Unlocking Test Performance: Migrating from Mocha to Jest

Gary Borton from Airbnb reveals how switching test frameworks slashed their CI build time by 62%. What started as a 12+ minute ordeal became a 4.5 minute sprint—and nearly eliminated test flakiness.

Airbnb's engineering team migrated thousands of test files from Mocha to Jest, tackling performance bottlenecks in their massive JavaScript test suite. The migration required minimal code changes but delivered dramatic improvements across speed, reliability, and developer experience.

Key Takeaways

• Test suite runtime dropped from 45 minutes locally to 14.5 minutes
• CI builds improved from 12+ minutes to 4.5 minutes with Jest parallelization
• Flaky build rate decreased from 12% to just 1% through test isolation
• Jest's built-in babel transform cache eliminated redundant CPU-intensive work
• Developers reported 3-hour testing tasks now complete in 30 minutes

The team discovered one global configuration file was secretly sabotaging their performance gains, costing over a minute of wasted setup time.