Add rudimentary implementation

.editorconfig

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+root = true
+
+[*]
+indent_style = tab
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[*.yml]
+indent_style = space
+indent_size = 2

.github/CODE_OF_CONDUCT.md

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+# Contributor Covenant Code of Conduct
+
+Translations: [Español](https://github.com/avajs/ava-docs/blob/master/es_ES/code-of-conduct.md), [Français](https://github.com/avajs/ava-docs/blob/master/fr_FR/code-of-conduct.md), [Italiano](https://github.com/avajs/ava-docs/blob/master/it_IT/code-of-conduct.md), [日本語](https://github.com/avajs/ava-docs/blob/master/ja_JP/code-of-conduct.md), [Português](https://github.com/avajs/ava-docs/blob/master/pt_BR/code-of-conduct.md), [Русский](https://github.com/avajs/ava-docs/blob/master/ru_RU/code-of-conduct.md), [简体中文](https://github.com/avajs/ava-docs/blob/master/zh_CN/code-of-conduct.md)
+
+## Our Pledge
+
+In the interest of fostering an open and welcoming environment, we as
+contributors and maintainers pledge to making participation in our project and
+our community a harassment-free experience for everyone, regardless of age, body
+size, disability, ethnicity, gender identity and expression, level of experience,
+nationality, personal appearance, race, religion, or sexual identity and
+orientation.
+
+## Our Standards
+
+Examples of behavior that contributes to creating a positive environment
+include:
+
+* Using welcoming and inclusive language
+* Being respectful of differing viewpoints and experiences
+* Gracefully accepting constructive criticism
+* Focusing on what is best for the community
+* Showing empathy towards other community members
+
+Examples of unacceptable behavior by participants include:
+
+* The use of sexualized language or imagery and unwelcome sexual attention or
+advances
+* Trolling, insulting/derogatory comments, and personal or political attacks
+* Public or private harassment
+* Publishing others' private information, such as a physical or electronic
+  address, without explicit permission
+* Other conduct which could reasonably be considered inappropriate in a
+  professional setting
+
+## Our Responsibilities
+
+Project maintainers are responsible for clarifying the standards of acceptable
+behavior and are expected to take appropriate and fair corrective action in
+response to any instances of unacceptable behavior.
+
+Project maintainers have the right and responsibility to remove, edit, or
+reject comments, commits, code, wiki edits, issues, and other contributions
+that are not aligned to this Code of Conduct, or to ban temporarily or
+permanently any contributor for other behaviors that they deem inappropriate,
+threatening, offensive, or harmful.
+
+## Scope
+
+This Code of Conduct applies both within project spaces and in public spaces
+when an individual is representing the project or its community. Examples of
+representing a project or community include using an official project e-mail
+address, posting via an official social media account, or acting as an appointed
+representative at an online or offline event. Representation of a project may be
+further defined and clarified by project maintainers.
+
+## Enforcement
+
+Instances of abusive, harassing, or otherwise unacceptable behavior may be
+reported by contacting the project team at [email protected]. All
+complaints will be reviewed and investigated and will result in a response that
+is deemed necessary and appropriate to the circumstances. The project team is
+obligated to maintain confidentiality with regard to the reporter of an incident.
+Further details of specific enforcement policies may be posted separately.
+
+Project maintainers who do not follow or enforce the Code of Conduct in good
+faith may face temporary or permanent repercussions as determined by other
+members of the project's leadership.
+
+## Attribution
+
+This Code of Conduct is adapted from the [Contributor Covenant][homepage], version 1.4,
+available at [http://contributor-covenant.org/version/1/4][version]
+
+[homepage]: http://contributor-covenant.org
+[version]: http://contributor-covenant.org/version/1/4/

.github/CONTRIBUTING.md

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+# Contributing to AVA
+
+✨ Thanks for contributing to AVA! ✨
+
+Please note that this project is released with a [Contributor Code of Conduct](CODE_OF_CONDUCT.md). By participating in this project you agree to abide by its terms.
+
+This repository is a part of AVA. Start with reading AVA's [contributing guide](https://github.com/avajs/ava/blob/master/.github/CONTRIBUTING.md). Issue labels may be a little different in this repository but otherwise the same applies.

.github/ISSUE_TEMPLATE/bug.md

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+---
+name: Bug Report
+about: If something isn't working the way you expect it to
+labels: needs triage
+---
+
+Please provide details about:
+
+* What you're trying to do
+* What happened
+* What you expected to happen
+
+Please share relevant sample code. Or better yet, provide a link to a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example).
+
+We'll also need your AVA configuration (in `package.json` or `ava.config.*` configuration files) and how you're invoking AVA. Share the installed AVA version (get it by running `npx ava --version`) and `@ava/babel` version (from your `package.json` file).

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true
+contact_links:
+  - name: AVA on Spectrum
+    url: https://spectrum.chat/ava
+    about: Ask questions and discuss in our Spectrum community
+  - name: Stack Overflow
+    url: https://stackoverflow.com/questions/tagged/ava
+    about: Tag your question on Stack Overflow

.github/ISSUE_TEMPLATE/feature.md

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+---
+name: Feature Request
+about: Suggestions for new or different behavior
+labels: question
+---
+
+Please provide details about:
+
+* What you're trying to do
+* Why you can't use AVA's Babel support for this
+* And maybe how you think AVA could handle this

.github/workflows/ci.yml

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+name: Install and test @ava/typescript
+on:
+  push:
+    branches:
+    - master
+  pull_request:
+    paths-ignore:
+    - '*.md'
+jobs:
+  nodejs:
+    name: Node.js
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        node-version: [^10.18.0, ^12.14.0, ^13.5.0]
+        os: [ubuntu-latest, windows-latest]
+    steps:
+      - uses: actions/checkout@v1
+        with:
+          fetch-depth: 1
+      - uses: actions/setup-node@v1
+        with:
+          node-version: ${{ matrix.node-version }}
+      - run: npm install --no-audit
+      - run: npm test
+      - uses: codecov/codecov-action@v1
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}

.gitignore

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+/.nyc_output
+/coverage
+/node_modules

.npmrc

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+package-lock=false

README.md

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+# @ava/typescript
+
+Adds rudimentary [TypeScript](https://www.typescriptlang.org/) support to [AVA](https://avajs.dev).
+
+This is designed to work for projects that precompile TypeScript. It allows AVA to load the compiled JavaScript, while configuring AVA to treat the TypeScript files as test files.
+
+In other words, say you have a test file at `src/test.ts`. You've configured TypeScript to output to `build/`. Using `@ava/typescript` you can run the test using `npx ava src/test.ts`.
+
+## Enabling TypeScript support
+
+Add this package to your project:
+
+```console
+npm install --save-dev @ava/typescript
+```
+
+Then, enable TypeScript support either in `package.json` or `ava.config.*`:
+
+**`package.json`:**
+
+```json
+{
+	"ava": {
+		"typescript": {
+			"rewritePaths": {
+				"src/": "build/"
+			}
+		}
+	}
+}
+```
+
+Both keys and values of the `rewritePaths` object must end with a `/`. Paths are relative to your project directory.
+
+Output files are expected to have the `.js` extension.
+
+## Add additional extensions
+
+You can configure AVA to recognize additional file extensions. To add (partial†) JSX support:
+
+**`package.json`:**
+
+```json
+{
+	"ava": {
+		"typescript": {
+			"extensions": [
+				"ts",
+				"tsx"
+			],
+			"rewritePaths": {
+				"src/": "build/"
+			}
+		}
+	}
+}
+```
+
+See also AVA's [`extensions` option](https://github.com/avajs/ava/blob/master/docs/06-configuration.md#options).
+
+† Note that the [*preserve* mode for JSX](https://www.typescriptlang.org/docs/handbook/jsx.html) is not (yet) supported.

index.js

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+'use strict';
+const path = require('path');
+
+const escapeStringRegexp = require('escape-string-regexp');
+const isPlainObject = require('is-plain-object');
+
+const pkg = require('./package.json');
+
+function isValidExtensions(extensions) {
+	return Array.isArray(extensions) &&
+		extensions.length > 0 &&
+		extensions.every(ext => typeof ext === 'string' && ext !== '') &&
+		new Set(extensions).size === extensions.length;
+}
+
+function isValidRewritePaths(rewritePaths) {
+	if (!isPlainObject(rewritePaths)) {
+		return false;
+	}
+
+	return Object.entries(rewritePaths).every(([from, to]) => {
+		return from.endsWith('/') && typeof to === 'string' && to.endsWith('/');
+	});
+}
+
+module.exports = ({negotiateProtocol}) => {
+	const protocol = negotiateProtocol(['ava-3'], {version: pkg.version});
+	if (protocol === null) {
+		return;
+	}
+
+	return {
+		main({config}) {
+			let valid = false;
+			if (isPlainObject(config)) {
+				const keys = Object.keys(config);
+				if (keys.every(key => key === 'extensions' || key === 'rewritePaths')) {
+					valid =
+						(config.extensions === undefined || isValidExtensions(config.extensions)) &&
+						isValidRewritePaths(config.rewritePaths);
+				}
+			}
+
+			if (!valid) {
+				throw new Error(`Unexpected Typescript configuration for AVA. See https://github.com/avajs/typescript/blob/v${pkg.version}/README.md for allowed values.`);
+			}
+
+			const {
+				extensions = ['ts'],
+				rewritePaths
+			} = config;
+
+			return {
+				async compile() {
+					return {
+						extensions: extensions.slice(),
+						rewritePaths: Object.entries(rewritePaths).map(([from, to]) => [
+							path.join(protocol.projectDir, from),
+							path.join(protocol.projectDir, to)
+						])
+					};
+				},
+
+				get extensions() {
+					return extensions.slice();
+				}
+			};
+		},
+
+		worker({extensionsToLoadAsModules, state: {extensions, rewritePaths}}) {
+			const testFileExtension = new RegExp(`\\.(${extensions.map(ext => escapeStringRegexp(ext)).join('|')})$`);
+
+			return {
+				canLoad(ref) {
+					return testFileExtension.test(ref) && rewritePaths.some(([from]) => ref.startsWith(from));
+				},
+
+				async load(ref, {requireFn}) {
+					for (const extension of extensionsToLoadAsModules) {
+						if (ref.endsWith(`.${extension}`)) {
+							throw new Error('@ava/typescript cannot yet load ESM files');
+						}
+					}
+
+					const [from, to] = rewritePaths.find(([from]) => ref.startsWith(from));
+					// TODO: Support JSX preserve mode — https://www.typescriptlang.org/docs/handbook/jsx.html
+					const rewritten = `${to}${ref.slice(from.length)}`.replace(testFileExtension, '.js');
+					return requireFn(rewritten);
+				}
+			};
+		}
+	};
+};

package.json

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+{
+  "name": "@ava/typescript",
+  "version": "0.0.0",
+  "description": "TypeScript provider for AVA",
+  "engines": {
+    "node": ">=10.18.0 <11 || >=12.14.0 <13 || >=13.5.0"
+  },
+  "files": [
+    "index.js"
+  ],
+  "author": "Mark Wubben (https://novemberborn.net)",
+  "repository": "avajs/typescript",
+  "license": "MIT",
+  "keywords": [
+    "ava",
+    "typescript"
+  ],
+  "scripts": {
+    "test": "xo && nyc ava"
+  },
+  "dependencies": {
+    "escape-string-regexp": "^2.0.0",
+    "is-plain-object": "^3.0.0"
+  },
+  "devDependencies": {
+    "ava": "^3.0.0",
+    "execa": "^4.0.0",
+    "nyc": "^15.0.0",
+    "xo": "^0.25.3"
+  },
+  "nyc": {
+    "reporter": [
+      "html",
+      "lcov",
+      "text"
+    ]
+  }
+}

test/base.js

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+const test = require('ava');
+const makeProvider = require('..');
+
+test('bails when negotiating protocol returns `null`', t => {
+	const provider = makeProvider({negotiateProtocol: () => null});
+	t.is(provider, undefined);
+});

test/fixtures/file.js

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+console.log('logged in file.js');

test/fixtures/install-and-load.js

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+const path = require('path');
+const makeProvider = require('../..');
+
+const provider = makeProvider({
+	negotiateProtocol() {
+		return {identifier: process.argv[2], ava: {version: '3.0.0'}, projectDir: path.resolve(__dirname, '..')};
+	}
+});
+
+const worker = provider.worker({
+	extensionsToLoadAsModules: [],
+	state: JSON.parse(process.argv[3])
+});
+
+const ref = path.resolve(process.argv[4]);
+
+if (worker.canLoad(ref)) {
+	worker.load(ref, {requireFn: require});
+}