add eslint

.eslintignore

@@ -0,0 +1,2 @@
+node_modules
+dist

.eslintrc.js

@@ -0,0 +1,132 @@
+//
+// SlackAPI JavaScript style
+// ---
+// This style helps maintainers enforce safe and consistent programming practices in this project. It is not meant to be
+// comprehensive on its own or vastly different from existing styles. The goal is to inherit and aggregate as many of
+// the communities' recommended styles for the technologies used as we can. When, and only when, we have a stated need
+// to differentiate, we add more rules (or modify options). Therefore, the fewer rules directly defined in this file,
+// the better.
+
+module.exports = {
+  // This is a root of the project, ESLint should not look through parent directories to find more config
+  root: true,
+
+  ignorePatterns: [
+    // Ignore all build outputs and artifacts (node_modules, dotfiles, and dot directories are implicitly ignored)
+    '/dist',
+    '/coverage',
+  ],
+
+  // These environments contain lists of global variables which are allowed to be accessed
+  env: {
+    // According to https://node.green, the target node version (v10) supports all important ES2018 features. But es2018
+    // is not an option since it presumably doesn't introduce any new globals over ES2017.
+    es2017: true,
+    node: true,
+  },
+
+  extends: [
+    // ESLint's recommended built-in rules: https://eslint.org/docs/rules/
+    'eslint:recommended',
+
+    // Node plugin's recommended rules: https://github.com/mysticatea/eslint-plugin-node
+    'plugin:node/recommended',
+
+    // AirBnB style guide (without React) rules: https://github.com/airbnb/javascript.
+    'airbnb-base',
+
+    // JSDoc plugin's recommended rules
+    'plugin:jsdoc/recommended',
+  ],
+
+  rules: {
+    // JavaScript rules
+    // ---
+    // The top level of this configuration contains rules which apply to JavaScript (and will also be inherited for
+    // TypeScript). This section does not contain rules meant to override options or disable rules in the base
+    // configurations (ESLint, Node, AirBnb). Those rules are added in the final override.
+
+    // Eliminate tabs to standardize on spaces for indentation. If you want to use tabs for something other than
+    // indentation, you may need to turn this rule off using an inline config comments.
+    'no-tabs': 'error',
+
+    // Bans use of comma as an operator because it can obscure side effects and is often an accident.
+    'no-sequences': 'error',
+
+    // This repo uses console.log, which is fine to do in GitHub actions
+    'no-console': 'off',
+
+    // Disallow the use of process.exit()
+    'node/no-process-exit': 'error',
+
+    // Allow safe references to functions before the declaration. Overrides AirBnB config. Not located in the override
+    // section below because a distinct override is necessary in TypeScript files.
+    'no-use-before-define': ['error', 'nofunc'],
+  },
+
+  overrides: [
+    {
+      files: ['**/*.js'],
+      rules: {
+        // Override rules
+        // ---
+        // This level of this configuration contains rules which override options or disable rules in the base
+        // configurations in JavaScript.
+
+        // Increase the max line length to 120. The rest of this setting is copied from the AirBnB config.
+        'max-len': ['error', 120, 2, {
+          ignoreUrls: true,
+          ignoreComments: false,
+          ignoreRegExpLiterals: true,
+          ignoreStrings: true,
+          ignoreTemplateLiterals: true,
+        }],
+
+        // Restrict the use of backticks to declare a normal string. Template literals should only be used when the
+        // template string contains placeholders. The rest of this setting is copied from the AirBnb config.
+        quotes: ['error', 'single', { avoidEscape: true, allowTemplateLiterals: false }],
+
+        // the server side Slack API uses snake_case for parameters often
+        // for mocking and override support, we need to allow snake_case
+        // Allow leading underscores for parameter names, which is used to acknowledge unused variables in TypeScript.
+        // Also, enforce camelCase naming for variables. Ideally, the leading underscore could be restricted to only
+        // unused parameter names, but this rule isn't capable of knowing when a variable is unused. The camelcase and
+        // no-underscore-dangle rules are replaced with the naming-convention rule because this single rule can serve
+        // both purposes, and it works fine on non-TypeScript code.
+        camelcase: 'off',
+        'no-underscore-dangle': 'off',
+
+        // Remove the minProperties option for enforcing line breaks between braces. The AirBnB config sets this to 4,
+        // which is arbitrary and not backed by anything specific in the style guide. If we just remove it, we can
+        // rely on the max-len rule to determine if the line is too long and then enforce line breaks. Overrides AirBnB
+        // styles.
+        'object-curly-newline': ['error', { multiline: true, consistent: true }],
+      },
+    },
+    {
+      files: ['src/test/*.js'],
+      rules: {
+        // Test-specific rules
+        // ---
+        // Rules that only apply to JavaScript _test_ source files
+
+        // With Mocha as a test framework, it is sometimes helpful to assign
+        // shared state to Mocha's Context object, for example in setup and
+        // teardown test methods. Assigning stub/mock objects to the Context
+        // object via `this` is a common pattern in Mocha. As such, using
+        // `function` over the the arrow notation binds `this` appropriately and
+        // should be used in tests. So: we turn off the prefer-arrow-callback
+        // rule.
+        // See https://github.com/slackapi/bolt-js/pull/1012#pullrequestreview-711232738
+        // for a case of arrow-vs-function syntax coming up for the team
+        'prefer-arrow-callback': 'off',
+
+        // Using ununamed functions (e.g., null logger) in tests is fine
+        'func-names': 'off',
+        // In tests, don't force constructing a Symbol with a descriptor, as
+        // it's probably just for tests
+        'symbol-description': 'off',
+      },
+    },
+  ],
+};