Add test-specific ruleset and disable prefer-arrow-callback for tests.

Allow use of "as Type" syntax for type assertions in test code.

Dont force symbol descriptors in test code

.eslintrc.js

@@ -115,9 +115,10 @@ module.exports = {
  // Require class properties and methods to explicitly use accessibility modifiers (public, private, protected)
  '@typescript-eslint/explicit-member-accessibility': 'error',
 
- // Forbids an object literal to appear in a type assertion expression unless its used as a parameter. This
- // allows the typechecker to perform validation on the value as an assignment, instead of allowing the type
- // assertion to always win.
+ // Forbids an object literal to appear in a type assertion expression unless its used as a parameter (we violate
+ // this rule for test code, to allow for looser property matching for objects - more in the test-specific rules
+ // section below). This allows the typechecker to perform validation on the value as an assignment, instead of
+ // allowing the type assertion to always win.
  // Requires use of `as Type` instead of `<Type>` for type assertion. Consistency.
  '@typescript-eslint/consistent-type-assertions': ['error', {
  assertionStyle: 'as',
@@ -224,7 +225,7 @@ module.exports = {
  },
  {
  'selector': 'objectLiteralProperty',
- format: ['camelCase', 'snake_case', 'PascalCase'] 
+ format: ['camelCase', 'snake_case', 'PascalCase'],
  },
  {
  selector: ['enumMember'],
@@ -256,6 +257,36 @@ module.exports = {
 
  },
  },
+ {
+ files: ['src/**/*.spec.ts'],
+ rules: {
+ // Test-specific rules
+ // ---
+ // Rules that only apply to Typescript _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',
+
+ // Unlike non-test-code, where we require use of `as Type` instead of `<Type>` for type assertion,
+ // in test code using the looser `as Type` syntax leads to easier test writing, since only required
+ // properties must be adhered to using the `as Type` syntax.
+ '@typescript-eslint/consistent-type-assertions': ['error', {
+ assertionStyle: 'as',
+ objectLiteralTypeAssertions: 'allow',
+ }],
+ // In tests, don't force constructing a Symbol with a descriptor, as
+ // it's probably just for tests
+ 'symbol-description': 'off',
+ },
+ },
  ],
 };