Snippet to detect Konami Code (and then do something)

konamiCode.js

/******************************************************************************
* This script detects the Konami code 
* (Full Konami Code obtained from: http://en.wikipedia.org/wiki/Konami_Code)
* If the Konami code is detected the function called "startUpKonami" is invoked.
* You can do whatever you want on "startUpKonami". Right now this method just:
*	- Detachs the Konami code detection (so no further detections can happen).
*	- Invokes an external function.
*
* Note: In case the user presses keys like "shift", "caps lock", "Ctrl"...
*		the code will have to be reentered from scratch 
*		as after detecting such keys the detected Konami code is cleaned, 
*		so a new Konami code detection starts again.
*
* Note2: The Konami Code sequence is the following (has to be entered in that order):
*		up,up,down,down,left,right,left,right,B,A
*
* Note3: The Konami Code detection doesn't care about upper case letters, so it's the same
*		to enter "up,up,down,down,left,right,left,right,b,a" or
*		to enter "up,up,down,down,left,right,left,right,B,A"
*
* Licence: WTFPL (http://es.wikipedia.org/wiki/WTFPL)
*
*
* Developed by @nestoralvaro
*
*******************************************************************************/
(function() {
    "use strict";
    // Some key codes that are used
    var up = 38,
	    down = 40,
	    left = 37,
	    right = 39,
	    A = 65,
	    B = 66;
    // Full Konami Code obtained from: http://en.wikipedia.org/wiki/Konami_Code
    var	konamiCode = [up,up,down,down,left,right,left,right,B,A];
    // Deteted sequence. Empty by default
    var konamiDetected = [];

    // Attachs the function on an element (for a certain event)
    function attachCustomEvent(el, eventName, desiredFunction) {
	    if (el.addEventListener) {
		    el.addEventListener(eventName,desiredFunction,false);
	    // Old IE
	    } else {
		    el.attachEvent('on' + eventName,desiredFunction);
	    }
    }

    // Detachs the function on an element (for a certain event)
    function detachCustomEvent(el, eventName, desiredFunction) {
	    if (el.removeEventListener) {
		    el.removeEventListener(eventName,desiredFunction,false);
	    // Old IE
	    } else {
		    el.detachEvent('on' + eventName,desiredFunction);
	    }
    }

    // Function that is invoked after detecting the Konami Code
    function startUpKonami() {
	    // Prevent further detection (When removing this line the Konami code can be entered multiple times)
	    detachCustomEvent(document,"keydown",isKonamiKey);
	    konamiIsDetected();
    }

    // Function to detect whether the pressed key is part of the Konami Code
    function isKonamiKey(e) {
	    var evt = e || window.event;
        var key = evt.keyCode ? evt.keyCode : evt.which;
	    // Set to true before checking everything    
	    var codeOk = true;
        // Push the key
        konamiDetected.push(key);
        // Check if the key is valid or not
        if (konamiDetected.length < konamiCode.length) {
		    // Check that the values are Ok so far. If not clear the array
		    for (var i = 0, max = konamiDetected.length; i < max ; i++) {
        		if(konamiDetected[i] !== konamiCode[i]) {
	        		codeOk = false;
        		}
        	}
        	if (!codeOk) {
        		// Clean the array
        		konamiDetected = [];
        		// Push the just detected value inside the array
        		konamiDetected.push(key);
        	}
        } else if (konamiDetected.length === konamiCode.length) {
        	for (var j = 0, max = konamiDetected.length; j < max ; j++) {
        		if(konamiDetected[j] !== konamiCode[j]) {
	        		codeOk = false;
        		}
        	}
        	// Clean the array
        	konamiDetected = [];
        	if (codeOk) {
	        	startUpKonami();
        	}
        // This should never happen, but if it happens we clean the array
        } else {
	        konamiDetected = [];
        }
        // After everything has been checked show the resulting array after pressing such key
	    // console.log(konamiDetected);
    }

    // Attach the event detection to the whole document
    attachCustomEvent(document,"keydown",isKonamiKey);
})();

// Function that is invoked after the konami code has been entered
function konamiIsDetected() {
	alert("yay, do something cool here!");
}

An alternative: https://www.sitepoint.com/jquery-konami-code-listener/

if ( window.addEventListener ) {
    var kkeys = [], konami = "38,38,40,40,37,39,37,39,66,65";
    window.addEventListener("keydown", function(e){
        kkeys.push( e.keyCode );
        if ( kkeys.toString().indexOf( konami ) >= 0 )
            // User entered Konami Code, do something cool!!!
            kkeys = [];
    }, true);
}

function konami(callback) {
    let kkeys = [];
    // up,up,down,down,left,right,left,right,B,A
    const konami = '38,38,40,40,37,39,37,39,66,65';
    return event => {
        kkeys.push(event.keyCode);
        if (kkeys.toString().indexOf(konami) >= 0) {
            callback(event);
            kkeys = [];
        }
    };
}
const handler = konami(() => {
    console.log('konami 1');
});
window.addEventListener('keydown', handler);
window.addEventListener('keydown', konami(event => {
    console.log('konami 2');
}));

@jRimbault thanks much shorter!