feat: Initial implementation

.c8rc.json

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+{
+	"include": ["index.js"],
+	"branches": 100,
+	"functions": 100,
+	"statements": 100,
+	"lines": 100
+}

.github/workflows/test.yml

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+name: Tests
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+
+env:
+  CI: true
+
+jobs:
+  test:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+        node-version: [13]
+    runs-on: ${{matrix.os}}
+    steps:
+    - uses: actions/checkout@v2
+    - name: Node.js ${{matrix.node-version}} on ${{matrix.os}}
+      uses: actions/setup-node@v1
+      with:
+        node-version: ${{matrix.node-version}}
+    - run: npm install
+    - name: Lint
+      run: npm run -s pretest
+    - name: Tests
+      run: npm run -s tests-only
+    - name: Coverage
+      run: npm run -s posttest

.gitignore

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+*.tgz
+package
+.nyc_output
+coverage
+node_modules

.npmignore

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+.github
+.c8rc.json
+*.tgz
+package
+coverage
+fixtures
+tap-snapshots
+test

.npmrc

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+package-lock=false
+access=public

LICENSE

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+ISC License
+
+Copyright (c) 2020, Contributors
+
+Permission to use, copy, modify, and/or distribute this software
+for any purpose with or without fee is hereby granted, provided
+that the above copyright notice and this permission notice
+appear in all copies.
+
+THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES
+OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE
+LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES
+OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS,
+WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION,
+ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

README.md

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+# @istanbuljs/esm-loader-hook
+
+![Tests][tests-status]
+[![NPM Version][npm-image]][npm-url]
+[![NPM Downloads][downloads-image]][downloads-url]
+[![ISC][license-image]](LICENSE)
+
+This [loader hook](https://nodejs.org/dist/latest/docs/api/esm.html#esm_hooks)
+makes it relatively easy to use NYC to check coverage of ESM running in node.js
+13.7.0.  Note this makes use of **experimental** node.js features and thus may
+stop working upon release of new versions of node.js.  Until the node.js feature
+is stabilized breakage should not be unexpected.
+
+For more stable options to test coverage you can:
+* Use [c8]
+* Pre-instrument your code (run `nyc instrument` then test the output)
+
+
+## Adding to processes
+
+To install this hook into a process the module must be provided through the
+`--experimental-loader` flag.
+
+The following can be used for your `npm test` script to enable live instrumentation
+of ES modules being tested with mocha:
+
+```sh
+cross-env 'NODE_OPTIONS=--experimental-loader @istanbuljs/esm-loader-hook' nyc mocha
+```
+
+
+## Configuration
+
+This module executes [babel-plugin-istanbul] in a transformSource loader hook.  No
+options are provided to the babel plugin and babel configuration files are not honored.
+Normally configuration will be provided by the currently running instance of nyc.  If
+this module is run outside nyc then it will use `@istanbuljs/load-nyc-config` to load
+options, defaults from `@istanbuljs/schema` will apply to missing options or if no
+configuration is found.
+
+
+[tests-status]: https://github.com/cfware/node-preload/workflows/Tests/badge.svg
+[npm-image]: https://img.shields.io/npm/v/@istanbuljs/esm-loader-hook.svg
+[npm-url]: https://npmjs.org/package/@istanbuljs/esm-loader-hook
+[downloads-image]: https://img.shields.io/npm/dm/@istanbuljs/esm-loader-hook.svg
+[downloads-url]: https://npmjs.org/package/@istanbuljs/esm-loader-hook
+[license-image]: https://img.shields.io/github/license/istanbuljs/esm-loader-hook
+
+[babel-plugin-istanbul]: https://github.com/istanbuljs/babel-plugin-istanbul#readme
+[c8]: https://github.com/bcoe/c8#readme

fixtures/hooked.js

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+export default () => 'value';

index.js

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+import {fileURLToPath} from 'url';
+import babel from '@babel/core';
+import loader from '@istanbuljs/load-nyc-config';
+import schema from '@istanbuljs/schema';
+import TestExclude from 'test-exclude';
+
+function nycEnvironmentConfig() {
+	try {
+		return JSON.parse(process.env.NYC_CONFIG);
+	} catch {
+		return null;
+	}
+}
+
+const initialCWD = process.env.NYC_CWD || process.cwd();
+let nycConfig = nycEnvironmentConfig();
+let testExclude;
+let babelConfig;
+
+export async function transformSource(source, context) {
+	if (nycConfig === null) {
+		nycConfig = {
+			...schema.defaults.nyc,
+			...await loader.loadNycConfig({cwd: initialCWD})
+		};
+	}
+
+	if (!testExclude) {
+		testExclude = new TestExclude(nycConfig);
+		babelConfig = {
+			...nycConfig,
+			// Skip/optimize the test-exclude used within babel-plugin-istanbul
+			include: [],
+			exclude: [],
+			extension: [],
+			excludeNodeModules: false
+		};
+	}
+
+	if (Buffer.isBuffer(source)) {
+		source = source.toString();
+	} else if (typeof source !== 'string') {
+		return {source};
+	}
+
+	const filename = fileURLToPath(context.url);
+	/* babel-plugin-istanbul does this but the early check optimizes */
+	if (!testExclude.shouldInstrument(filename)) {
+		return {source};
+	}
+
+	/* Can/should this handle inputSourceMap? */
+	const {code} = await babel.transformAsync(source, {
+		babelrc: false,
+		configFile: false,
+		filename,
+		/* Revisit this if transformSource adds support for returning sourceMap object */
+		sourceMaps: babelConfig.produceSourceMap ? 'inline' : false,
+		compact: babelConfig.compact,
+		comments: babelConfig.preserveComments,
+		parserOpts: {
+			sourceType: 'module',
+			plugins: babelConfig.parserPlugins
+		},
+		plugins: [
+			['babel-plugin-istanbul', babelConfig]
+		]
+	});
+
+	return {source: code};
+}

package.json

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+{
+	"name": "@istanbuljs/esm-loader-hook",
+	"version": "0.1.0",
+	"description": "Loader hook for ESM instrumentation (experimental!!)",
+	"type": "module",
+	"main": "index.js",
+	"exports": "./index.js",
+	"scripts": {
+		"pretest": "cfware-lint .",
+		"tests-only": "c8 -r none node test/index.js | tap-yaml-summary",
+		"test": "npm run -s tests-only",
+		"posttest": "c8 report --check-coverage",
+		"snap": "cross-env TAP_SNAPSHOT=1 npm test",
+		"release": "standard-version"
+	},
+	"engines": {
+		"node": ">=13.7.0"
+	},
+	"license": "ISC",
+	"repository": {
+		"type": "git",
+		"url": "git+https://github.com/istanbuljs/esm-loader-hook.git"
+	},
+	"bugs": {
+		"url": "https://github.com/istanbuljs/esm-loader-hook/issues"
+	},
+	"homepage": "https://github.com/istanbuljs/esm-loader-hook#readme",
+	"dependencies": {
+		"@babel/core": "^7.8.7",
+		"@istanbuljs/load-nyc-config": "^1.0.0",
+		"@istanbuljs/schema": "^0.1.2",
+		"babel-plugin-istanbul": "^6.0.0",
+		"test-exclude": "^6.0.0"
+	},
+	"devDependencies": {
+		"@cfware/lint": "^1.4.0",
+		"c8": "^7.1.0",
+		"cross-env": "^7.0.2",
+		"libtap": "^0.3.0",
+		"standard-version": "^7.1.0",
+		"tap-yaml-summary": "^0.1.0"
+	}
+}

tap-snapshots/test-transform-hook.js-TAP.test.cjs

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+/* IMPORTANT
+ * This snapshot file is auto-generated, but designed for humans.
+ * It should be checked into source control and tracked carefully.
+ * Re-generate by setting TAP_SNAPSHOT=1 and running tests.
+ * Make sure to inspect the output below.  Do not ignore changes!
+ */
+'use strict'
+exports[`test/transform-hook.js TAP > coverage after run 1`] = `
+Object {
+  "_coverageSchema": "",
+  "b": Object {},
+  "branchMap": Object {},
+  "f": Object {
+    "0": 1,
+  },
+  "fnMap": Object {
+    "0": Object {
+      "decl": Object {
+        "end": Object {
+          "column": 16,
+          "line": 1,
+        },
+        "start": Object {
+          "column": 15,
+          "line": 1,
+        },
+      },
+      "line": 1,
+      "loc": Object {
+        "end": Object {
+          "column": 28,
+          "line": 1,
+        },
+        "start": Object {
+          "column": 21,
+          "line": 1,
+        },
+      },
+      "name": "(anonymous_0)",
+    },
+  },
+  "hash": "",
+  "path": "hooked.js",
+  "s": Object {
+    "0": 1,
+  },
+  "schema": "",
+  "statementMap": Object {
+    "0": Object {
+      "end": Object {
+        "column": 28,
+        "line": 1,
+      },
+      "start": Object {
+        "column": 21,
+        "line": 1,
+      },
+    },
+  },
+}
+`
+
+exports[`test/transform-hook.js TAP > initial coverage 1`] = `
+Object {
+  "_coverageSchema": "",
+  "b": Object {},
+  "branchMap": Object {},
+  "f": Object {
+    "0": 0,
+  },
+  "fnMap": Object {
+    "0": Object {
+      "decl": Object {
+        "end": Object {
+          "column": 16,
+          "line": 1,
+        },
+        "start": Object {
+          "column": 15,
+          "line": 1,
+        },
+      },
+      "line": 1,
+      "loc": Object {
+        "end": Object {
+          "column": 28,
+          "line": 1,
+        },
+        "start": Object {
+          "column": 21,
+          "line": 1,
+        },
+      },
+      "name": "(anonymous_0)",
+    },
+  },
+  "hash": "",
+  "path": "hooked.js",
+  "s": Object {
+    "0": 0,
+  },
+  "schema": "",
+  "statementMap": Object {
+    "0": Object {
+      "end": Object {
+        "column": 28,
+        "line": 1,
+      },
+      "start": Object {
+        "column": 21,
+        "line": 1,
+      },
+    },
+  },
+}
+`