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.
Recent articles
- OpenAI's new open weight (Apache 2) models are really good - 5th August 2025
- ChatGPT agent's user-agent - 4th August 2025
- The ChatGPT sharing dialog demonstrates how difficult it is to design privacy preferences - 3rd August 2025