Add TypeScript ruleset & config

[robertrossmann]

There is a parser for TS: https://github.com/eslint/typescript-eslint-parser
And a ruleset: https://github.com/nzakas/eslint-plugin-typescript
And There is this thing, although with not much info about usage: https://github.com/JamesHenry/eslint-plugin-tslint

[robertrossmann]

For now, here are the steps necessary to make it work. Everything seems to be working correctly. I have not tested if the ESLint's rules actually work on TS-specific code constructs, but they should definitely work fine on constructs present in normal JS. 🎉

I'll be incorporating this into the module as a new ruleset later.

npm i -D eslint@latest @strv/eslint-config-javascript@latest eslint-plugin-typescript@latest typescript-eslint-parser@latest

// .eslintrc.js

'use strict'

module.exports = {
    parser: 'typescript-eslint-parser',

    extends: [
        '@strv/javascript/environments/nodejs/v10',
        '@strv/javascript/environments/nodejs/optional',
        '@strv/javascript/coding-styles/recommended',
    ],

    plugins: [
        'typescript',
    ],

    env: {
        es6: true,
    },

    parserOptions: {
        sourceType: 'module'
    },

    rules: {
        // TS code is mostly self-documented and having JSDoc directives for everything is redundant
        // when you can easily infer return values and argument types from the code itself.
        'valid-jsdoc': 'off',

        // Disabled because it generates false positives with interface declarations and TypeScript
        // blows up anyway during compilation when it encouters an undefined variable.
        'no-undef': 'off',

        // Prevent TypeScript-specific constructs from being erroneously flagged as unused
        'typescript/no-unused-vars': 'error',

        // @TODO: Add the remaining rules from eslint-plugin-typescript
    },

    overrides: [{
        files: ['**/*.test.ts'],
        env: {
            mocha: true
        },
    }],
}

# .eslintignore
node_modules
.nyc_output
coverage
docs

# Choose whichever you use
out
dist

To run ESLint on all source files: npx eslint --ext ts .