Code coverage

Bun's test runner now supports built-in code coverage reporting. This makes it easy to see how much of the codebase is covered by tests, and find areas that are not currently well-tested.

Enabling coverage

bun:test supports seeing which lines of code are covered by tests. To use this feature, pass --coverage to the CLI. It will print out a coverage report to the console:

$ bun test --coverage
-------------|---------|---------|-------------------
File         | % Funcs | % Lines | Uncovered Line #s
-------------|---------|---------|-------------------
All files    |   38.89 |   42.11 |
 index-0.ts  |   33.33 |   36.84 | 10-15,19-24
 index-1.ts  |   33.33 |   36.84 | 10-15,19-24
 index-10.ts |   33.33 |   36.84 | 10-15,19-24
 index-2.ts  |   33.33 |   36.84 | 10-15,19-24
 index-3.ts  |   33.33 |   36.84 | 10-15,19-24
 index-4.ts  |   33.33 |   36.84 | 10-15,19-24
 index-5.ts  |   33.33 |   36.84 | 10-15,19-24
 index-6.ts  |   33.33 |   36.84 | 10-15,19-24
 index-7.ts  |   33.33 |   36.84 | 10-15,19-24
 index-8.ts  |   33.33 |   36.84 | 10-15,19-24
 index-9.ts  |   33.33 |   36.84 | 10-15,19-24
 index.ts    |  100.00 |  100.00 |
-------------|---------|---------|-------------------

To always enable coverage reporting by default, add the following line to your bunfig.toml:

[test]

# always enable coverage
coverage = true

By default coverage reports will include test files and exclude sourcemaps. This is usually what you want, but it can be configured otherwise in bunfig.toml.

[test]
coverageSkipTestFiles = true       # default false

Coverage thresholds

It is possible to specify a coverage threshold in bunfig.toml. If your test suite does not meet or exceed this threshold, bun test will exit with a non-zero exit code to indicate the failure.

[test]

# to require 90% line-level and function-level coverage
coverageThreshold = 0.9

# to set different thresholds for lines and functions
coverageThreshold = { lines = 0.9, functions = 0.9, statements = 0.9 }

Setting any of these thresholds enables fail_on_low_coverage, causing the test run to fail if coverage is below the threshold.

Exclude test files from coverage

By default, test files themselves are included in coverage reports. You can exclude them with:

[test]
coverageSkipTestFiles = true   # default false

This will exclude files matching test patterns (e.g., _.test.ts, __spec.js) from the coverage report.

Sourcemaps

Internally, Bun transpiles all files by default, so Bun automatically generates an internal source map that maps lines of your original source code onto Bun's internal representation. If for any reason you want to disable this, set test.coverageIgnoreSourcemaps to true; this will rarely be desirable outside of advanced use cases.

[test]
coverageIgnoreSourcemaps = true   # default false

Coverage defaults

By default, coverage reports:

1. Exclude node_modules directories
2. Exclude files loaded via non-JS/TS loaders (e.g., .css, .txt) unless a custom JS loader is specified
3. Include test files themselves (can be disabled with coverageSkipTestFiles = true as shown above)

Coverage reporters

By default, coverage reports will be printed to the console.

For persistent code coverage reports in CI environments and for other tools, you can pass a --coverage-reporter=lcov CLI option or coverageReporter option in bunfig.toml.

[test]
coverageReporter  = ["text", "lcov"]  # default ["text"]
coverageDir = "path/to/somewhere"  # default "coverage"

lcov coverage reporter

To generate an lcov report, you can use the lcov reporter. This will generate an lcov.info file in the coverage directory.

[test]
coverageReporter = "lcov"