Here is the source code.
You start filling the holes at the bottom row with pegs, and each time
you have completed a guess (i.e. filled all four holes) the computer
will respond by giving you one peg for each correct color - black if your
guess was also in the right place, otherwise white. (But you will not
be informed about which colors were correct. That part you will have
to figure out for yourself.)
If you haven't managed to guess the sequence by your 8th guess, the
computer will reveal it to you and the game ends.
In case you wish to start over, just press 'End' to discontinue the game.
First select which colors you want to play with. (The default set contains
blue, green, red and yellow. That's moderately difficult. For
obvious reasons you are not allowed to select fewer than two.
Clicking on a color in the
field on the right will toggle the circle around it on/off, indicating
whether it will be part of the set the computer draws from. Then press
'Start' to begin.
Disclaimer: the name "Mastermind", the toy and distinctive likenesess
thereof are the properties of Hasbro Inc.
This applet is an unofficial, not-for-profit version and neither it nor this
site is endorsed by Hasbro in any way.