[OP-18993] Add TypeScript tooling (1/3)

.github/workflows/ci.yml

@@ -33,3 +33,4 @@ jobs:
     - run: npm run build
     - run: npm test
     - run: npm run lint
+    - run: npm run typecheck

README.md

@@ -56,6 +56,15 @@ See https://community.openproject.org/work_packages/47084 for context.
 Now the webpack development mode is building the files and outputting them to `frontend/src/vendor/ckeditor/*` in the
 core repository, overriding anything in there.
 
+### Type checking
+
+The source is being converted to TypeScript. Files ending in `.ts` are checked in strict mode; files still ending in
+`.js` are not checked. Webpack and jest strip types without checking them, so run the checker separately:
+
+```
+npm run typecheck
+```
+
 ## Migration Notes
 
 ### jQuery Removal

babel.config.js

@@ -8,6 +8,15 @@ module.exports = {
 				},
 			},
 		],
+		[
+			"@babel/preset-typescript",
+			{
+				// Match tsc: keep imports unless written as `import type`,
+				// and let `declare` fields emit nothing.
+				onlyRemoveTypeImports: true,
+				allowDeclareFields: true,
+			},
+		],
 	],
 	plugins: ["@babel/plugin-transform-modules-commonjs"]
 }

eslint.config.mjs

@@ -1,6 +1,43 @@
 import globals from "globals";
 import eslint from '@eslint/js';
 import jestPlugin from 'eslint-plugin-jest';
+import tseslint from 'typescript-eslint';
+
+const tsFiles = ["src/**/*.ts", "tests/**/*.ts"];
+
+const tsRules = {
+	// TypeScript reports undefined names itself.
+	"no-undef": "off",
+	"no-cond-assign": "off",
+	// TODO(OP-18993): re-enable once the conversion has merged. It changes
+	// types only, and these would demand edits to runtime code that the
+	// JavaScript rules never asked for.
+	"prefer-const": "off",
+	"no-var": "off",
+	"prefer-rest-params": "off",
+	"prefer-spread": "off",
+	"@typescript-eslint/no-this-alias": "off",
+	"@typescript-eslint/no-unused-expressions": "off",
+	"no-unused-vars": "off",
+	"@typescript-eslint/no-unused-vars": [
+		"error",
+		{
+			"argsIgnorePattern": "^_",
+			"caughtErrorsIgnorePattern": "^_",
+			"varsIgnorePattern": "^_"
+		}
+	],
+	"@typescript-eslint/no-explicit-any": "error",
+	"@typescript-eslint/ban-ts-comment": [
+		"error",
+		{
+			"ts-expect-error": "allow-with-description",
+			"ts-ignore": true,
+			"ts-nocheck": true,
+			"minimumDescriptionLength": 10
+		}
+	]
+};
 
 export default [
 	eslint.configs.recommended,
@@ -75,4 +112,28 @@ export default [
 			"no-undef": "error"
 		}
 	},
+	...tseslint.configs.recommended.map(config => ({ ...config, files: tsFiles })),
+	{
+		files: ["src/**/*.ts"],
+		languageOptions: {
+			globals: {
+				...globals.browser
+			}
+		},
+		rules: tsRules
+	},
+	{
+		files: ["tests/**/*.ts"],
+		plugins: {
+			jest: jestPlugin
+		},
+		languageOptions: {
+			globals: {
+				...globals.browser,
+				...globals.node,
+				...globals.jest
+			}
+		},
+		rules: tsRules
+	},
 ];

jest.config.js

@@ -17,7 +17,7 @@ module.exports = {
 	],
 	// A map from regular expressions to paths to transformers
 	transform: {
-		'^.+\\.js$': 'babel-jest',
+		'^.+\\.(js|ts)$': 'babel-jest',
 	},
 	// A map from regular expressions to module names or to arrays of module names that allow to stub out resources with a single module
 	moduleNameMapper: {