September 5, 2018

Konami Code - Easter egg code in JavaScript

The Konami Code (コナミコマンド) is a cheat code that appears in many Konami video games. The code is also present as an Easter egg on a number of websites.

UP, UP, DOWN, DOWN, LEFT, RIGHT, LEFT, RIGHT, B, A

(try it)

The main concerns when implementing such an Easter egg into a website are:

Don't run the Easter egg unintentionally
Don't break the site

What we need to implement the Konami Code is an event listener that captures the key events and checks if the combination represents the specified code.

For this purpose I used an array. I chose an array over a concated string approach for ease of use and code readability. The first iteration of the code just added to the array. And added, and added, ... This is not good, as it fills the memory after a while.

65
65,66
65,66,67
65,66,67,49
65,66,67,49,50
65,66,67,49,50,51
65,66,67,49,50,51,38
65,66,67,49,50,51,38,38
65,66,67,49,50,51,38,38,40
65,66,67,49,50,51,38,38,40,40
65,66,67,49,50,51,38,38,40,40,37
65,66,67,49,50,51,38,38,40,40,37,39
65,66,67,49,50,51,38,38,40,40,37,39,37
65,66,67,49,50,51,38,38,40,40,37,39,37,39
65,66,67,49,50,51,38,38,40,40,37,39,37,39,66
65,66,67,49,50,51,38,38,40,40,37,39,37,39,66,65

The next iteration fixed this bug by always removing the first element of the array which captures all the key events.

65
65,66
65,66,67
65,66,67,49
65,66,67,49,50
65,66,67,49,50,51
65,66,67,49,50,51,38
65,66,67,49,50,51,38,38
65,66,67,49,50,51,38,38,40
65,66,67,49,50,51,38,38,40,40
66,67,49,50,51,38,38,40,40,37
67,49,50,51,38,38,40,40,37,39
49,50,51,38,38,40,40,37,39,37
50,51,38,38,40,40,37,39,37,39
51,38,38,40,40,37,39,37,39,66
38,38,40,40,37,39,37,39,66,65

A further improvement unbinds the keydown event listener once the Konami Code was fired.

The finished code:

$(document).ready(function(){
    var konamiUserKeyCodeStorage = [];
    var konamiCode = [38,38,40,40,37,39,37,39,66,65];
    $(document).keydown(function(e){
        konamiUserKeyCodeStorage.push(e.keyCode); //log the entered key code
        if(konamiUserKeyCodeStorage.toString() == konamiCode.toString()){
            $(document).unbind('keydown',arguments.callee); //destruct
            alert('Easter egg');
        }
        if(konamiUserKeyCodeStorage.length >= konamiCode.length){
            konamiUserKeyCodeStorage.shift(); //prevent filling the storage
        }
    });
});

Dependencies: jQuery

Tags: javascript, easter egg, konami

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About the author

human, software engineer, tech enthusiast, security researcher

E-Mail: blog@cipher.digital