Being the Trope Namer, the Joker had this from day one. He was originally conceived as a one-off villain (co-creator Bill Finger worried that Batman and law enforcement would look pretty incompetent if the villains kept returning), and Batman didn't even have his no-kill code back in those early days, but the Joker proved too good a villain to waste by killing after one issue so a last minute edit had him survive. He's been laughing at readers ever since.
There was even a comic book from the 1940s where the Joker got the death penalty and was brought Back from the Dead, only to be conveniently ignored later on when he couldn't be punished again for the same crimes.
Jason Todd: Why? I'm not talking about killing Penguin or Scarecrow or Dent. I'm talking about him. Just him.
Batman knows he's always this close to permanently snapping; he probably thinks Jason could have a point on anyone else, but even if he wanted to, he just couldn't go back after killing intentionally a first time. Besides, for the victims of Two-Face or Scarecrow and their loved ones, it's dubious the Joker's superior bodycount would make much of a difference on the subject.
Death of the Family has a new reason: Batman confides to Alfred that the main reason he refuses to kill Joker is because he sincerely believes killing Joker wouldn't make things any better. Gotham would just send someone worse, or bring Joker back from the dead, or something. To Bruce, the Joker is just one facet of the true Big Bad of his story: Gotham City itself. Still hasn't explained why the police have not killed the Joker the moment he resists arrest.
Batman: Give me one good reason why I shouldn't finish you off once and for all, right now! Joker: Merchandising. You can't afford to lose your best villain.
The Elseworld comic Kingdom Come's backstory in fact starts when a rising Superhero violates the Joker's own Joker Immunity. The Joker had just killed the entire staff of the Daily Planet - Lois Lane included. Superman apprehends him, but while in custody of the Metropolis Police, Magog shoots and kills the Joker as he's being taken in by the cops, in a scene that mirrors the shooting of Lee Harvey Oswald. When Magog is acquitted, and most civilians agree with Magog's move, Superman leaves in disgust. Magog's example is then used by all the new generation of superheroes as inspiration that they do not have to pull their punches. The Novelization explains that Lois Lane's Heroic Sacrifice (stalling The Joker til Supes arrived) became a Senseless Sacrifice thanks to Magog, and that is why Superman finally gave up.
Explicitly lampshaded in the Knightfall novelization, when Batman listed several of the times Joker should've died:
Would the world finally be rid of the Joker? No way to be sure. Batman had seen him survive explosions, gunfire, electrocution falling from aircraft, and yes, even plunging to the bottom of the Gotham Rivernote This is when Batman is in a damaged, soon-to-collapse tunnel under the Gotham River, looking for Joker and the Mayor, and Bruce has to decide whether to leave them to die and escape, or to keep pursuing.. What reason was there to believe the odds would finally catch up with him?
Surprisingly, the Joker is resistant to death in Batman: The Animated Series, as well. He can survive long falls and explosions that would kill just about anyone else. One would suspect that, like Team Rocket, the Joker is actually immortal, if he wasn't ironically one of the few characters to actually die in the show's continuity, although in Batman Beyond: Return of the Joker rather than shown in the show proper. Granted, Return of the Joker has Tim be transformed into a clone of the Joker through a microchip, so the Joker returns once again, but it's far from unexplained, and the resurrected Mr. J. is quickly destroyed himself with no reprieve. However, due to Batman Beyond existing in a floating timeline where it's always pushed further into the future to make room for more Bruce Wayne as Batman stories, the Joker retains his immunity in the present-day DCAU stories, unable to die thanks to Return of the Joker.
"Laughter After Midnight", a story by Paul Dini in The Batman Adventures Annual #1 uses this trope. It begins with the Joker falling out of a police blimp after a climactic fight with Batman, and proceeds to show how he spends the rest of his night getting back to one of his lairs. First he survives by falling into a park's lake. Understandably angry that his archenemy threw him from a blimp, he begins a massacre of Gotham's midnight denizens while buying donuts and a paper. He asks Harley to pick him up, but the police are with her. A Red Shirt patrolman tries to arrest him and the Joker steals his patrol car. Batman is The Only One who can stop Joker, but he believes the Joker's dead for some reason despite the fact that the Joker has survived all of the other times he should have died. For some hours, Joker is unstoppable. The comic ends in an eerie scene with the Joker trying to get home.
Joker:Oh who cares? I've been blown up, thrown down smokestacks, fed to sharks; I'm the Joker! I always survive!
Averted once again in Batman (1989), where Joker unambiguously dies by falling off the top of Gotham Cathedral and breaking his skull on the pavement. They even have a long, rotating Dies Wide Open shot to hammer it in. Subverted in that while lying there he appears to still be laughing until a police officer on the scene checks his pockets and finds a recorder that's making the chuckling sound.
In The Batman vs. Dracula, the Joker appears to have died from electrocution by falling into a river after using his Electric Joybuzzer. Of course, he comes back halfway into the film.
The Dark Knight: In a bit of dark, bitter irony, the Joker survives the events of the film, but the character can't come back either way because of actor Heath Ledger's real life demise.
In Suicide Squad (2016), the Joker is seemingly killed when his helicopter is shot down and blown up with him in it, but he shows up alive and well by the end in dramatic fashion.
Batman: How many people have I murdered by letting you live? (comic book) Batman: No more! All the people I've murdered... by letting you live. (animatedadaptation)
In fact, most Batman Elseworlds comic books seem to enjoy killing off Mr. J, as they're not in continuity. The Red Rain trilogy, the one where Batman and Joker were both pirates, etc.
Turned on its head in the Batman: Vengeance videogame, where Joker attempts to revoke his own Joker Immunity by killing himself. Only your saving him will prevent a Non-Standard Game Over.
In one Batman/Punisher crossover, Batman stops The Punisher from killing the Joker (although Batman fails to provide a convincing reason why the Joker shouldn't be killed). At least Frank managed to wipe the smile from the Joker's face on realizing Frank really was going to pull the trigger.
There was one vigilante named the Wyld Carde whose family was killed by the Joker. Problem with the Wyld Carde that he was so obsessed with killing the Joker that he didn't know what to do once the Joker was killed. Upon confronting the Joker, the Wyld Carde kept hesitating to pull the gun trigger (leaving a Joker enough time to escape, and infect the opponent with Joker gas).
In fact, the DC wiki lists the Joker's powers as Cheating Death and "Comic Awareness".
Spider-Man comes very close to killing him in a Batman/Spider-Man crossover. The Joker taunts him when he refuses to go through with it. Spider-Man decides that beating the crap out of him is justified however.
Batman's moral code isn't the only flawed aspect of this, Joker has been spared the death penalty in the past due to being insane and as such its seen as unjustified to execute him. Now becomes Hilarious in Hindsight: Gotham is said to be in New Jersey. As of 2007, New Jersey has abolished capital punishment, so there is some justification for this.
Rationalized in an issue of The Spectre. The Spectre is the embodied Wrath of God, and his whole shtick is executing murderers in ironic ways. When the Joker guest stars in his comic, the writers have to explain why the Spectre doesn't just kill him (by turning his smile inside-out or somesuch). The Spectre ends up discovering that the Joker has no functioning conscience, and thus can't tell right from wrong — and it would be unjust to kill him when he isn't consciously evil. (There's almost no good reason to believe that the Joker can't tell right from wrong, though.) Still doesn't explain why he doesn't go after Lex Luthor, though...
The Batman: Arkham Series as a whole does a Decon-Recon Switch of this trope. True to form, Batman won't kill Joker, but there's a good inadvertent reason for that: the other supercriminals are terrified of him after what he did to Black Mask. Joker's hilarious chaos has resulted in all of Gotham's criminals becoming paranoid and turning on each other at the slightest provocation; thus, ironically, Joker's continued existence makes Gotham safer. This is further looked into in Batman: Arkham City and into Batman: Arkham Knight: With Joker dead by his own foolishness, all it takes is one year for Gotham's criminals to put aside their differences and launch a full-scale military invasion of Gotham City. And even then, Joker arguably attempts a return from beyond the grave, through the blood of the Joker's Infected. Batman himself nearly becomes the Joker.
Injustice: Gods Among Us averts this big time in the very beginning of the game. The Joker creates a plan that ultimately leads to Supermankilling Lois Lanenote it involves the use of the Scarecrow's fear toxin laced with Kryptonite which makes Superman see Lois as Doomsday. He carries "Doomsday" out to space, which leads to her death. To make matters worse, Joker implanted a nuclear bomb detonator into her chest. When Lois dies, the bomb goes off. As for the location of the bomb, it's inside Metropolis. Sometime later during Batman's interrogation of the Joker, Superman interrupts them by promptly stabbing the latter withhis hand, killing him! Of course, this all took place in an Alternate Universe, where Superman eventually becomes a Fallen Hero. In the main universe of the game, the Joker is alive and remains that way.
In the final episode of the second game of Batman: The Telltale Series with the Joker as a villain, he seems to die after a fight with Bruce but is resurrected by him.
During Batman: No Man's Land, after the Joker murders Gordon's wife, Batman still refuses to execute the villain, but he tells Gordon he will not stop him from doing so. (And Gordon almost does. He backs down after deciding there's been too much death already.)
In an interview Grant Morrison interpreted that Batman did kill The Joker at the end of The Killing Joke. Although this is a bit odd, as The Killing Joke is an in-canon story, and the Joker isn’t dead.
This is all nicely parodied in one strip of The Far Side, where the Joker is gunned down by a random Gotham citizen who simply says it was about time someone did it.
Parodied in a Robot Chicken skit featuring Hamill as The Joker. Batman literally beats him within an inch of his life before lamenting that he's promised to let the justice system do its job and pondering what he should do. The scene then cuts to Joker having been given the death sentence after a testimony from the Batman, to which he says that it's now out of his hands.
Even Garth Ennis wasn't able to overcome this; in the first arc of the ongoing series for Hitman (1993), Tommy Monaghan is hired to assassinate the Joker. Everyone except for Batman thinks it's a good idea, or at least is really, really uncomfortable with stopping him. But Hitman can't go through with it. Because doing so will allow a pair of demon lords with dominion over guns known as the Arkannone to forcibly claim his soul and make him into their herald.
In Batman #37 it's revealed that the Joker might be straight up immortal....maybe...he survives a gunshot to the heart...
Double subverted in the Elseworlds story The Batman of Arkham, where Bruce Wayne runs Arkham Asylum to try and cure the inmates he turns in as Batman during the early 20th century. When Batman foils the Joker's plan to make all of Gotham go crazy from inhaling his gas, he nearly lets the Joker burn to death when his balloon explodes in flames, but realizes that letting the Joker die goes against his belief that the mentally ill can be cured, so he saves the Joker's life and has him sent to Arkham like the rest of his enemies.
Inverted in one issue of Robin, which was just Joker talking to doctors in Arkham. He reasons that if there are replacement Robins, then there might be replacement Batmen, and he actually did kill him all those times. While he likes the work, he says, he'd really like to see a death stick for a change.
Adrian Tullberg's "Improv"; a bunch of cops decide to execute Joker and make it look like a failed escape attempt. It fails.
The 2001 crossover Joker's Last Laugh was originally intended to have the Joker kill the Elongated Man. Out of disgust, Superman would kill the Joker. The Clown Prince of Crime would then be replaced by a psychic who could help bring out people's worst fears.
Batman: Hush provides a reason why Batman just can't kill the Joker: because if he does, Gordon will just consider him yet another of the mad-dog costumed maniacs running around Gotham and will bring him down by any means necessary. Acknowledging that he needs Gordon to have a degree of effectiveness in his war, he doesn't beats him to death like he wanted to because the Joker apparently killed Bruce's childhood friend Thomas... serious emphasis on "apparently".
Horrifically averted and exploited in Dark Nights: Metal. From the depths of the Dark Multiverse (a multiverse where worlds are made out of decisions from hopes and fears of individuals before rotting away into oblivion), one world shows Batman being forced to kill the Joker out of sheer rage due to the latter ravaging Gotham and killing Commissioner Gordon before his very eyes. All of this was because he was apparently dying and wanted to makes something new for the "both of [us]". But when some form of nanotoxin seeping from the Joker's mouth entered Batman's body, his mind was later rewritten to become The Batman Who Laughs, where his moral compass is twisted as the Joker's whilst maintaining his intellectual and combat prowess like Batman. He then proceeds to gun down his Batfamily in cold blood when he realises that they'll be the first ones to recognise his transformation (and that's just BEFORE he becomes twisted both physically and mentally), then slaughtered the entire Justice League, saving Superman as the last, and then finally (albeit offscreen) armies of villains and alien tyrants. He even manages to take down the Wrath of God with just only precision and blood.
With Joker as the Villain Protagonist of his own self-titled film from 2019, it's kind of a Foregone Conclusion that this trope would come into play. The most noteworthy example came at the end during the clown riots, where a hijacked ambulance rams into the police car Joker was arrested in. It manages to kill the cops driving the car, but Arthur Fleck himself was merely unconsious. Eventually he managed to wake up, and despite being shaken up and severely injured, he's still well enough to perform a dance for the rioters.
Zig-zagged in Batman: Damned, a follow-up to Brian Azzarello's Joker graphic novel where the premise involves Batman allegedly resorting to killing the Joker after realizing that letting his enemy live will only lead to more suffering and death, the ending having Batman surrender his life so that the Joker can come back to life.
Averted in DCeased, which has the Joker become a zombie after being infected by the Anti-Life Equation and killed by Harley Quinn shortly afterwards.
The Interactive Fiction short DC Showcase – Batman: Death in the Family actually enables the viewer to defy Joker Immunity by allowing them to make decisions that have the outcome of the Joker dying at Jason Todd's hand, the point of divergence starting at when the viewer chooses whether or not Jason sticks to Batman's wishes that he not kill the Joker after the path where Batman sacrifices himself to save Jason. Letting Jason's desire for vengeance eat away at him ends with Jason meeting a stranger at a cafe who turns out to be the Joker in disguise and killing him by stabbing him in the eye with a knife the moment he recognizes him, while choosing to respect Batman's dying wish results in Jason becoming the Red Hood and confronting the Joker to find out that he actually had been killing criminals while suppressing his memories of doing so, at which point the viewer gets to choose whether or not Jason Todd goes through with killing the Joker.
Countdown to Final Crisis featured an aversion that showed a good example of how Batman killing the Joker can go wrong, where Jason Todd encounters a Batman from a world where the Dark Knight was driven to take the Joker's life in retribution for the demise of his world's Jason Todd. The bad news is that Batman escalated towards wiping out all the other supervillains, did the same to the other heroes when they didn't approve of his methods and is overtly hostile and distrustful towards the Jason Todd visiting him.
Averted in Earth 2, where the second Batman (Thomas Wayne) sees the Joker in a stasis chamber and immediately chooses to shoot him dead rather than recruit him like the other two prisoners at the Arkham Base he came for (Aquawoman and Jimmy Olsen).
Pointedly defied in the Tales from the Dark Multiverse take on The Death of Superman, where Lois Lane takes the opportunity to vaporize the Joker shortly after becoming the Eradicator and calls out Batman for being complicit in the lives the Joker has taken by refusing to kill the Joker himself.
Averted in the Elseworld story Batman: Two Faces, a retelling of The Strange Case of Dr. Jekyll and Mr. Hyde with the twist of Bruce Wayne playing the part of Dr. Jekyll with the Joker as Mr. Hyde, effectively making Batman and the Joker the same person in this continuity. Bruce Wayne can't bear the thought of his alter ego continuing to cause havoc in Gotham City, so he forces Harvey Dent to let him fall to his death. On the bright side, Harvey honors his friend's wishes of taking the Twilight Orchid serum to cure himself of his insanity and honors Bruce's memory by becoming the new Batman.
Chip Zdarsky's run manages to retroactively apply this to several incarnations of the Joker where it was initially averted, as Red Mask resurrects several deceased Jokers in the multiverse, including his DCAU, Burtonverse and Arkhamverse incarnations.
This trope doesn't only apply for the Joker. Most of Batman's Rogues Gallery never get killed off (in principle) no matter what happens to them. The common in-story explanation is Batman realizes he's quite capable of killing opponents, but doesn't trust himself not to come up with excuses to do it again if he can rationalize it the first time. Often forgotten is that other characters have been insistent on stopping Batman if they think he's really been tempted. Jim Gordon explicitly informs Batman that he, the police, and citizens of Gotham tolerate him because of his moral code, and would not hesitate to deal with him if this was broken. Still doesn't explain why the GCPD does not kill these super villains the moment they resist arrest, and present a clear, and present danger, as the police are legally authorized to do.
This is deconstructed in The LEGO Batman Movie where Batman is unable to reduce the crime rate.
In DC's Flashpoint alternate reality, Batman has privatized the Gotham City Police Department and has killed off Killer Croc, Hush, Scarecrow, and Poison Ivy. However, even that extremely bitter version of Batman can't bring himself to kill Joker for the excellent reason that Bruce was the Wayne that got killed in Crime Alley that fateful night; Thomas became Batman and Martha... well, perhaps you can guess.
Occasionally a Batman villain DOES get killed off (i.e. Ventriloquist, KGBeast, Blockbuster I, Clayface II, Black Spider II, Ten-Eyed Man, Magpie etc.), by someone other than Batman, but, alas, being a comic, Death Is Cheap and they usually end up coming back anyway.
The KGBeast was originally an aversion to this trope, made to upheld the trope, and then became an aversion again. Batman realized that the villains sheer physical, and mental cunning made him too dangerous to leave alive. Thus Batman left the KGBeast locked inside a sewer room. The implication was that the KGBeast starved to death. Later comics rebooted the event to state that Batman later came back and took him to jail. Eventually, the villain was killed with shocking ease by an even more minor villain, the second Tally Man, who did it to frame a temporarily reformed Two-Face.
Hugo Strange has appeared to die multiple times, including being killed by both his own hand and other villains, and then come back because he had been using his yoga skills to appear lifeless, been Actually a Doombot, etc.
Maintained with the New 52 version of Harley Quinn, and Deadshot who were shot through the spine and then completely healed with a Lazarus Pit injection from Amanda Waller.
in the webcomic Shortpacked!.