The Joker is a fictional supervillain created by Bill Finger, Bob Kane, and Jerry Robinson who first appeared in the debut issue of the comic book Batman (April 25, 1940), published by DC Comics. Credit for the Joker's creation is disputed; Kane and Robinson claimed responsibility for the Joker's design, while acknowledging Finger's writing contribution. Although the Joker was planned to be killed off during his initial appearance, he was spared by editorial intervention, allowing the character to endure as the archenemy of the superhero Batman. In his comic book appearances, the Joker is portrayed as a criminal mastermind.

The Joker debuted in Batman #1 (Spring 1940) as the eponymous character's first villain, shortly after Batman's debut in Detective Comics #27 (May 1939). The Joker initially appeared as a remorseless serial killer, modeled after a joker playing card with a mirthless grin, who killed his victims with "Joker venom": a toxin which left their faces smiling grotesquely. The character was intended to be killed in his second appearance in Batman #1, after being stabbed in the heart. Finger wanted the Joker to die because of his concern that recurring villains would make Batman appear inept, but was overruled by then-editor Whitney Ellsworth; a hastily drawn panel, indicating that the Joker was still alive, was added to the comic. The Joker went on to appear in nine of Batman's first twelve issues.  The character's regular appearances quickly defined him as the archenemy of the dynamic duo, Batman and Robin; he killed dozens of people, and even derailed a train. By issue #13, Kane's work on the syndicated Batman newspaper strip left him little time for the comic book; artist Dick Sprang assumed his duties, and editor Jack Schiff collaborated with Finger on stories. Around the same time, DC Comics found it easier to market its stories to children without the more mature pulp elements that had originated many superhero comics. During this period, the first changes in the Joker began to appear, portraying him more as a prankster than threat; when he kidnaps Robin, Batman pays the ransom by check, meaning that the Joker cannot cash it without being arrested. Comic book writer Mark Waid suggests that the 1942 story "The Joker Walks the Last Mile" was the beginning point for the character's transformation into a more goofy incarnation, a period that Grant Morrison considered lasted the following thirty years.  The 1942 cover of Detective Comics #69, known as "Double Guns" (with the Joker emerging from a genie lamp, aiming two guns at Batman and Robin), is considered one of the greatest superhero comic covers of the Golden Age and is the only image of the character using traditional guns. Robinson said that other contemporary villains used guns, and the creative team wanted the Joker--as Batman's adversary--to be more resourceful.

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