Fix #9 lint, fix #7 negative cases, fix #8 fetch reference error

Author: boly38

.github/CONTRIBUTING.md

@@ -9,6 +9,15 @@ Please create an [issue](https://github.com/boly38/dropbox-refresh-token/issues)
 If you want to push some code :
 - following next `HowTo push some code` guide,
 - create a [pull request](https://github.com/boly38/dropbox-refresh-token/pulls) that link your issue using `#<issue_id>`.
+- execute linter (you could suggest test)
+
+## linter
+*  launch lint using `npm run lint`.
+
+About linter :
+- locally ESLint 9.0 is used as dev dependencies and rely on `eslint.config.js` ([doc](https://eslint.org/docs/latest/use/configure/configuration-files))
+- on Github PR, [HoundCi service](https://houndci.com) is triggered and rely on [`.hound.yml`](../.hound.yml) file and derived file. HoundCi is yet not compatible with 9.0 config file ([src](http://help.houndci.com/en/articles/2461415-supported-linters) - [eslint 8.0 config file doc](https://eslint.org/docs/v8.x/use/configure/configuration-files).
+
 
 
 # Maintainers HowTos

.hound.yml

@@ -0,0 +1,8 @@
+jshint:
+  config_file: .jshintrc
+# ESLint supported version
+# http://help.houndci.com/en/articles/2461415-supported-linters
+eslint:
+  enabled: true
+  version: 8.1.0
+  config_file: .hound_eslint_config.json

.hound_eslint_config.json

@@ -0,0 +1,19 @@
+{
+    "root": true,
+    "extends": [
+        "eslint:recommended"
+    ],
+    "env" : {
+        "browser" : true,
+        "node" : true,
+        "es6" : true
+    },
+    "parserOptions": {
+        "sourceType": "module",
+        "ecmaVersion": 2022
+    },
+    "plugins": [
+    ],
+    "rules": {
+    }
+}

eslint.config.js

@@ -0,0 +1,33 @@
+// https://eslint.org/docs/latest
+import js from "@eslint/js";
+import globals from "globals";
+import mochaPlugin from 'eslint-plugin-mocha'; // https://www.npmjs.com/package/eslint-plugin-mocha
+import stylisticJs from '@stylistic/eslint-plugin-js'
+export default [
+    js.configs.recommended, // Recommended config applied to all files
+    mochaPlugin.configs.flat.recommended, // or `mochaPlugin.configs.flat.all` to enable all
+    {
+        "files": ["src/**/*.js"],
+        "languageOptions": {
+            ecmaVersion: 2022,
+            sourceType: "module",
+            globals: {
+                ...globals.node,
+                "console": true,
+                "process": true,
+                "btoa": true
+            },
+            "parserOptions": {
+                "sourceType": "module",
+                "allowImportExportEverywhere": true
+            },
+        },
+        plugins: {
+            '@stylistic/js': stylisticJs
+        },
+        "rules": {
+            "mocha/no-mocha-arrows": "off", /* pff */
+            '@stylistic/js/no-extra-semi': "error", /* to match houndci : https://eslint.org/docs/latest/rules/no-extra-semi */
+        }
+    }
+];