- Added 'clean-and-run-downlevel-dts' script for running 'clean', 'do…

…wnlevel-dts', 'copy' steps for downlevel-dts output directory ('./ts3.5').

- Added generated 'downleveled dts' files (keeping this in the git cache
since we're already keeping the library's artifacts there - more
consistent).

package.json

@@ -4,9 +4,18 @@
   "description": "Rollup Sass files.",
   "main": "dist/index.js",
   "typings": "dist/index.d.ts",
+  "typesVersions": {
+    "<4.0": {
+      "*": [
+        "ts3.5/*"
+      ]
+    }
+  },
   "scripts": {
     "prepare": "npm run build && npm test",
-    "build": "tsc --project tsconfig.json",
+    "build": "npm run build-downlevel-dts && tsc --project tsconfig.json",
+    "build-downlevel-dts": "node scripts/clean-and-run-downlevel-dts.js",
+    "downlevel-dts": "downlevel-dts . ts3.5 [--to=3.5]",
     "test": "nyc --reporter=html --reporter=text ava ./test/*.test.ts -s && npm run test:rollup.config.spec.ts",
     "coverage": "nyc report --reporter=text-lcov | coveralls",
     "test:rollup.config.spec.ts": "tsc --project tsconfig.spec.json --noEmit"
@@ -59,6 +68,7 @@
     "@types/sass": "^1.16.1",
     "ava": "^4.3.1",
     "coveralls": "^3.1.1",
+    "downlevel-dts": "^0.10.0",
     "eslint": "^7.32.0",
     "jsdom": "^17.0.0",
     "nyc": "^15.1.0",

scripts/clean-and-run-downlevel-dts.js

@@ -0,0 +1,47 @@
+/**
+ * @script clean-and-run-downlevel-dts.js
+ * @description Removes '{repo-root}/ts3.5' directory, re-creates it, runs
+ *  `npm run downlevel-dts` from repo-root, and copies the root tsconfig.json file into new directory.
+ */
+ const fs = require('fs').promises,
+  path = require('path'),
+  {spawn} = require('child_process'),
+
+  {log, warn, error} = console,
+
+  rootDir = path.join(__dirname, '..'),
+  outputDir = path.resolve(path.join(__dirname, '../ts3.5')),
+
+  tsConfigFilePath = path.join(rootDir, 'tsconfig.json'),
+  tsConfigOutFilePath = path.join(outputDir, 'tsconfig.json');
+
+(async () =>
+    fs.rmdir(outputDir, {recursive: true})
+
+      .then(() => fs.mkdir(outputDir))
+
+      .then(() => fs.copyFile(tsConfigFilePath, tsConfigOutFilePath))
+
+      // Run downlevel-dts
+      .then(() => new Promise((resolve, reject) => {
+
+        // Start downlevel-dts package script
+        const subProcess = spawn('npm', ['run', 'downlevel-dts'], {cwd: rootDir});
+
+        // Log child process buffer data as `string`
+        subProcess.stdout.on('data', data => log(data.toString().trim() + '\n'));
+
+        // Log stderr, child process, buffer data as a `string`
+        subProcess.stderr.on('data', data => warn(data.toString().trim() + '\n'));
+
+        // Handle process end
+        subProcess.on('close', (code) => code !== 0 ?
+          reject(`Child process existed with code ${code}.\n`) :
+          resolve('Process completed successfully.\n')
+        );
+
+        // Catch process start errors
+        subProcess.on('error', reject);
+      }))
+
+)().then(log, error);