@@ -1,36 +1,24 @@
-# How to Contribute
-
-## Pull Requests
-
-We also accept [pull requests][pull-request]!
-
-Generally we like to see pull requests that
-
-- Maintain the existing code style
-- Are focused on a single change (i.e. avoid large refactoring or style adjustments in untouched code if not the primary goal of the pull request)
-- Have [good commit messages](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html)
-- Have tests
-- Don't decrease the current code coverage (see coverage/lcov-report/index.html)
-
-## Building
+## Building and testing
 
 ```
 yarn
 yarn test
 ```
 
-Running `yarn test -- dev` will watch for tests within Node and `karma start` may be used for manual testing in browsers.
+To run tests in a *browser* (for instance to test compatibility with Firefox, with Safari, or with old browser versions), run `yarn karma start`, then open http://localhost:9876/ in the browser you want to test in. Results of the test run will appear in the terminal where `yarn karma start` is running.
 
 If you notice any problems, please report them to the GitHub issue tracker at
 [http://github.com/kpdecker/jsdiff/issues](http://github.com/kpdecker/jsdiff/issues).
 
 ## Releasing
 
+Run a test in Firefox via the procedure above before releasing.
+
 A full release may be completed by first updating the `"version"` property in package.json, then running the following:
 
 ```
 yarn clean
-yarn grunt release
+yarn build
 yarn publish
 ```
 
 
@@ -0,0 +1,182 @@
+// @ts-check
+
+import eslint from '@eslint/js';
+import tseslint from 'typescript-eslint';
+import globals from "globals";
+
+export default tseslint.config(
+  {
+    ignores: [
+      "**/*", // ignore everything...
+      "!src/**/", "!src/**/*.ts", // ... except our TypeScript source files...
+      "!test/**/", "!test/**/*.js", // ... and our tests
+    ],
+  },
+  eslint.configs.recommended,
+  tseslint.configs.recommended,
+  {
+    files: ['src/**/*.ts'],
+    languageOptions: {
+      parserOptions: {
+        projectService: true,
+        tsconfigRootDir: import.meta.dirname,
+      },
+    },
+    extends: [tseslint.configs.recommendedTypeChecked],
+    rules: {
+      // Not sure if these actually serve a purpose, but they provide a way to enforce SOME of what
+      // would be imposed by having "verbatimModuleSyntax": true in our tsconfig.json without
+      // actually doing that.
+      "@typescript-eslint/consistent-type-imports": 2,
+      "@typescript-eslint/consistent-type-exports": 2,
+
+      // Things from the recommendedTypeChecked shared config that are disabled simply because they
+      // caused lots of errors in our existing code when tried. Plausibly useful to turn on if
+      // possible and somebody fancies doing the work:
+      "@typescript-eslint/no-unsafe-argument": 0,
+      "@typescript-eslint/no-unsafe-assignment": 0,
+      "@typescript-eslint/no-unsafe-call": 0,
+      "@typescript-eslint/no-unsafe-member-access": 0,
+      "@typescript-eslint/no-unsafe-return": 0,
+    }
+  },
+  {
+    languageOptions: {
+      globals: {
+        ...globals.browser,
+      },
+    },
+
+    rules: {
+      // Possible Errors //
+      //-----------------//
+      "comma-dangle": [2, "never"],
+      "no-console": 1, // Allow for debugging
+      "no-debugger": 1, // Allow for debugging
+      "no-extra-parens": [2, "functions"],
+      "no-extra-semi": 2,
+      "no-negated-in-lhs": 2,
+      "no-unreachable": 1, // Optimizer and coverage will handle/highlight this and can be useful for debugging
+
+      // Best Practices //
+      //----------------//
+      curly: 2,
+      "default-case": 1,
+      "dot-notation": [2, {
+        allowKeywords: false,
+      }],
+      "guard-for-in": 1,
+      "no-alert": 2,
+      "no-caller": 2,
+      "no-div-regex": 1,
+      "no-eval": 2,
+      "no-extend-native": 2,
+      "no-extra-bind": 2,
+      "no-floating-decimal": 2,
+      "no-implied-eval": 2,
+      "no-iterator": 2,
+      "no-labels": 2,
+      "no-lone-blocks": 2,
+      "no-multi-spaces": 2,
+      "no-multi-str": 1,
+      "no-native-reassign": 2,
+      "no-new": 2,
+      "no-new-func": 2,
+      "no-new-wrappers": 2,
+      "no-octal-escape": 2,
+      "no-process-env": 2,
+      "no-proto": 2,
+      "no-return-assign": 2,
+      "no-script-url": 2,
+      "no-self-compare": 2,
+      "no-sequences": 2,
+      "no-throw-literal": 2,
+      "no-unused-expressions": 2,
+      "no-warning-comments": 1,
+      radix: 2,
+      "wrap-iife": 2,
+
+      // Variables //
+      //-----------//
+      "no-catch-shadow": 2,
+      "no-label-var": 2,
+      "no-undef-init": 2,
+
+      // Node.js //
+      //---------//
+
+      // Stylistic //
+      //-----------//
+      "brace-style": [2, "1tbs", {
+        allowSingleLine: true,
+      }],
+      camelcase: 2,
+      "comma-spacing": [2, {
+        before: false,
+        after: true,
+      }],
+      "comma-style": [2, "last"],
+      "consistent-this": [1, "self"],
+      "eol-last": 2,
+      "func-style": [2, "declaration"],
+      "key-spacing": [2, {
+        beforeColon: false,
+        afterColon: true,
+      }],
+      "new-cap": 2,
+      "new-parens": 2,
+      "no-array-constructor": 2,
+      "no-lonely-if": 2,
+      "no-mixed-spaces-and-tabs": 2,
+      "no-nested-ternary": 1,
+      "no-new-object": 2,
+      "no-spaced-func": 2,
+      "no-trailing-spaces": 2,
+      "quote-props": [2, "as-needed", {
+        keywords: true,
+      }],
+      quotes: [2, "single", "avoid-escape"],
+      semi: 2,
+      "semi-spacing": [2, {
+        before: false,
+        after: true,
+      }],
+      "space-before-blocks": [2, "always"],
+      "space-before-function-paren": [2, {
+        anonymous: "never",
+        named: "never",
+      }],
+      "space-in-parens": [2, "never"],
+      "space-infix-ops": 2,
+      "space-unary-ops": 2,
+      "spaced-comment": [2, "always"],
+      "wrap-regex": 1,
+      "no-var": 2,
+
+      // Typescript //
+      //------------//
+      "@typescript-eslint/no-explicit-any": 0, // Very strict rule, incompatible with our code
+
+      // We use these intentionally - e.g.
+      //     export interface DiffCssOptions extends CommonDiffOptions {}
+      // for the options argument to diffCss which currently takes no options beyond the ones
+      // common to all diffFoo functions. Doing this allows consistency (one options interface per
+      // diffFoo function) and future-proofs against the API having to change in future if we add a
+      // non-common option to one of these functions.
+      "@typescript-eslint/no-empty-object-type": [2, {allowInterfaces: 'with-single-extends'}],
+    },
+  },
+  {
+    files: ['test/**/*.js'],
+    languageOptions: {
+      globals: {
+        ...globals.node,
+        ...globals.mocha,
+      },
+    },
+    rules: {
+      "no-unused-expressions": 0, // Needs disabling to support Chai `.to.be.undefined` etc syntax
+      "@typescript-eslint/no-unused-expressions": 0, // (as above)
+    },
+  }
+);