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ES6 polyfill vs. feature performance tests.

Report is located at


npm run test:sauce

Test against all registered Sauce Labs browsers.

npm run test:node

Tests against the current node version.

npm run profile:node -- --testName=$name --type=$type --count=$iterationCount

Profiles a given test within the current Node environment. Type may be one of:

  • babel
  • babel-runtime
  • babel-loose
  • typescript
  • es5
  • es6
npm run build

Builds test files into build directory.

npm start

Starts a server instance for manual browser testing. Tests may be accessed via http://machineName:9999/ and the # component may be used to filter the tests to be executed, i.e. http://machineName:9999/#promises

Profiling of specific tests may be done through http://machineName:9999/profile.html?testName=$testName&type=$type&count=$number, i.e. http://localhost:9999/profile.html?testName=generator&type=babel&count=1000000.

Firefox browsers need to use /moz/index.html and /moz/profile.html respectively to enable all supported ES6 features.

npm run report

Generates the data report.

Testing methodology

For each of the ES6 features in question, an ES5 implementation of that functionality was written along with an ES6 version. It should be noted that the functionality is frequently the same, but in some cases the "common" vs. "correct" version was written, i.e. using x[key] = value vs. defineProperty which is faster but can be hit but a particular nasty edge case for those who deem it fun to extend Object.prototype.

Babel, in both loose+runtime and runtime mode, and Babel was then used to compile the ES6 version to an ES5 compliant version, utilizing the runtime over polyfill to maintain test isolation and avoid native implementations where possible.

All of these test instances were then benchmarked in the given JavaScript engine using Benchmark.js and then the operations per second compared to the ES5 implementation. Cross browser and cross execution comparisons are avoided as much as possible to isolate environmental issues when executing on VMs in the cloud.

Test Steps

  1. ./bin/
  2. npm run report
  3. Checkin changes to site sub-repository.

VM Setup

The Windows 10 VM used must be manually setup to ensure the proper state prior to testing. This can be done with this command:

mkdir browsers
./node_modules/.bin/browser-downloader vm ./browsers

After this the image should be restarted a few times until all setup and update processes have completed and then a snapshot named six-speed taken from the idle desktop screen. The script will check that the VM image is up to date and will halt execution if the image is not setup properly, as a sanity check.

Adding a custom browser to the report

  • Run the server (npm run build && npm start)
  • Visit the test url in your browser to perform the tests
  • This updates data.json to include the results
  • Update notes.json to include the useragent and engine
  • Run the report builder (npm run report)
    • Report is generated in site/index.html



Thanks to BrowserStack and Sauce Labs for providing open source accounts which the majority of this testing was performed on.


ES6 polyfill vs. feature performance tests







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