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Configuring Babel

Translations: Français

There are multiple options for configuring how AVA transpiles your tests using Babel.

AVA's default transpiler behavior

By default, AVA transpiles your tests (and only your tests) using the es2015-node4 on Node.js 4 or node6 on Node.js 6 and higher, and stage-2 Babel presets. This is a great option for small modules where you do not desire a build step to transpile your source before deploying to npm.

Customizing how AVA transpiles your tests

You can override the default Babel configuration AVA uses for test transpilation in package.json. For example, the configuration below adds the Babel rewire plugin, and opts to only use the Babel stage-3 preset (which is a subset of stage-2).

{
  "ava": {
    "babel": {
      "plugins": ["rewire"],
      "presets": ["es2015-node4", "stage-3"]
    }
  }
}

Transpiling Sources

To transpile your sources, you will need to define a babel config in package.json or a .babelrc file. Also, you will need to tell AVA to load babel-register in every forked process, by adding it to the require section of your AVA config:

package.json

{
  "ava": {
    "require": ["babel-register"]
  },
  "babel": {
    "presets": ["es2015-node4"]
  }
}

Note that loading babel-register in every forked process has a non-trivial performance cost. If you have lots of test files, you may want to consider using a build step to transpile your sources before running your tests. This isn't ideal, since it complicates using AVA's watch mode, so we recommend using babel-register until the performance penalty becomes too great. Setting up a precompilation step is out of scope for this document, but we recommend you check out one of the many build systems that support Babel. There is an open issue discussing ways we could make this experience better.

Transpiling tests and sources the same way

Using the "inherit" shortcut will cause your tests to be transpiled the same as your sources (as specified in your babelrc). AVA will add a few additional internal plugins when transpiling your tests, but they won't affect the behavior of your test code.

package.json:

{
  "ava": {
    "require": "babel-register",
    "babel": "inherit"
  },
  "babel": {
    "presets": ["es2015-node4", "react"]
  }
}

In the above example, both tests and sources will be transpiled using the es2015-node4 and react presets.

Extend your source transpilation configuration

When specifying the Babel config for your tests, you can set the babelrc option to true. This will merge the specified plugins with those from your babelrc.

package.json:

{
  "ava": {
    "require": "babel-register",
    "babel": {
      "babelrc": true,
      "plugins": ["custom-plugin-name"],
      "presets": ["custom-preset"]
    }
  },
  "babel": {
    "presets": ["es2015-node4", "react"]
  }
}

In the above example, sources are compiled use es2015-node4 and react, tests use those same plugins, plus the additional custom plugins specified.

Extend an alternate config file.

If, for some reason, your Babel config is not specified in one of the default locations (.babelrc or package.json, you can set the extends option to the alternate config you want to use during testing.

package.json:

{
  "ava": {
    "require": "babel-register",
    "babel": {
      "extends": "./babel-test-config.json",
      "plugins": ["custom-plugin-name"],
      "presets": ["custom-preset"]
    }
  }
}

The above uses babel-test-config.json as the transpilition config for sources, and as the base config for tests. For tests, it extends that base config with the custom plugins and presets specified.

Notes

AVA always adds a few custom Babel plugins when transpiling your plugins. They serve a variety of functions:

  • Enable power-assert support.
  • Rewrite require paths internal AVA dependencies like babel-runtime (important if you are still using npm@2).
  • ava-throws-helper helps AVA detect and report improper use of the t.throws assertion.
  • Generate test metadata to determine which files should be run first (future).
  • Static analysis of dependencies for precompilation (future).