./tool is a script in the root of this project. ./tool test runs the tests in this directory.

To make it work: run yarn install in the flow directory. This is needed both for tool to run, and also for flow check to work in the newtests directory.

• Tests should pair small examples with the expectations for each example.
• We should dogfood Flow

Check out tool_test_example, which is an example test.

• A test is a file named ./**/test.js.
• Each test.js file exports a Suite by default
• Each Suite contains a list of Tests.
• Each Test contains a list of TestSteps

The only way to create a Suite is to call the suite() function. The suite() function takes a callback, like so

import {suite} from 'flow-dev-tools/src/test/Tester';
import type TestStep from 'flow-dev-tools/src/test/TestStep';
module.exports = suite((emptyTestStep: TestStep) => [ < List of Tests >]);

(Why the suite() function? Why not just export the callback directly? Well, it removes the need for type annotations!)

The only way to create a Test is to call the test() function. The test() function takes a test name and a list of TestSteps, like so

const {suite, test} = require('flow-dev-tools/src/test/Tester');
import type TestStep from 'flow-dev-tools/src/test/TestStep';
module.exports = suite((emptyTestStep: TestStep) => [
  test('My first test, [ < List of TestSteps > ]'),
]);

A TestStep is made up of 0 or more actions and 0 or more assertions. The emptyTestStep passed to suite()'s callback is a TestStep with 0 actions and 0 assertions. TestSteps are immutable, so when you call emptyTestStep.addFile('foo.js') you get back a new TestStep with 1 action and 0 assertions. So a test looks like

const {suite, test} = require('flow-dev-tools/src/test/Tester');
import type TestStep from 'flow-dev-tools/src/test/TestStep';
module.exports = suite((emptyTestStep: TestStep) => [
  test('My first test', [
    emptyTestStep
      .addCode('var x = 123')
      .noNewErrors(),
    emptyTestStep
      .addCode('var y = "hello"')
      .noNewErrors(),
  ]),
]);

More concisely, this can be written

const {suite, test} = require('flow-dev-tools/src/test/Tester');
module.exports = suite(({addCode}) => [
  test('My first test', [
    addCode('var x = 123')
      .noNewErrors(),
    addCode('var y = "hello"')
      .noNewErrors(),
  ]),
]);

Note: You cannot add actions to a TestStep after an assertion because @gabelevi felt like messing around with the type system to prevent it :)