Jest

Debugging Commands

Debug failing tests with Node.js inspector, verbose output, and Jest's built-in debugging tools. Find and fix flaky tests, memory leaks, and timing issues.

10 commands

Pro Tips

Debug with Chrome DevTools: 'node --inspect-brk node_modules/.bin/jest --runInBand' then open chrome://inspect.

Use 'jest --verbose' to see individual test names and status — helps identify which test in a suite is failing.

Find flaky tests with 'jest --detectOpenHandles --forceExit' — shows async operations that aren't cleaned up.

Common Mistakes

Using '--forceExit' masks real issues. It hides tests that don't clean up async operations properly. Fix the root cause.

Timer mocking with 'jest.useFakeTimers()' can cause confusing failures. Always call 'jest.useRealTimers()' in afterEach.

Commands

Debug mode

$ jest --debug

Print debug information.

Node inspector

$ jest --inspect

Run tests with Node inspector.

Break on start

$ jest --inspect-brk

Pause before test execution.

Debug with Node

$ node --inspect-brk node_modules/.bin/jest --runInBand

Debug Jest with Node debugger.

Skip coverage

$ jest --no-coverage

Run tests without coverage collection.

Set timeout

$ jest --testTimeout=10000

Set test timeout in milliseconds.

Max concurrency

$ jest --maxConcurrency=5

Limit concurrent tests in describe.

Log heap usage

$ jest --logHeapUsage

Log heap usage after each test.

Expand diff output

$ jest --expand

Show full diff output for failures.

Show version

$ jest --version

Display Jest version number.