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0.3.0 • Public • Published


View deoptimizations of your JavaScript in V8

Sample image of the results of running v8-deopt-viewer

You may not need this tool...

V8 only optimizes code that runs repeatedly. Often for websites this code is your framework's code and not your app code. If you are looking to improve your website's performance, first check out tools like Lighthouse or webhint, and follow other general website performance guidance.


Install NodeJS 14.x or greater.

npx v8-deopt-viewer program.js --open

If you want run this against URLs, also install puppeteer:

npm i -g puppeteer

The main usage of this repo is through the CLI. Download the v8-deopt-viewer package through NPM or use npx to run the CLI. Options for the CLI can be found using the --help.

$ npx v8-deopt-viewer --help

    Generate and view deoptimizations in JavaScript code running in V8

    $ v8-deopt-viewer [file] [options]

    -i, --input         Path to an already generated v8.log file
    -o, --out           The directory to output files too  (default current working directory)
    -t, --timeout       How long in milliseconds to keep the browser open while the webpage runs  (default 5000)
    --keep-internals    Don't remove NodeJS internals from the log
    --skip-maps         Skip tracing internal maps of V8
    --open              Open the resulting webapp in a web browser
    -v, --version       Displays current version
    -h, --help          Displays this message

    $ v8-deopt-viewer examples/simple/adder.js
    $ v8-deopt-viewer examples/html-inline/adders.html -o /tmp/directory
    $ v8-deopt-viewer https://google.com
    $ v8-deopt-viewer -i v8.log
    $ v8-deopt-viewer -i v8.log -o /tmp/directory

Running the CLI will run the script or webpage provided with V8 flags to output a log of optimizations and deoptimizations. That log is saved as v8.log. We'll then parse that log into a JSON object filtering out the useful log lines and extending the information with such as the severity of the deopts. This data is saved in a JavaScript file (v8-data.js). We copy over the files from the webapp for viewing the data (index.html, v8-deopt-webapp.js, v8-deopt-webapp.css). Finally, open the index.html file in a modern browser to view the results of the run.

Prior work

  • thlorenz/deoptigate

    This project started out as a fork of the awesome deoptigate but as the scope of what I wanted to accomplish grew, I figured it was time to start my own project that I could re-architect to meet my requirements




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  • drewigg
  • marvinhagemeister