Normally it will be:
target.addEventListener(type, listener[, useCapture]);
And the definition of the parameters of addEventlistener are:
type ：A string representing the event type to listen out for.
listener ：The object which receives a notification (an object that implements the Event interface) when an event of the specified type
occurs. This must be an object implementing the EventListener
interface, or a JavaScript function.
(From MDN)
But I think there is one thing should be remarked:
When you use Javascript function as the listener, the object that implements the Event interface(object event) will be automatically assigned to the "first parameter" of the function.So,if you use function(e) ,the object will be assigned to "e" because "e" is the only parameter of the function(definitly the first one !),then you can use e.preventDefault to prevent something....
let us try the example as below:
<p>Please click on the checkbox control.</p>
<form>
<label for="id-checkbox">Checkbox</label>
<input type="checkbox" id="id-checkbox"/>
</div>
</form>
<script>
document.querySelector("#id-checkbox").addEventListener("click", function(e,v){
//var e=3;
var v=5;
var t=e+v;
console.log(t);
e.preventDefault();
}, false);
</script>
the result will be : [object MouseEvent]5 and you will prevent the click event.
but if you remove the comment sign like :
<script>
document.querySelector("#id-checkbox").addEventListener("click", function(e,v){
var e=3;
var v=5;
var t=e+v;
console.log(t);
e.preventDefault();
}, false);
</script>
you will get : 8 and an error:"Uncaught TypeError: e.preventDefault is not a function
at HTMLInputElement. (VM409:69)".
Certainly,the click event will not be prevented this time.Because the "e" was defined again in the function.
However,if you change the code to:
<script>
document.querySelector("#id-checkbox").addEventListener("click", function(e,v){
var e=3;
var v=5;
var t=e+v;
console.log(t);
event.preventDefault();
}, false);
</script>
every thing will work propertly again...you will get 8 and the click event be prevented...
Therefore, "e" is just a parameter of your function and you need an "e" in you function() to receive the "event object" then perform e.preventDefault(). This is also the reason why you can change the "e" to any words that is not reserved by js.

It's a reference to the current event object

this will be my first stackoverflow help but I am confident that my answer will help anyone reading this.
Basically, e is just an object containing information about the EVENT which has just occured.
if it is 'click', then the object will contain about the click,
if it is 'submit', then the object will contain about the submit,
and they are typically found in addEventListener.
clickMe.addEventListener('click', e => {
console.log(e)
}
meaning, whenever I 'click' the button, it will console.log the INFOMRATION about the event that happened which is I did is to 'click' it, this will print information about the click event.
e is very useful because you can access and use the event to your very own project such as printing the location of x value... like
clickMe.addEventListener('click', e => {
console.log(e.clientX)
}
then it will print the location where you 'click' that event.. mine it returns 32
if you prefer video, please watch this
https://www.youtube.com/watch?v=_BVkOvpyRI0
video is not mine
upvoting me will truly help me since I am a student and looking for opportunity to help here. Love lots!

$(this).click(function(e) {
// does something
});
In reference to the above code
$(this) is the element which as some variable.
click is the event that needs to be performed.
the parameter e is automatically passed from js to your function which holds the value of $(this) value and can be used further in your code to do some operation.

getModifierState() returned as an error

I am trying to debug a function using Firebug, for the JavaScript getModifierState() method seems to be not working. Here is the beginning of the function:
function kbdEq () {
$(document).on ('keypress', function (e) {
e.preventDefault();
debugger;
var x = e.charCode || e.keyCode;
// etc.
Jumping into Firebug is straightforward, but trying to test
e.getModifierState('Alt')
(or any other keyboard event as a parameter, with or without quotes) results in this message from Firebug:
e.getModifierState is not a function
I am using Firefox 44.0.2.
What mistake am I (presumably) making here?

As you are using jQuery, note that the event parameter (e in your case) is a jQuery specific object wrapping the actual event object.
To access the original event it has a property originalEvent. So, to access the modifier state you have to write this:
e.originalEvent.getModifierState("Alt");
Note that the jQuery event object has some simpler way to check whether the Alt, Ctrl, Shift or Meta key is pressed. This can be done via e.altKey, e.ctrlKey, e.shiftKey and e.metaKey, respectively.

JS function when keyboard key is pressed?

Is there a way to run a JavaScript function when a key is pressed and released?
For example, how would I run a function example() when the T key is pressed? I've seen examples of these before, but they were long and messy, and I couldn't get them to work. Would something like this just go in a <script> in the <head>?

Part 1: Where to put the scriptblock?
To capture over the entire page, like as a page-help-function (maybe you want to capture F1?) then you would put your script block in the <head> tag, inside a script. But if you want to capture a DOM element, then you have to execute the code after the DOM element occurs (because the script is interpreted as it's found, if the DOM element doesn't exist yet, the selector engine can't find it. If this doesn't make sense say something, and an article shall be found).
But here's something for you to consider: Good javascript programmer mentors today recommend all javascript be loaded at the end of the page. The only things you might want to load at the head of the document are libraries like jQuery, because those are widely cached, especially if you're using a CDN version of jQuery, as that generally tends to not impact load times.
So that answers the question of "where do I put the codeblock, in the <head>?": No. At the end.
Now, as to how to actually capture the keystroke, let's do it in three parts:
Part 2: Capturing all keyboard events on the window:
<html>
<head>
<title>blah blah</title>
<meta "woot, yay StackOverflow!">
</head>
<body>
<h1>all the headers</h1>
<div>all the divs</div>
<footer>All the ... ... footers?</footer>
<script>
/* the last example replaces this one */
function keyListener(event){
//whatever we want to do goes in this block
event = event || window.event; //capture the event, and ensure we have an event
var key = event.key || event.which || event.keyCode; //find the key that was pressed
//MDN is better at this: https://developer.mozilla.org/en-US/docs/DOM/event.which
if(key===84){ //this is for 'T'
doThing();
}
}
/* the last example replace this one */
var el = window; //we identify the element we want to target a listener on
//remember IE can't capture on the window before IE9 on keypress.
var eventName = 'keypress'; //know which one you want, this page helps you figure that out: http://www.quirksmode.org/dom/events/keys.html
//and here's another good reference page: http://unixpapa.com/js/key.html
//because you are looking to capture for things that produce a character
//you want the keypress event.
//we are looking to bind for IE or non-IE,
//so we have to test if .addEventListener is supported,
//and if not, assume we are on IE.
//If neither exists, you're screwed, but we didn't cover that in the else case.
if (el.addEventListener) {
el.addEventListener('click', keyListener, false);
} else if (el.attachEvent) {
el.attachEvent('on'+eventName, keyListener);
}
//and at this point you're done with registering the function, happy monitoring
</script>
</body>
</html>
Part 3: Capturing all keyboard events on a specific element
This line: var el = window; //we identify the element we want to target a listener on might also be var el = document.getElementByTagName('input'); or some other document selector. The example still works the same that way.
Part 4: An 'elegant' solution
var KeypressFunctions = [];
KeypressFunctions['T'.charCodeAt(0)] = function _keypressT() {
//do something specific for T
}
KeypressFunctions['t'.charCodeAt(0)] = function _keypresst() {
//do something specific for t
}
//you get the idea here
function keyListener(event){
//whatever we want to do goes in this block
event = event || window.event; //capture the event, and ensure we have an event
var key = event.key || event.which || event.keyCode; //find the key that was pressed
//MDN is better at this: https://developer.mozilla.org/en-US/docs/DOM/event.which
KeypressFunctions[key].call(); //if there's a defined function, run it, otherwise don't
//I also used .call() so you could supply some parameters if you wanted, or bind it to a specific element, but that's up to you.
}
What does all this do?
The KeypressFunctions is an array, that we can populate with various values but have them be somewhat human readable. Each index into the array is done as 'T'.charCodeAt(0) which gives the character code (event.which || event.keyCode look familiar?) for the index position into the array that we're adding a function for. So in this case our array only has two defined index-values, 84 (T) and 116 (t). We could've written that as KeypressFunctions[84] = function ... but that's less human-readable, at the expense of the human-readable is longer. Always write code for yourself first, the machine is often smarter than you give it credit for. Don't try and beat it with logic, but don't code excessive if-else blocks when you can be slightly elegant.
gah! I forgot to explain something else!
The reason for the _keypressT and _keypresst is so that when this gets called as an anonymous function, or as part of a callstack (it will, one day) then you can identify the function. This is a really handy practice to get into the habit of, making sure that all potentially anonymous functions still get "named" even though they have a proper name elsewhere. Once again, good javascript mentors suggest things that help folks ;-).
gah! I forgot to explain something else!
Notice you could just as easily do:
function doThing() //some pre-defined function before our code
var KeypressFunctions = [];
KeypressFunctions['T'.charCodeAt(0)] = doThing
KeypressFunctions['t'.charCodeAt(0)] = doThing
and then for either T or t, the doThing function is run. Notice that we just passed the name of the function and we didn't try to run the function by doThing() (this is a HUGE difference and a big hint if you're going to do this sort of thing)
I can't believe I forgot this one!
Part 5: jQuery:
Because the emphasis today is on jQuery, here's a block you can put anywhere in your app after the jQuery library has loaded (head, body, footer, whatever):
<script>
function doTheThingsOnKeypress(event){
//do things here! We've covered this before, but this time it's simplified
KeypressFunctions[event.which].call();
}
$(document).on('keypress','selector',doTheThingsOnKeypress);
// you could even pass arbitrary data to the keypress handler, if you wanted:
$(document).on('keypress','selector',{/* arbitrary object here! */},doTheThingsOnKeypress);
//this object is accessible through the event as data: event.data
</script>
If you're going to use the KeypressFunctions as from before, ensure they are actually defined before this.

Use the onkeydown event and the keyCode property (where T code is 84):
document.onkeydown = function(e){
e = e || window.event;
var key = e.which || e.keyCode;
if(key===84){
example();
}
}
I just suggest you to use addEventListener/attachEvent methods instead of the onkeydown property
EDIT:
As T.J. Crowder requested, here's the addEventListener/attachEvent usage, with a compatibility check:
var addEvent = document.addEventListener ? function(target,type,action){
if(target){
target.addEventListener(type,action,false);
}
} : function(target,type,action){
if(target){
target.attachEvent('on' + type,action,false);
}
}
addEvent(document,'keydown',function(e){
e = e || window.event;
var key = e.which || e.keyCode;
if(key===84){
example();
}
});
And for a list of the key codes, check this page

Bind a keyup/down/press event handler to the document object.
Check which key was pressed (by looking at key or keyCode on the event object
Call the function if it matches the key want you want
It doesn't matter where the script runs, the document object is (effectively) always available.