Simon Willison’s Weblog

2nd June 2025 - Link Blog

Directive prologues and JavaScript dark matter (via) Tom MacWright does some archaeology and describes the three different magic comment formats that can affect how JavaScript/TypeScript files are processed:

"a directive"; is a directive prologue, most commonly seen with "use strict";.

/** @aPragma */ is a pragma for a transpiler, often used for /** @jsx h */.

//# aMagicComment is usually used for source maps - //# sourceMappingURL=<url> - but also just got used by v8 for their new explicit compile hints feature.