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Viscosity

Profile-guided decisions

Static analysis can’t always tell how big a collection gets: the size may depend on input or on how often a function is called. Profiling runs your original program once with light instrumentation that records each collection’s final length:

Terminal window
viscosity profile app.js -o profile.json
viscosity build app.js --profile profile.json -o app.vsc.js

explain marks measured sizes (measured). A collection measured below --min-size is left as objects.

The profile is a JSON object mapping collection names to { "maxLength": n }. If you write or edit one by hand, each maxLength must be a non-negative number; otherwise --profile is an error, not silently ignored (see Errors and validation).

Two limits to know about:

  • JavaScript only. viscosity profile runs the instrumented program with Node, so it profiles .js programs (and .ts ones with the TypeScript CLI), in both CLIs; Node.js must be on the PATH. It refuses .py files with a clear error; Python profiling isn’t supported yet. If the program fails, there is no profile.
  • Keyed by variable name. A profile records sizes per collection name, so two collections with the same name in different functions share one entry.