The Beefeater in a bearskin hat, tipping his cap. Scaffolding tools came and went, but nobody else in JavaScript ever picked a Yeoman Warder as their mascot.
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BABEL hand-lettered in yellow marker, deliberately rough. A compiler logo that looks like it was scrawled on a whiteboard, which suits a tool most people never think about.
Flow's angular F in overlapping yellows and oranges. Meta's other type checker, comprehensively beaten by TypeScript, and increasingly a collector's item.
The yellow square with two black letters. No foundation designed it and no one owns it — the community adopted a logo somebody posted on GitHub and never looked back.
The dark red jester's hat, bells and all. Testing framework branding rarely bothers with a character; Jest committed to the pun and drew the whole hat.