Using ESLint and Prettier in a TypeScript Project

Using ESLint and Prettier in a TypeScript Project

ESLint's large set of linting rules and the increased commitment to use ESLint by the TypeScript team makes ESLint a great tool for linting.

ESLint's large set of linting rules and the increased commitment to use ESLint by the TypeScript team makes ESLint a great tool for linting

When it comes to linting TypeScript code, there are two major linting options to choose from: TSLint and ESLint. TSLint is a linter than can only be used for TypeScript, while ESLint supports both JavaScript and TypeScript.

In the TypeScript 2019 Roadmap, the TypeScript core team explains that ESLint has a more performant architecture than TSLint and that they will only be focusing on ESLint when providing editor linting integration for TypeScript. For that reason, I would recommend using ESLint for linting TypeScript projects.

Setting up ESLint to work with TypeScript

First, install all the required dev dependencies:

yarn add eslint @typescript-eslint/parser @typescript-eslint/eslint-plugin --dev


  • [eslint](https://www.npmjs.com/package/eslint "eslint"): The core ESLint linting library
  • [@typescript-eslint/parser](https://www.npmjs.com/package/@typescript-eslint/parser "@typescript-eslint/parser"): The parser that will allow ESLint to lint TypeScript code
  • [@typescript-eslint/eslint-plugin](https://www.npmjs.com/package/@typescript-eslint/eslint-plugin "@typescript-eslint/eslint-plugin"): A plugin that contains a bunch of ESLint rules that are TypeScript specific

Next, add an .eslintrc.js configuration file in the root project directory. Here is a sample configuration for a TypeScript project:

module.exports =  {
  parser:  '@typescript-eslint/parser',  // Specifies the ESLint parser
  extends:  [
    'plugin:@typescript-eslint/recommended',  // Uses the recommended rules from the @typescript-eslint/eslint-plugin
  ],
 parserOptions:  {
    ecmaVersion:  2018,  // Allows for the parsing of modern ECMAScript features
    sourceType:  'module',  // Allows for the use of imports
  },
  rules:  {
    // Place to specify ESLint rules. Can be used to overwrite rules specified from the extended configs
    // e.g. "@typescript-eslint/explicit-function-return-type": "off",
  },
};


Note: I prefer using a JavaScript file for the .eslintrc file (instead of a JSON file) as it supports comments that can be used to better describe rules.

If using TypeScript with React, the [eslint-plugin-react](https://www.npmjs.com/package/eslint-plugin-react "eslint-plugin-react") dev dependency should be installed and the following configuration can be used:

module.exports =  {
  parser:  '@typescript-eslint/parser',  // Specifies the ESLint parser
  extends:  [
    'plugin:react/recommended',  // Uses the recommended rules from @eslint-plugin-react
    'plugin:@typescript-eslint/recommended',  // Uses the recommended rules from @typescript-eslint/eslint-plugin
  ],
  parserOptions:  {
  ecmaVersion:  2018,  // Allows for the parsing of modern ECMAScript features
  sourceType:  'module',  // Allows for the use of imports
  ecmaFeatures:  {
    jsx:  true,  // Allows for the parsing of JSX
  },
  },
  rules:  {
    // Place to specify ESLint rules. Can be used to overwrite rules specified from the extended configs
    // e.g. "@typescript-eslint/explicit-function-return-type": "off",
  },
  settings:  {
    react:  {
      version:  'detect',  // Tells eslint-plugin-react to automatically detect the version of React to use
    },
  },
};


Ultimately it’s up to you to decide what rules you would like to extend from and which ones to use within the rules object in your .eslintrc.js file.

Adding Prettier to the mix

What works well along with ESLint is prettier, which does a great job at handling code formatting. Install the required dev dependencies to get prettier working with ESLint:

yarn add prettier eslint-config-prettier eslint-plugin-prettier --dev

  • [eslint](https://www.npmjs.com/package/eslint "eslint"): The core ESLint linting library
  • [@typescript-eslint/parser](https://www.npmjs.com/package/@typescript-eslint/parser "@typescript-eslint/parser"): The parser that will allow ESLint to lint TypeScript code
  • [@typescript-eslint/eslint-plugin](https://www.npmjs.com/package/@typescript-eslint/eslint-plugin "@typescript-eslint/eslint-plugin"): A plugin that contains a bunch of ESLint rules that are TypeScript specific

In order to configure prettier, a .prettierrc.js file is required at the root project directory. Here is a sample .prettierrc.js file:

module.exports =  {
  semi:  true,
  trailingComma:  'all',
  singleQuote:  true,
  printWidth:  120,
  tabWidth:  4,
};


Next, the .eslintrc.js file needs to be updated:

module.exports =  {
  parser:  '@typescript-eslint/parser',  // Specifies the ESLint parser
  extends:  [
    'plugin:@typescript-eslint/recommended',  // Uses the recommended rules from the @typescript-eslint/eslint-plugin
    'prettier/@typescript-eslint',  // Uses eslint-config-prettier to disable ESLint rules from @typescript-eslint/eslint-plugin that would conflict with prettier
    'plugin:prettier/recommended',  // Enables eslint-plugin-prettier and displays prettier errors as ESLint errors. Make sure this is always the last configuration in the extends array.
  ],
  parserOptions:  {
    ecmaVersion:  2018,  // Allows for the parsing of modern ECMAScript features
    sourceType:  'module',  // Allows for the use of imports
  },
};


The advantage of having prettier setup as an ESLint rule using eslint-plugin-prettier is that code can automatically be fixed using ESLint’s --fix option.

Automatically Fixing Code (VS Code)

For a good developer experience, it’s useful to setup your editor to automatically run ESLint’s automatic fix command (i.e. eslint --fix) whenever a file is saved. Since i’m using VS Code, here is the config required in the settings.json file in VS Code to get automatic fixing whenever saving a file:

"eslint.autoFixOnSave":  true,
"eslint.validate":  [
  "javascript",
  "javascriptreact",
  {"language":  "typescript",  "autoFix":  true  },
  {"language":  "typescriptreact",  "autoFix":  true  }
],


If you’ve also set the editor.formatOnSave option to true in your settings.json, you’ll need to add the following config to prevent running 2 formatting commands on save for JavaScript and TypeScript files:

"editor.formatOnSave":  true,
"[javascript]":  {
  "editor.formatOnSave":  false,
},
"[javascriptreact]":  {
  "editor.formatOnSave":  false,
},
"[typescript]":  {
  "editor.formatOnSave":  false,
},
"[typescriptreact]":  {
  "editor.formatOnSave":  false,
},


And there you have it. That’s how you can lint a TypeScript project using ESLint. If you want to make sure all the files you commit to git pass the ESLint checks, take a look at lint-staged, which can run ESLint on files being commited.

What TypeScript taught me about JavaScript

What TypeScript taught me about JavaScript

What TypeScript taught me about JavaScript. TypeScript was designed to make the most sense out of any JavaScript code. How void behaves in both TypeScript and JavaScript. What Symbols are and why they can be unique. Why substitutability is such an important concept for TypeScript

TypeScript was designed to make the most sense out of any JavaScript code. Given the dynamic nature of JavaScript, this can lead to some very interesting typings that may seem odd at a first glance. In this talk, we will look at JavaScript scenarios that are easy to understand, but complex to define. We then see what tools TypeScript provides to make the most dynamical behaviour predictable, in the most elegant way possible.

Join us and learn:

  • How void behaves in both TypeScript and JavaScript
  • What Symbols are and why they can be unique
  • The constructor interface pattern, and why classes are more complex than you might think
  • Why substitutability is such an important concept for TypeScript
    ... and much more!

Introduction Boolean in JavaScript and TypeScript

Introduction Boolean in JavaScript and TypeScript

Boolean values are supported by both JavaScript and TypeScript and stored as true/false values .This is because, while JavaScript coerces an object to its primitive type, the TypeScript type system does not. TypeScript treats it like an object type.

Boolean in JavaScript

boolean can take the values of true and false. Values from other types can be truthy or falsy, like undefined or null.

let b = true
if(b) console.log('logged')

b = false
if(b) console.log('not logged')

b = undefined
if(b) console.log('not logged')

b = null
if(b) console.log('not logged')

Values other than undefined, null or false considered falsy are "" (empty string), -0 and 0, as well as NaN.

To get the boolean value of any value, you can use the Boolean function:

Boolean(false) // false
Boolean(true) // true
Boolean("false") // true ❗️
Boolean("Hey folks") // true
Boolean({}) // true
Boolean([]) // true
Boolean(123.4) // true
Boolean(Symbol()) // true
Boolean(function() {}) // true
Boolean(undefined) // false
Boolean(null) // false
Boolean(NaN) // false
Boolean(0) // false
Boolean("") // false

Rule of thumb: All empty values evaluate to false. Empty object {} and empty array [] (which is an object itself) do have value as they are containers for other values.

The Boolean function is really good to filter empty values from collections:

const collection = [
  { name: 'Stefan Baumgartner', age: 37 },
  undefined,
  { name: 'D.', age: 36 },
  false
  { name: 'C.', age: 2},
  false
]

collection.filter(Boolean) // handy!

Together with Number – which converts all values into their number counterpart or NaN, this is a really cool way of getting to actual values quickly:

const x = ["1.23", 2137123, "wut", false, "lol", undefined, null]
  .map(Number)
  .filter(Boolean) // [1.23, 2137123] 👍

Boolean exists as a constructor and has the same conversion rules as the Boolean function. However, with new Boolean(...) you create a wrapping object, making value comparisions truthy, but reference comparisions falsy:

const value = Boolean("Stefan") // true
const reference = new Boolean("Stefan") // [Boolean: true]

value == reference // true
value === reference // false

You get to the value via .valueOf():

value === reference.valueOf() // true

I have a REPL for you to check. The use of Boolean as a function is obviously great, but new Boolean has very limited use. If you know a practical use case, please let me know.

Boolean in TypeScript

boolean in TypeScript is a primitive type. Be sure to use the lower case version and don’t refer to
object instances from Boolean

const boolLiteral: boolean = false // 👍
const boolObject: Boolean = false // 👎

It works, but it’s bad practice as we really rarely need new Boolean objects.

You can assign true, false and undefined and null to boolean in TypeScript without strict null checks.

const boolTrue: boolean = true // 👍
const boolFalse: boolean = false // 👍
const boolUndefined: boolean = undefined // 👍
const boolNull: boolean = null // 👍

With that, boolean is the only one we can express fully through union types:

type MyBoolean = true | false | null | undefined // same as boolean

const mybool: MyBoolean = true
const yourbool: boolean = false

When we enable the strictNullChecks compiler flag, the set of values reduces to true and false.

const boolTrue: boolean = true // 👍
const boolFalse: boolean = false // 👍
const boolUndefined: boolean = undefined // 💥
const boolNull: boolean = null // 💥

So our set reduces to two values in total.

type MyStrictBoolean = true | false

We can also get rid of null values with the NonNullable helper type:

type NonNullable<T> = T extends null | undefined
  ? never
  : T;

type MyStrictBoolean = NonNullable<MyBoolean> // true | false

The fact that boolean consists of a limited set of values only used in conditions, allows for interesting conditional types.

Think of an mutation in a datastore through a function. You set a flag in a function that updates e.g. the user id. You have to provide the user ID then:

type CheckUserId<Properties, AddUserId> = 
    AddUserId extends true 
    ? Properties & { userId: string }
    : Properties & { userId?: string }

Depending on the value of our generic AddUserId, we expect the property userId to be set or to be optional.

We can make this type more explicit by extending our generics from the types we expect

- type CheckUserId<Properties, AddUserId> = 
+ type CheckuserId<
+  Properties extends {},
+  AddUserId extends boolean
+ >
     AddUserId extends true 
     ? Properties & { userId: string }
     : Properties & { userId?: string }

In use, it might declare a function like this:

declare function mutate<P, A extends boolean = false>
  (props: CheckUserId<P, A>, addUserId?: A): void

Note that I even set a default value for A to make sure CheckUserId gives the correct info depending on addUserId to be set or not.

The function in action:

mutate({}) // 👍
mutate({ data: 'Hello folks' }) // 👍
mutate({ name: 'Stefan' }, false) // 👍
mutate({ name: 'Stefan' }, true) // 💥 userId is missing
mutate({ name: 'Stefan', userId: 'asdf' }, true) // 👍 userId is here

Handy if your code relies a lot on truthy and falsy values. As always, there’s playground for you.

Originally published at fettblog.eu on 10 September 2019

Understanding void in JavaScript and TypeScript

Understanding void in JavaScript and TypeScript

void exists in both JavaScript as an operator and in TypeScript as a primitive type. And in both worlds void works a little bit different than most people are used to.

If you come from traditional, strongly typed languages you might be familiar with the concept of void: A type telling you that functions and methods return nothing when called.

void in JavaScript

void in JavaScript is an operator which evaluates the expression next to it. No matter which expression is evaluated, void always returns undefined.

let i = void 2; // i === undefined

Why would we need something like this? First, in earlier times, people were able to override undefined and giving it an actual value. void always returned the real undefined.

Second, it’s a nice way to call immediately invoked functions:

void function() {
  console.log('What')
}()

All without polluting the global namespace:

void function aRecursion(i) {
  if(i > 0) {
    console.log(i--)
    aRecursion(i)
  }
}(3)

console.log(typeof aRecursion) // undefined

Since void always returns undefined, and void always evaluates the expression next to it, you have a very terse way of returning from a function without returning a value, but still calling a callback for example:

// returning something else than undefined would crash the app
function middleware(nextCallback) {
  if(conditionApplies()) {
    return void nextCallback();
  }
}

Which brings me to the most important use case of void: It’s a security gate for your app. When your function is always supposed to return undefined, you can make sure that this is always the case.

button.onclick = () => void doSomething();

void in TypeScript

void in TypeScript is a subtype of undefined. Functions in JavaScript always return something. Either it’s a value, or undefined

function iHaveNoReturnValue(i) {
  console.log(i)
} // returns undefined

Since functions without a return value always return undefined, and void always returns undefined in JavaScript, void in TypeScript is a proper type for telling developers that this function returns undefined:

declare function iHaveNoReturnValue(i: number): void

void as type can also be used for parameters and all other declarations. The only value that can be passed is undefined:

declare function iTakeNoParameters(x: void): void

iTakeNoParameters() // 👍
iTakeNoParameters(undefined) // 👍
iTakeNoParameters(void 2) // 👍

So void and undefined are pretty much the same. There’s one little difference though, and this difference is significant: void as a return type can be substituted with different types, to allow for advanced callback patterns:

function doSomething(callback: () => void) {
  let c = callback() // at this position, callback always returns undefined
  //c is also of type undefiend
}

// this function returns a number
function aNumberCallback(): number {
  return 2;
}

// works 👍 type safety is ensured in doSometing
doSomething(aNumberCallback) 

This is desired behaviour and often used in JavaScript applications. Read more on this pattern called substitutability in my other articles.

If you want to make sure to pass functions who only return undefined (as in “nothing”), be sure to adapt your callback method signature:

- function doSomething(callback: () => void) {
+ function doSomething(callback: () => undefined) { /* ... */ }

function aNumberCallback(): number { return 2; }

// 💥 types don't match
doSomething(aNumberCallback)

You’ll propably be good with void most of the time.

Originally published at fettblog.eu on 06 September 2019