@rushstack/eslint-config/mixins/friendly-locals.js

// Copyright (c) Microsoft Corporation. All rights reserved. Licensed under the MIT license.
// See LICENSE in the project root for license information.

// For the first 5 years of Rush, our lint rules required explicit types for most declarations
// such as function parameters, function return values, and exported variables.  Although more verbose,
// declaring types (instead of relying on type inference) encourages engineers to create interfaces
// that inspire discussions about data structure design.  It also makes source files easier
// to understand for code reviewers who may be unfamiliar with a particular project.  Once developers get
// used to the extra work of declaring types, it turns out to be a surprisingly popular practice.
//
// However in 2020, to make adoption easier for existing projects, this rule was relaxed.  Explicit
// type declarations are now optional for local variables (although still required in other contexts).
// See this GitHub issue for background:
//
//  https://github.com/microsoft/rushstack/issues/2206
//
// If you are onboarding a large existing code base, this new default will make adoption easier.
//
// On the other hand, if your top priority is to make source files more friendly for other
// people to read, enable the "@rushstack/eslint-config/mixins/friendly-locals" mixin.
// It will restore the requirement that local variables should have explicit type declarations.
//
// IMPORTANT: Your .eslintrc.js "extends" field must load mixins AFTER the profile.
module.exports = {
  overrides: [
    {
      files: ['*.ts', '*.tsx'],
      rules: {
        '@rushstack/typedef-var': 'off', // <--- disabled by the mixin

        '@typescript-eslint/typedef': [
          'warn',
          {
            arrayDestructuring: false,
            arrowParameter: false,
            memberVariableDeclaration: true,
            objectDestructuring: false,
            parameter: true,
            propertyDeclaration: true,

            variableDeclaration: true, // <--- reenabled by the mixin

            variableDeclarationIgnoreFunction: true
          }
        ]
      }
    },
    // Note that the above block also applies to *.test.ts, so we need to
    // reapply those overrides.
    {
      files: [
        // Test files
        '*.test.ts',
        '*.test.tsx',
        '*.spec.ts',
        '*.spec.tsx',

        // Facebook convention
        '**/__mocks__/*.ts',
        '**/__mocks__/*.tsx',
        '**/__tests__/*.ts',
        '**/__tests__/*.tsx',

        // Microsoft convention
        '**/test/*.ts',
        '**/test/*.tsx'
      ],
      rules: {
        '@typescript-eslint/typedef': [
          'warn',
          {
            arrayDestructuring: false,
            arrowParameter: false,
            memberVariableDeclaration: true,
            objectDestructuring: false,
            parameter: true,
            propertyDeclaration: true,
            variableDeclaration: false, // <--- special case for test files
            variableDeclarationIgnoreFunction: true
          }
        ]
      }
    }
  ]
};