event.key === "ArrowRight"...

More recent and much cleaner: use event.key. No more arbitrary number codes! If you are transpiling or know your users are all on modern browsers, use this!

node.addEventListener('keydown', (event) => {
    const key = event.key; // "ArrowRight", "ArrowLeft", "ArrowUp", or "ArrowDown"
});

Verbose Handling:

switch (event.key) {
    case "ArrowLeft":
        // Left pressed
        break;
    case "ArrowRight":
        // Right pressed
        break;
    case "ArrowUp":
        // Up pressed
        break;
    case "ArrowDown":
        // Down pressed
        break;
}

Modern Switch Handling:

const callback = {
    "ArrowLeft"  : leftHandler,
    "ArrowRight" : rightHandler,
    "ArrowUp"    : upHandler,
    "ArrowDown"  : downHandler,
}[event.key]
callback?.()

NOTE: The old properties (.keyCode and .which) are Deprecated.

"w", "a", "s", "d" for direction, use event.code

To support users who are using non-qwerty/English keyboard layouts, you should instead use event.code. This will preserve physical key location, even if resulting character changes.

event.key would be , on Dvorak and z on Azerty, making your game unplayable.

const {code} = event
if (code === "KeyW") // KeyA, KeyS, KeyD

Optimally, you also allow key remapping, which benefits the player regardless of their situation.

P.S. event.code is the same for arrows

key Mozilla Docs

code Mozilla Docs

Supported Browsers

On key up and down call function. There are different codes for each key.

document.onkeydown = checkKey;

function checkKey(e) {

    e = e || window.event;

    if (e.keyCode == 38) {
        // up arrow
    }
    else if (e.keyCode == 40) {
        // down arrow
    }
    else if (e.keyCode == 37) {
       // left arrow
    }
    else if (e.keyCode == 39) {
       // right arrow
    }

}

Possibly the tersest formulation:

document.onkeydown = function(e) {
    switch (e.keyCode) {
        case 37:
            alert('left');
            break;
        case 38:
            alert('up');
            break;
        case 39:
            alert('right');
            break;
        case 40:
            alert('down');
            break;
    }
};

Demo (thanks to user Angus Grant): http://jsfiddle.net/angusgrant/E3tE6/

This should work cross-browser. Leave a comment if there is a browser where it does not work.

There are other ways to get the key code (e.which, e.charCode, and window.event instead of e), but they should not be necessary. You can try most of them out at http://www.asquare.net/javascript/tests/KeyCode.html. Note that event.keycode does not work with onkeypress in Firefox, but it does work with onkeydown.

Use keydown, not keypress for non-printable keys such as arrow keys:

function checkKey(e) {
    e = e || window.event;
    alert(e.keyCode);
}

document.onkeydown = checkKey;

The best JavaScript key event reference I've found (beating the pants off quirksmode, for example) is here: http://unixpapa.com/js/key.html

Modern answer since keyCode is now deprecated in favor of key:

document.onkeydown = function (e) {
    switch (e.key) {
        case 'ArrowUp':
            // up arrow
            break;
        case 'ArrowDown':
            // down arrow
            break;
        case 'ArrowLeft':
            // left arrow
            break;
        case 'ArrowRight':
            // right arrow
    }
};

I believe the most recent method would be:

document.addEventListener("keydown", function(event) {
  event.preventDefault();
  const key = event.key; // "ArrowRight", "ArrowLeft", "ArrowUp", or "ArrowDown"
  switch (key) { // change to event.key to key to use the above variable
    case "ArrowLeft":
      // Left pressed
      <do something>
      break;
    case "ArrowRight":
      // Right pressed
      <do something>
      break;
    case "ArrowUp":
      // Up pressed
      <do something>
      break;
    case "ArrowDown":
      // Down pressed
      <do something>
      break;
  }
});

This assumes the developer wants the code to be active anywhere on the page and the client should ignore any other key presses. Eliminate the event.preventDefault(); line if keypresses, including those caught by this handler should still be active.

function checkArrowKeys(e){
    var arrs= ['left', 'up', 'right', 'down'], 
    key= window.event? event.keyCode: e.keyCode;
    if(key && key>36 && key<41) alert(arrs[key-37]);
}
document.onkeydown= checkArrowKeys;

Here's an example implementation:

var targetElement = $0 || document.body;

function getArrowKeyDirection (keyCode) {
  return {
    37: 'left',
    39: 'right',
    38: 'up',
    40: 'down'
  }[keyCode];
}

function isArrowKey (keyCode) {
  return !!getArrowKeyDirection(keyCode);
}

targetElement.addEventListener('keydown', function (event) {
  var direction,
      keyCode = event.keyCode;

  if (isArrowKey(keyCode)) {
    direction = getArrowKeyDirection(keyCode);

    console.log(direction);
  }
});

Here's how I did it:

var leftKey = 37, upKey = 38, rightKey = 39, downKey = 40;
var keystate;
document.addEventListener("keydown", function (e) {
    keystate[e.keyCode] = true;
});
document.addEventListener("keyup", function (e) {
    delete keystate[e.keyCode];
});

if (keystate[leftKey]) {
//code to be executed when left arrow key is pushed.
}
if (keystate[upKey]) {
//code to be executed when up arrow key is pushed.
}
if (keystate[rightKey]) {
//code to be executed when right arrow key is pushed.
}
if (keystate[downKey]) {
//code to be executed when down arrow key is pushed.
}

Arrow Keys are triggered on keyup

$(document).on("keyup", "body", function(e) {
 if (e.keyCode == 38) {
    // up arrow
    console.log("up arrow")
  }
  if (e.keyCode == 40) {
      // down arrow
      console.log("down arrow")
  }
  if (e.keyCode == 37) {
    // left arrow
    console.log("lefy arrow")
  }
  if (e.keyCode == 39) {
    // right arrow
    console.log("right arrow")
  }
})

onkeydown allows ctrl, alt, shits

onkeyup allows tab, up arrows, down arrows, left arrows, down arrows

I've been able to trap them with jQuery:

$(document).keypress(function (eventObject) {
    alert(eventObject.keyCode);
});

An example: http://jsfiddle.net/AjKjU/

That is the working code for chrome and firefox

<html>
<head>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.1/jquery.min.js"></script>

<script type="text/javascript">

    function leftArrowPressed() {
      alert("leftArrowPressed" );
      window.location = prevUrl  
    }

    function rightArrowPressed() {
      alert("rightArrowPressed" );
      window.location = nextUrl  
    }
    function topArrowPressed() {
      alert("topArrowPressed" );
      window.location = prevUrl  
    }

    function downArrowPressed() {
      alert("downArrowPressed" );
      window.location = nextUrl  
    }

        document.onkeydown = function(evt) {
                        var nextPage = $("#next_page_link")
                        var prevPage = $("#previous_page_link")
                        nextUrl = nextPage.attr("href")
                        prevUrl = prevPage.attr("href")
        evt = evt || window.event;
        switch (evt.keyCode) {
                case 37:
                leftArrowPressed(nextUrl);
                break;

                case 38:
                topArrowPressed(nextUrl);
                break;

                 case 39:
                rightArrowPressed(prevUrl);
                break;

                case 40:
                downArrowPressed(prevUrl);
                break;

        }
    };


</script>
</head>
<body>
<p>
<a id="previous_page_link" href="http://www.latest-tutorial.com">Latest Tutorials</a> 
<a id="next_page_link" href="http://www.zeeshanakhter.com">Zeeshan Akhter</a>
 </p>
</body>
</html>

I was also looking for this answer until I came across this post.

I've found another solution to know the keycode of the different keys, courtesy to my problem. I just wanted to share my solution.

Just use keyup/keydown event to write the value in the console/alert the same using event.keyCode. like-

console.log(event.keyCode) 

// or

alert(event.keyCode)

- rupam

That's shorter.

function IsArrows (e) { return (e.keyCode >= 37 && e.keyCode <= 40); }

This library rocks! https://craig.is/killing/mice

Mousetrap.bind('up up down down left right left right b a enter', function() {
    highlight([21, 22, 23]);
});

You need to press the sequence a bit fast to highlight the code in that page though.

With key and ES6.

This gives you a separate function for each arrow key without using switch and also works with the 2,4,6,8 keys in the numpad when NumLock is on.

const element = document.querySelector("textarea"),
  ArrowRight = k => {
    console.log(k);
  },
  ArrowLeft = k => {
    console.log(k);
  },
  ArrowUp = k => {
    console.log(k);
  },
  ArrowDown = k => {
    console.log(k);
  },
  handler = {
    ArrowRight,
    ArrowLeft,
    ArrowUp,
    ArrowDown
  };

element.addEventListener("keydown", e => {
  const k = e.key;

  if (handler.hasOwnProperty(k)) {
    handler[k](k);
  }
});

<p>Click the textarea then try the arrows</p>
<textarea></textarea>

In the off chance you're still developing with desktops in mind in 2023:

KeyboardEvent: keyCode property has been deprecated

https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/keyCode

Use the key property instead

// run snippet and click a key to see which key you clicked:

document.addEventListener('keydown', (event) => { 
 alert(`You clicked ${event.key}`);
});

Re answers that you need keydown not keypress.

Assuming you want to move something continuously while the key is pressed, I find that keydown works for all browsers except Opera. For Opera, keydown only triggers on 1st press. To accommodate Opera use:

document.onkeydown = checkKey;
document.onkeypress = checkKey;
function checkKey(e)
{ etc etc

If you use jquery then you can also do like this,

 $(document).on("keydown", '.class_name', function (event) {
    if (event.keyCode == 37) {
        console.log('left arrow pressed');
    }
    if (event.keyCode == 38) {
        console.log('up arrow pressed');
    }
    if (event.keyCode == 39) {
        console.log('right arrow pressed');
    }
    if (event.keyCode == 40) {
        console.log('down arrow pressed');
    }
 });

control the Key codes %=37 and &=38... and only arrow keys left=37 up=38

function IsArrows (e) {
   return ( !evt.shiftKey && (e.keyCode >= 37 && e.keyCode <= 40)); 
}

If you want to detect arrow keypresses but not need specific in Javascript

function checkKey(e) {
   if (e.keyCode !== 38 || e.keyCode !== 40 || e.keyCode !== 37 || e.keyCode !== 39){
    // do something
   };
}