added eslint@6.0.0, gulp-eslint, babel-eslint dev dependencies. added linting to the concatenated source. removed a duplicate function to allow it to build.

gulpfile.js

@@ -2,6 +2,7 @@ const { task, src, dest, series, watch } = require("gulp");
 const concat = require("gulp-concat");
 const del = require("del");
 const vinylPaths = require("vinyl-paths");
+const eslint = require("gulp-eslint");
 var sass = require("gulp-sass");
 sass.compiler = require("dart-sass");
 
@@ -20,7 +21,12 @@ const gulpSrc = [
 ];
 
 task("cat", function () {
-  return src(gulpSrc).pipe(concat("monkeytype.js")).pipe(dest("./dist/js"));
+  return src(gulpSrc)
+    .pipe(concat("monkeytype.js"))
+    .pipe(eslint(eslintConfig))
+    .pipe(eslint.format())
+    .pipe(eslint.failAfterError())
+    .pipe(dest("./dist/js"));
 });
 
 task("sass", function () {
@@ -44,3 +50,69 @@ task("watch", function () {
 });
 
 task("rebuild", series("clean", "build"));
+
+let eslintConfig = {
+  parser: "babel-eslint",
+  extends: "eslint:recommended",
+  globals: ["jQuery", "$", "firebase"],
+  envs: ["es6", "browser", "node"],
+  rules: {
+    "constructor-super": "error",
+    "for-direction": "error",
+    "getter-return": "error",
+    "no-async-promise-executor": "error",
+    "no-case-declarations": "error",
+    "no-class-assign": "error",
+    "no-compare-neg-zero": "error",
+    "no-cond-assign": "error",
+    "no-const-assign": "error",
+    "no-constant-condition": "error",
+    "no-control-regex": "error",
+    "no-debugger": "error",
+    "no-delete-var": "error",
+    "no-dupe-args": "error",
+    "no-dupe-class-members": "error",
+    "no-dupe-else-if": "error",
+    "no-dupe-keys": "error",
+    "no-duplicate-case": "error",
+    "no-empty": "error",
+    "no-empty-character-class": "error",
+    "no-empty-pattern": "error",
+    "no-ex-assign": "error",
+    "no-extra-boolean-cast": "error",
+    "no-extra-semi": "error",
+    "no-fallthrough": "error",
+    "no-func-assign": "error",
+    "no-global-assign": "error",
+    "no-import-assign": "error",
+    "no-inner-declarations": "error",
+    "no-invalid-regexp": "error",
+    "no-irregular-whitespace": "error",
+    "no-misleading-character-class": "error",
+    "no-mixed-spaces-and-tabs": "error",
+    "no-new-symbol": "error",
+    "no-obj-calls": "error",
+    "no-octal": "error",
+    "no-prototype-builtins": "error",
+    "no-redeclare": "error",
+    "no-regex-spaces": "error",
+    "no-self-assign": "error",
+    "no-setter-return": "error",
+    "no-shadow-restricted-names": "error",
+    "no-sparse-arrays": "error",
+    "no-this-before-super": "error",
+    "no-undef": "error",
+    "no-unexpected-multiline": "error",
+    "no-unreachable": "error",
+    "no-unsafe-finally": "error",
+    "no-unsafe-negation": "error",
+    "no-unused-labels": "error",
+    "no-unused-vars": "error",
+    "no-useless-catch": "error",
+    "no-useless-escape": "error",
+    "no-with": "error",
+    "require-yield": "error",
+    "use-isnan": "error",
+    "valid-typeof": "error",
+  },
+};

package.json

@@ -13,11 +13,14 @@
     "node": "10"
   },
   "devDependencies": {
+    "babel-eslint": "^10.1.0",
     "concurrently": "^5.3.0",
     "dart-sass": "^1.25.0",
     "del": "^6.0.0",
+    "eslint": "^6.0.0",
     "gulp": "^4.0.2",
     "gulp-concat": "^2.6.1",
+    "gulp-eslint": "^6.0.0",
     "gulp-sass": "^4.1.0",
     "husky": "^4.3.0",
     "prettier": "2.1.2",

src/js/commandline.js

@@ -1,7 +1,3 @@
-function capitalizeFirstLetter(str) {
-  return str.charAt(0).toUpperCase() + str.slice(1);
-}
-
 function addChildCommands(
   unifiedCommands,
   commandItem,