Parsing JavaScript - better lazy than eager?
summary
V8 uses an eager parser for top-level code and a fast pre-parser ~2x faster for lazy functions, trading full AST/scope work for speed; mobile parsing averages ≈1 MB/s and real pages spend 15–20% of V8 time (≈370 ms) parsing at startup. Streaming and code cache speed script load but a single large bundle invalidates cache for small updates; nested functions force context allocation and cause repeated reparsing of inner functions, and forcing eager parse with the parens hack or optimize-just sometimes fixes startup latency.
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chapters (20)
- 0:00Intro
- 1:01Parsing
- 4:05Why should you care
- 4:52Parsing modes
- 6:05Parse example
- 7:02Other heuristics
- 7:19Lazy vs eager cases
- 8:26Why lazy parsing
- 9:38Force eager parsing
- 10:20Function context allocation
- 11:53Lazy parsing with names
- 12:55Lazy parsing with nested functions
- 14:13Why is parsing hard
- 16:11A typical parser bug
- 17:55Benchmarking
- 19:43What can you do
- 20:51Code Cache
- 21:36Streaming
- 22:12Evolve
- 22:56Bonus Content
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