babel-plugin-spock

babel-plugin-spock

Structured JS test cases, inspired by Spock Framework

Note: This module is in an early stage of development and currently provides only a small set of features.

Example test case

test('basic arithmetic', () => {
  expect: {
    1 + 2 === 3;
    3 * 3 >= 4 * 4; // falsy
  }
});

Expected value to be (operator: ==):
  true
Received:
  false

  assert(3 * 3 >= 4 * 4)
           |   |    |
           |   |    16
           9   false

Installation

npm install --save-dev babel-plugin-spock power-assert

This module is a Babel plugin that needs to transform your test sources in order to generate assertions, so your test runner will need support for Babel. Babel integrates quite nicely into most modern test runners. Check the documentation of your test runner for instructions on how to configure Babel (e.g. for Jest, AVA etc.) or consult Babel's own documentation.

Once Babel is set up for your test files, simply add "babel-plugin-spock" to the plugins array in your babel configuration and you're good to go.

Note: The last version of this plugin that supports Babel 6 is 0.1.0

Usage

Assertion blocks

Inside of a block labeled with expect: or then:, all statements will be considered assertions and evaluated to check for truth:

expect: {
  1 < 2;
}

when-then blocks can be particularly useful and expressive for code with side effects:

// The 'when' label here does not have a special meaning
// It is used simply to make the test more structured
when: {
  abc.setXyz(1);
}

then: {
  abc.getXyz() === 1;
}

Single labeled statements are also possible:

expect: 'a' + 'b' === 'ab';

Note that these blocks can only contain statements that can be evaluated as expressions. For example, an if statement would not be valid:

// BAD
expect: {
  if (x < 1) x === 0.5;
  else x === 2;
}

However, you can nest an assertion block into other structures:

// GOOD
if (x < 1) expect: x === 0.5;
else expect: x === 2;

If you want to perform more complicated checks, it might be helpful to look for small helper libraries on npm. For example, deep-strict-equal can help perform deep equality checks on object structures. In the future, this plugin might provide special syntax for such use cases.

Of course, you still have the option to use your native assertion library alongside assertion blocks wherever you consider it appropriate. Some assertion libraries may provide features on top of what this plugin supports.

Linters

The test you will write using this plugin often employ syntax that is otherwise uncommon in JavaScript code. For this reason, if you use a linter such as ESLint, you will likely see annoying warnings all over your tests. To work around this, most linters will give you multiple options:

• Disable the problematic rules with special annotations in your test files. This can be a hassle because it needs to be done for every file.
• Completely disable the rules in the configuration. This means that they will no longer apply to production code either.
• Create a separate config for tests that extends the base config, but disables the rules.

If you're using TypeScript, tsc might also complain about unused labels. allowUnusedLabels can turn those warnings off.

Configuration

You can configure this plugin using Babel's regular plugin configuration mechanism. The following options are available:

powerAssert

The plugin can seamlessly generate assertions that produce detailed mismatch messages to help you figure out what exactly about the assertion went wrong. Turning this feature off could be useful if you're tests appear to run slowly or you are experiencing other issues with your assertions.

This feature is powered by the awesome project power-assert.

type: boolean
default: true

autoImport

The plugin transforms your assertion blocks to calls to an assert function. Be default (true), this function is automatically imported from power-assert. You can set this option to a string containing the name of a module that exports an assert function as its default export to use that module for assertions instead.

You can also set this option to false to disable automatic imports. You will then have to provide a function named assert yourself in your test files wherever you use assertion blocks.

type: boolean | string
default: true ('power-assert')

staticTruthCheck

The plugin can try to statically evaluate your assertion expressions at compile-time and throw an error if they can be inferred to always be truthy or to always be falsy. Such expressions sometimes indicate a test that does not provide any value.

Here's an example of an assertion expression that can be inferred to always be truthy:

const x = 1;
expect: x === 1;

type: boolean
default: false

assertFunctionName

The plugin will automatically define an assert import or use an existing one. You can set this option to enforce a specific name for the generated assert calls.

type: string
default: empty string (generated identifier)

Changelog

0.5.0 (2018-04-27)

Features

• add assertFunctionName option (448b277)
• assert-function-name/auto-import: gracefully handle conflicts by renaming existing bindings (4bea5c8)

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