Doom cheats are typed straight into the running game — there is no console and no menu to open. You simply type the code on the keyboard while running through a level, and a confirmation message appears at the top of the screen after the last letter. If you are playing in the browser, click inside the game window first so the keyboard talks to the game and not to the page. This is a map set or modification built on top of Doom II. The engine is unchanged, so every original Doom code works.
CodeEffect
IDDQDGod mode — neither monsters nor the environment can take your health
IDKFAAll weapons, full ammo, armour and every key in the level
IDFAAll weapons, ammo and armour, but no keys
IDCLIPWalk through walls (noclip) — the Doom II and later variant
IDSPISPOPDWalk through walls (noclip) — works only in Doom 1 and Ultimate Doom
IDBEHOLD + letterPower-ups: S = berserk strength, I = partial invisibility, V = invulnerability, A = full area map, L = light amplification visor, R = radiation suit
IDCLEV##Warp instantly to a chosen level (IDCLEV12 = episode 1, map 2; in Doom II IDCLEV20 = MAP20)
IDDTReveals the automap; typed twice it also shows monsters and items (only works with the automap open)
IDMYPOSDisplays your current coordinates and viewing angle
IDMUS##Plays the music track from a chosen level (e.g. IDMUS05)
IDCHOPPERSGrants the chainsaw plus temporary invulnerability, with the message “Doesn’t suck – GM”
Doom cheats are not hidden in a menu, they are simply key sequences — which is why you can trigger one by accident. They are also worth saving for a second playthrough: IDDQD turns Doom into a shooting gallery and switches off the rule the whole game is built on, namely managing ammo and health.