Coding Guidelines

We use a resonably strict coding style to ensure that the code is easy to read and maintain. This document is a summary of the coding style that we use. We implemented these coding practices in our eslint configuration. You can find the eslint configuration in the .eslintrc.js file in the root of the project. We will keep updating this document as we find better ways to write code along side updating the eslint configuration.

If you see any piece of code that does not follow the guidelines, please feel free to fix it and submit a pull request.

This style guide is applicable to code written in the following languages:

• TypeScript (.ts)
• React TypeScript (.tsx)

Table of Contents

You can find the table of contents for this document on the right side of the page. It is recommended to read through the entire document to get a good understanding of the coding style. However, if you are looking for something specific, you can use the table of contents to jump to that section.

File and Folder Naming Conventions

• Try to use short and descriptive names for files and folders.
• If a name has multiple words in it, use a hyphen to separate the words.

# bad, uses camelCase
touch myFile.tsx;
 
# good, uses hyphen to separate words
touch my-file.tsx;
 
# good, uses hyphen to separate words
mkdir -p my-folder && touch my-folder/my-file.txt

Naming Conventions

• Always use PascalCase and suffix with the word Type when defining new types in TypeScript.

  // bad, not PascalCase and not suffixed with Type
  type user = {
  	name: string;
  	age: number;
  };
   
  // good
  type UserType = {
  	name: string;
  	age: number;
  };
   
  // good
  type CheckoutSessionType = {
  	session: {};
  };

• Avoid single letter names. Be descriptive with your naming.

  // bad
  function q() {
  	// ...
  }
   
  // good
  function query() {
  	// ...
  }

• Use camelCase when naming objects, functions, and instances.

  // bad
  const OBJEcttsssss = {};
  const this_is_my_object = {};
  function c() {}
   
  // good
  const thisIsMyObject = {};
  function thisIsMyFunction() {}

• Use PascalCase only when naming constructors or classes.

  // bad
  function user(options) {
  	this.name = options.name;
  }
   
  const bad = new user({
  	name: "nope",
  });
   
  // good
  class User {
  	constructor(options) {
  		this.name = options.name;
  	}
  }
   
  const good = new User({
  	name: "yup",
  });

• Do not use trailing or leading underscores. JavaScript does not have the concept of privacy in terms of properties or methods. Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract. This convention might lead developers to wrongly think that a change won’t count as breaking, or that tests aren’t needed. tl;dr: if you want something to be “private”, it must not be observably present.

  // bad
  this.__firstName__ = "Panda";
  this.firstName_ = "Panda";
  this._firstName = "Panda";
   
  // good
  this.firstName = "Panda";
   
  // good, in environments where WeakMaps are available
  const firstNames = new WeakMap();
  firstNames.set(this, "Panda");

• Don’t save references to this. Use arrow functions or Function#bind (opens in a new tab).

  // bad
  function foo() {
  	const self = this;
  	return function () {
  		console.log(self);
  	};
  }
   
  // bad
  function foo() {
  	const that = this;
  	return function () {
  		console.log(that);
  	};
  }
   
  // good
  function foo() {
  	return () => {
  		console.log(this);
  	};
  }

• A base filename should exactly match the name of its default export.

  // file 1 contents
  class CheckBox {
  	// ...
  }
  export default CheckBox;
   
  // file 2 contents
  export default function fortyTwo() {
  	return 42;
  }
   
  // file 3 contents
  export default function insideDirectory() {}
   
  // in some other file
  // bad
  import CheckBox from "@/checkBox"; // PascalCase import/export, camelCase filename
  import FortyTwo from "@/FortyTwo"; // PascalCase import/filename, camelCase export
  import InsideDirectory from "@/InsideDirectory"; // PascalCase import/filename, camelCase export
   
  // bad
  import CheckBox from "@/check_box"; // PascalCase import/export, snake_case filename
  import forty_two from "@/forty_two"; // snake_case import/filename, camelCase export
  import inside_directory from "@/inside_directory"; // snake_case import, camelCase export
  import index from "@/inside_directory/index"; // requiring the index file explicitly
  import insideDirectory from "@/insideDirectory/index"; // requiring the index file explicitly
   
  // good
  import CheckBox from "@/CheckBox"; // PascalCase export/import/filename
  import fortyTwo from "@/fortyTwo"; // camelCase export/import/filename
  import insideDirectory from "@/insideDirectory"; // camelCase export/import/directory name/implicit "index"
  // ^ supports both insideDirectory.js and insideDirectory/index.js

• Use camelCase when you export-default a function. Your filename should be identical to your function’s name.

  function makeStyleGuide() {
  	// ...
  }
   
  export default makeStyleGuide;

• Use PascalCase when you export a constructor / class / singleton / function library / bare object.

  const CheckoutSession = {
  	session: {},
  };
   
  export default CheckoutSession;

• Acronyms and initialisms should always be all uppercased, or all lowercased. Names are for readability, not to appease a computer algorithm.

  // bad
  import SmsContainer from "@/containers/SmsContainer";
   
  // bad
  const HttpRequests = [
  	// ...
  ];
   
  // good
  import SMSContainer from "@/containers/SMSContainer";
   
  // good
  const HTTPRequests = [
  	// ...
  ];
   
  // also good
  const httpRequests = [
  	// ...
  ];
   
  // best
  import TextMessageContainer from "@/containers/TextMessageContainer";
   
  // best
  const requests = [
  	// ...
  ];

• You may optionally uppercase a constant only if it (1) is exported, (2) is a const (it can not be reassigned), and (3) the programmer can trust it (and its nested properties) to never change. This is an additional tool to assist in situations where the programmer would be unsure if a variable might ever change. UPPERCASE_VARIABLES are letting the programmer know that they can trust the variable (and its properties) not to change.

  • What about all const variables? - This is unnecessary, so uppercasing should not be used for constants within a file. It should be used for exported constants however.
  • What about exported objects? - Uppercase at the top level of export (e.g. EXPORTED_OBJECT.key) and maintain that all nested properties do not change.

  // bad
  const PRIVATE_VARIABLE = "should not be unnecessarily uppercased within a file";
   
  // bad
  export const THING_TO_BE_CHANGED = "should obviously not be uppercased";
   
  // bad
  export let REASSIGNABLE_VARIABLE = "do not use let with uppercase variables";
   
  // ---
   
  // allowed but does not supply semantic value
  export const apiKey = "SOMEKEY";
   
  // better in most cases
  export const API_KEY = "SOMEKEY";
   
  // ---
   
  // bad - unnecessarily uppercases key while adding no semantic value
  export const MAPPING = {
  	KEY: "value",
  };
   
  // good
  export const MAPPING = {
  	key: "value",
  };

References

• Use const for all of your references; avoid using var. This ensures that you can't reassign your references, which can lead to bugs and difficult to comprehend code.

  // bad
  var a = 1;
  var b = 2;
   
  // good
  const a = 1;
  const b = 2;

• If you must reassign references, use let instead of var. Why? let is block-scoped rather than function-scoped like var.

  // bad
  var count = 1;
  if (true) {
  	count += 1;
  }
   
  // good, use the let.
  let count = 1;
  if (true) {
  	count += 1;
  }

• Both let and const are block-scoped, whereas var is function-scoped.

  // const and let only exist in the blocks they are defined in.
  {
  	let a = 1;
  	const b = 1;
  	var c = 1;
  }
  console.log(a); // ReferenceError
  console.log(b); // ReferenceError
  console.log(c); // Prints 1

  In the above code, you can see that referencing a and b will produce a ReferenceError, while c contains the number. This is because a and b are block scoped, while c is scoped to the containing function.

Objects

• Use the literal syntax for object creation.

  // bad
  const item = new Object();
   
  // good
  const item = {};

• Use object method shorthand.

  // bad
  const atom = {
  	value: 1,
   
  	addValue: function (value) {
  		return atom.value + value;
  	},
  };
   
  // good
  const atom = {
  	value: 1,
   
  	addValue(value) {
  		return atom.value + value;
  	},
  };

• Use property value shorthand as tt is shorter and descriptive.

  const lewisHamilton = "Lewis Hamilton";
   
  // bad
  const obj = {
  	lewisHamilton: lewisHamilton,
  };
   
  // good
  const obj = {
  	lewisHamilton,
  };

• Group your shorthand properties at the beginning of your object declaration because it’s easier to tell which properties are using the shorthand.

  const lewisHamilton = "Lewis Hamilton";
  const maxVerstappen = "Max Verstappen";
   
  // bad
  const obj = {
  	episodeOne: 1,
  	numberOfRaces: 2,
  	lewisHamilton,
  	episodeThree: 3,
  	mayTheFourth: 4,
  	maxVerstappen,
  };
   
  // good
  const obj = {
  	lewisHamilton,
  	maxVerstappen,
  	episodeOne: 1,
  	numberOfRaces: 2,
  	episodeThree: 3,
  	mayTheFourth: 4,
  };

• Only quote properties that are invalid identifiers. In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.

  // bad
  const bad = {
  	foo: 3,
  	bar: 4,
  	"data-blah": 5,
  };
   
  // good
  const good = {
  	foo: 3,
  	bar: 4,
  	"data-blah": 5,
  };

• Do not call Object.prototype methods directly, such as hasOwnProperty, propertyIsEnumerable, and isPrototypeOf. These methods may be shadowed by properties on the object in question - consider { hasOwnProperty: false } - or, the object may be a null object (Object.create(null)). In modern browsers that support ES2022, or with a polyfill such as https://npmjs.com/object.hasown (opens in a new tab), Object.hasOwn can also be used as an alternative to Object.prototype.hasOwnProperty.call.

  // bad
  console.log(object.hasOwnProperty(key));
   
  // good
  console.log(Object.prototype.hasOwnProperty.call(object, key));
   
  // better
  const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
  console.log(has.call(object, key));
   
  // best
  console.log(Object.hasOwn(object, key)); // only supported in browsers that support ES2022
   
  /* or */
  import has from "has"; // https://www.npmjs.com/package/has
  console.log(has(object, key));
  /* or */
  console.log(Object.hasOwn(object, key)); // https://www.npmjs.com/package/object.hasown

• Prefer the object spread syntax over Object.assign (opens in a new tab) to shallow-copy objects. Use the object rest parameter syntax to get a new object with certain properties omitted.

  // very bad
  const original = { a: 1, b: 2 };
  const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ
  delete copy.a; // so does this
   
  // bad
  const original = { a: 1, b: 2 };
  const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 }
   
  // good
  const original = { a: 1, b: 2 };
  const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }
   
  const { a, ...noA } = copy; // noA => { b: 2, c: 3 }

Arrays

• Use the literal syntax for array creation.

  // bad
  const items = new Array();
   
  // good
  const items = [];

• Use Array#push (opens in a new tab) instead of direct assignment to add items to an array.

  const someStack = [];
   
  // bad
  someStack[someStack.length] = "abracadabra";
   
  // good
  someStack.push("abracadabra");

• Use array spreads ... to copy arrays.

  // bad
  const len = items.length;
  const itemsCopy = [];
  let i;
   
  for (i = 0; i < len; i += 1) {
  	itemsCopy[i] = items[i];
  }
   
  // good
  const itemsCopy = [...items];

• Use Array.from (opens in a new tab) for converting an array-like object to an array.

  const arrLike = { 0: "foo", 1: "bar", 2: "baz", length: 3 };
   
  // bad
  const arr = Array.prototype.slice.call(arrLike);
   
  // good
  const arr = Array.from(arrLike);

• Use Array.from (opens in a new tab) instead of spread ... for mapping over iterables, because it avoids creating an intermediate array.

  // bad
  const baz = [...foo].map(bar);
   
  // good
  const baz = Array.from(foo, bar);

• Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, following 8.2.

  // good
  [1, 2, 3].map(x => {
  	const y = x + 1;
  	return x * y;
  });
   
  // good
  [1, 2, 3].map(x => x + 1);
   
  // bad - no returned value means `acc` becomes undefined after the first iteration
  [
  	[0, 1],
  	[2, 3],
  	[4, 5],
  ].reduce((acc, item, index) => {
  	const flatten = acc.concat(item);
  });
   
  // good
  [
  	[0, 1],
  	[2, 3],
  	[4, 5],
  ].reduce((acc, item, index) => {
  	const flatten = acc.concat(item);
  	return flatten;
  });
   
  // bad
  inbox.filter(msg => {
  	const { subject, author } = msg;
  	if (subject === "Mockingbird") {
  		return author === "Harper Lee";
  	} else {
  		return false;
  	}
  });
   
  // good
  inbox.filter(msg => {
  	const { subject, author } = msg;
  	if (subject === "Mockingbird") {
  		return author === "Harper Lee";
  	}
   
  	return false;
  });

• Use line breaks after opening array brackets and before closing array brackets, if an array has multiple lines

  // bad
  const arr = [
  	[0, 1],
  	[2, 3],
  	[4, 5],
  ];
   
  const objectInArray = [
  	{
  		id: 1,
  	},
  	{
  		id: 2,
  	},
  ];
   
  const numberInArray = [1, 2];
   
  // good
  const arr = [
  	[0, 1],
  	[2, 3],
  	[4, 5],
  ];
   
  const objectInArray = [
  	{
  		id: 1,
  	},
  	{
  		id: 2,
  	},
  ];
   
  const numberInArray = [1, 2];

Destructuring

• Use object destructuring when accessing and using multiple properties of an object. Destructuring saves you from creating temporary references for those properties, and from repetitive access of the object. Repeating object access creates more repetitive code, requires more reading, and creates more opportunities for mistakes. Destructuring objects also provides a single site of definition of the object structure that is used in the block, rather than requiring reading the entire block to determine what is used.

  // bad
  function getFullName(user) {
  	const firstName = user.firstName;
  	const lastName = user.lastName;
   
  	return `${firstName} ${lastName}`;
  }
   
  // good
  function getFullName(user) {
  	const { firstName, lastName } = user;
  	return `${firstName} ${lastName}`;
  }
   
  // best
  function getFullName({ firstName, lastName }) {
  	return `${firstName} ${lastName}`;
  }

• Use array destructuring.

  const arr = [1, 2, 3, 4];
   
  // bad
  const first = arr[0];
  const second = arr[1];
   
  // good
  const [first, second] = arr;

• Use object destructuring for multiple return values, not array destructuring because you can add new properties over time or change the order of things without breaking call sites.

  // bad
  function processInput(input) {
  	// then a miracle occurs
  	return [left, right, top, bottom];
  }
   
  // the caller needs to think about the order of return data
  const [left, __, top] = processInput(input);
   
  // good
  function processInput(input) {
  	// then a miracle occurs
  	return { left, right, top, bottom };
  }
   
  // the caller selects only the data they need
  const { left, top } = processInput(input);

Strings

• Use double quotes "" for strings.

  // bad
  const name = "Capt. Janeway";
   
  // bad - template literals should contain interpolation or newlines
  const name = `Capt. Janeway`;
   
  // good
  const name = "Capt. Janeway";

• Strings that cause the line to go over 100 characters should not be written across multiple lines using string concatenation because broken strings are painful to work with and make code less searchable.

  // bad
  const errorMessage =
  	"This is a super long error that was thrown because \
  of Batman. When you stop to think about how Batman had anything to do \
  with this, you would get nowhere \
  fast.";
   
  // bad
  const errorMessage =
  	"This is a super long error that was thrown because " +
  	"of Batman. When you stop to think about how Batman had anything to do " +
  	"with this, you would get nowhere fast.";
   
  // good
  const errorMessage =
  	"This is a super long error that was thrown because of Batman. When you stop to think about how Batman had anything to do with this, you would get nowhere fast.";

• When programmatically building up strings, use template strings instead of concatenation because template strings give you a readable, concise syntax with proper newlines and string interpolation features.

  // bad
  function sayHi(name) {
  	return "How are you, " + name + "?";
  }
   
  // bad
  function sayHi(name) {
  	return ["How are you, ", name, "?"].join();
  }
   
  // bad
  function sayHi(name) {
  	return `How are you, ${name}?`;
  }
   
  // good
  function sayHi(name) {
  	return `How are you, ${name}?`;
  }

• Never use eval() on a string; it opens too many vulnerabilities because JavaScript’s eval() function is potentially dangerous and is often misused. Using eval() on untrusted code can open a program up to several different injection attacks. The use of eval() in most contexts can be substituted for a better, alternative approach to a problem.

• Please do not unnecessarily escape characters in strings. We have seen a lot of experienced developers do this. Why? Backslashes harm readability, thus they should only be present when necessary.

  // bad
  const foo = "'this' is \"quoted\"";
   
  // good
  const foo = "'this' is \"quoted\"";
  const foo = `my name is '${name}'`;

Functions

• Use named function expressions instead of function declarations because function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module!

  // bad
  const foo = function () {
  	// ...
  };
   
  // good
  function foo() {
  	// ...
  }

• Wrap immediately invoked function expressions in parentheses. An immediately invoked function expression is a single unit - wrapping both it, and its invocation parens, in parens, cleanly expresses this. Note that in a world with modules everywhere, you almost never need an IIFE.

  // immediately-invoked function expression (IIFE)
  (function () {
  	console.log("Welcome to the Internet. Please follow me.");
  })();

• Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears.

  // bad
  if (currentUser) {
  	function test() {
  		console.log("Nope.");
  	}
  }
   
  // good
  let test;
  if (currentUser) {
  	test = () => {
  		console.log("Yup.");
  	};
  }

• Never name a parameter arguments. This will take precedence over the arguments object that is given to every function scope.

  // bad
  function foo(name, options, arguments) {
  	// ...
  }
   
  // good
  function foo(name, options, args) {
  	// ...
  }

• Never use arguments, opt to use rest syntax ... instead. ... is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like like arguments.

  // bad
  function concatenateAll() {
  	const args = Array.prototype.slice.call(arguments);
  	return args.join("");
  }
   
  // good
  function concatenateAll(...args) {
  	return args.join("");
  }

• Use default parameter syntax rather than mutating function arguments.

  // really bad
  function handleThings(opts) {
  	// No! We shouldn’t mutate function arguments.
  	// Double bad: if opts is falsy it'll be set to an object which may
  	// be what you want but it can introduce subtle bugs.
  	opts = opts || {};
  	// ...
  }
   
  // still bad
  function handleThings(opts) {
  	if (opts === void 0) {
  		opts = {};
  	}
  	// ...
  }
   
  // good
  function handleThings(opts = {}) {
  	// ...
  }

• Avoid side effects with default parameters. They are confusing to reason about.

  let b = 1;
  // bad
  function count(a = b++) {
  	console.log(a);
  }
  count(); // 1
  count(); // 2
  count(3); // 3
  count(); // 3

• Always put default parameters last. It’s a cleaner syntax.

  // bad
  function handleThings(opts = {}, name) {
  	// ...
  }
   
  // good
  function handleThings(name, opts = {}) {
  	// ...
  }

• Never use the Function constructor to create a new function. Creating a function like this evaluates a string similarly to eval(), which opens vulnerabilities.

  // bad
  const add = new Function("a", "b", "return a + b");
   
  // still bad
  const subtract = Function("a", "b", "return a - b");

• Spacing in a function signature because consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.

  // bad
  const f = function () {};
  const g = function () {};
  const h = function () {};
   
  // good
  const x = function () {};
  const y = function a() {};

• Never mutate parameters. Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.

  // bad
  function f1(obj) {
  	obj.key = 1;
  }
   
  // good
  function f2(obj) {
  	const key = Object.prototype.hasOwnProperty.call(obj, "key") ? obj.key : 1;
  }

• Never reassign parameters. Reassigning parameters can lead to unexpected behavior, especially when accessing the arguments object. It can also cause optimization issues, especially in V8.

  // bad
  function f1(a) {
  	a = 1;
  	// ...
  }
   
  function f2(a) {
  	if (!a) {
  		a = 1;
  	}
  	// ...
  }
   
  // good
  function f3(a) {
  	const b = a || 1;
  	// ...
  }
   
  function f4(a = 1) {
  	// ...
  }

• Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item.

  // bad
  function foo(bar, baz, quux) {
  	// ...
  }
   
  // good
  function foo(bar, baz, quux) {
  	// ...
  }
   
  // bad
  console.log(foo, bar, baz);
   
  // good
  console.log(foo, bar, baz);

Arrow Functions

• When you must use an anonymous function (as when passing an inline callback), use arrow function notation.

  // bad
  [1, 2, 3].map(function (x) {
  	const y = x + 1;
  	return x * y;
  });
   
  // good
  [1, 2, 3].map(x => {
  	const y = x + 1;
  	return x * y;
  });

• If the function body consists of a single statement returning an expression (opens in a new tab) without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement because it is syntactic sugar. It reads well when multiple functions are chained together.

  // bad
  [1, 2, 3].map(number => {
  	const nextNumber = number + 1;
  	`A string containing the ${nextNumber}.`;
  });
   
  // good
  [1, 2, 3].map(number => `A string containing the ${number + 1}.`);
   
  // good
  [1, 2, 3].map(number => {
  	const nextNumber = number + 1;
  	return `A string containing the ${nextNumber}.`;
  });
   
  // good
  [1, 2, 3].map((number, index) => ({
  	[index]: number,
  }));
   
  // No implicit return with side effects
  function foo(callback) {
  	const val = callback();
  	if (val === true) {
  		// Do something if callback returns true
  	}
  }
   
  let bool = false;
   
  // bad
  foo(() => (bool = true));
   
  // good
  foo(() => {
  	bool = true;
  });

• In case the expression spans over multiple lines, wrap it in parentheses for better readability. It shows clearly where the function starts and ends.

  // bad
  ["get", "post", "put"].map(httpMethod =>
  	Object.prototype.hasOwnProperty.call(httpMagicObjectWithAVeryLongName, httpMethod)
  );
   
  // good
  ["get", "post", "put"].map(httpMethod =>
  	Object.prototype.hasOwnProperty.call(httpMagicObjectWithAVeryLongName, httpMethod)
  );

• Always include parentheses around arguments for clarity and consistency because it minimizes diff churn when adding or removing arguments.

  // bad
  [1, 2, 3].map(x => x * x);
   
  // good
  [1, 2, 3].map(x => x * x);
   
  // bad
  [1, 2, 3].map(
  	number =>
  		`A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
  );
   
  // good
  [1, 2, 3].map(
  	number =>
  		`A long string with the ${number}. It’s so long that we don’t want it to take up space on the .map line!`
  );
   
  // bad
  [1, 2, 3].map(x => {
  	const y = x + 1;
  	return x * y;
  });
   
  // good
  [1, 2, 3].map(x => {
  	const y = x + 1;
  	return x * y;
  });

• Avoid confusing arrow function syntax (=>) with comparison operators (<=, >=).

  // bad
  const itemHeight = item => (item.height <= 256 ? item.largeSize : item.smallSize);
   
  // bad
  const itemHeight = item => (item.height >= 256 ? item.largeSize : item.smallSize);
   
  // good
  const itemHeight = item => (item.height <= 256 ? item.largeSize : item.smallSize);
   
  // good
  const itemHeight = item => {
  	const { height, largeSize, smallSize } = item;
  	return height <= 256 ? largeSize : smallSize;
  };

• Enforce the location of arrow function bodies with implicit returns.

  // bad
  foo => bar;
   
  foo => bar;
   
  // good
  foo => bar;
  foo => bar;
  foo => bar;

Modules

• Always use modules (import/export) over a non-standard module system. You can always transpile to your preferred module system.

  // bad
  const CheckoutSession = require("@/checkout-session");
  module.exports = CheckoutSession.session;
   
  // ok
  import CheckoutSession from "@/checkout-session";
  export default CheckoutSession.session;
   
  // best
  import { CheckoutSession } from "@/checkout-session";
  export default CheckoutSession;

• Do not use wildcard imports unless it is absolutely necessary.

  // bad
  import * as CheckoutSession from "@/checkout-session";
   
  // good
  import CheckoutSession from "@/checkout-session";

• Do not export directly from an import.

  // bad
  // filename session.js
  export { session as default } from "@/checkout-session";
   
  // good
  // filename session.js
  import { session } from "@/checkout-session";
  export default session;

• Only import from a path in one place.

  // bad
  import foo from "foo";
  // … some other imports … //
  import { named1, named2 } from "foo";
   
  // good
  import foo, { named1, named2 } from "foo";
   
  // good
  import foo, { named1, named2 } from "foo";

• Do not export mutable bindings. Mutation should be avoided in general, but in particular when exporting mutable bindings. While this technique may be needed for some special cases, in general, only constant references should be exported.

  // bad
  let foo = 3;
  export { foo };
   
  // good
  const foo = 3;
  export { foo };

• In modules with a single export, prefer default export over named export.

  // bad
  export function foo() {}
   
  // good
  export default function foo() {}

• Put all imports above non-import statements becauseimports are hoisted, keeping them all at the top prevents surprising behavior.

  // bad
  import foo from "foo";
  foo.init();
   
  import bar from "bar";
   
  // good
  import foo from "foo";
  import bar from "bar";
   
  foo.init();

• Multiline imports should be indented just like multiline array and object literals.

  // bad
  import { longNameA, longNameB, longNameC, longNameD, longNameE } from "path";
   
  // good
  import { longNameA, longNameB, longNameC, longNameD, longNameE } from "path";

• Do not include JavaScript filename extensions

  // bad
  import foo from "@/foo.js";
  import bar from "@/bar.jsx";
  import baz from "@/baz/index.jsx";
   
  // good
  import foo from "@/foo";
  import bar from "@/bar";
  import baz from "@/baz";

Iterators and Generators

• Don’t use iterators. Prefer JavaScript’s higher-order functions instead of loops like for-in or for-of. This enforces our immutable rule. Dealing with pure functions that return values is easier to reason about than side effects. Use map() / every() / filter() / find() / findIndex() / reduce() / some() / ... to iterate over arrays, and Object.keys() / Object.values() / Object.entries() to produce arrays so you can iterate over objects.

  ```typescript
  const numbers = [1, 2, 3, 4, 5];
  
  // bad
  let sum = 0;
  for (let num of numbers) {
  	sum += num;
  }
  sum === 15;
  
  // good
  let sum = 0;
  numbers.forEach(num => {
  	sum += num;
  });
  sum === 15;
  
  // best (use the functional force)
  const sum = numbers.reduce((total, num) => total + num, 0);
  sum === 15;
  
  // bad
  const increasedByOne = [];
  for (let i = 0; i < numbers.length; i++) {
  	increasedByOne.push(numbers[i] + 1);
  }
  
  // good
  const increasedByOne = [];
  numbers.forEach(num => {
  	increasedByOne.push(num + 1);
  });
  
  // best (keeping it functional)
  const increasedByOne = numbers.map(num => num + 1);
  ```

Properties

• Use dot notation when accessing properties.

  const luke = {
  	jedi: true,
  	age: 28,
  };
   
  // bad
  const isJedi = luke["jedi"];
   
  // good
  const isJedi = luke.jedi;

• Use bracket notation [] when accessing properties with a variable.

  const luke = {
  	jedi: true,
  	age: 28,
  };
   
  function getProp(prop) {
  	return luke[prop];
  }
   
  const isJedi = getProp("jedi");

• Use exponentiation operator ** when calculating exponentiations.

  // bad
  const binary = Math.pow(2, 10);
   
  // good
  const binary = 2 ** 10;

Variables

• Always use const or let to declare variables. Not doing so will result in global variables.

  // bad
  superPower = new SuperPower();
   
  // good
  const superPower = new SuperPower();

• Use one const or let declaration per variable or assignment. It’s easier to add new variable declarations this way, and you never have to worry about swapping out a ; for a , or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.

  // bad
  const items = getItems(),
  	goSportsTeam = true,
  	dragonball = "z";
   
  // bad
  // (compare to above, and try to spot the mistake)
  const items = getItems(),
  	goSportsTeam = true;
  dragonball = "z";
   
  // good
  const items = getItems();
  const goSportsTeam = true;
  const dragonball = "z";

• Group all your consts and then group all your lets. This is helpful when later on you might need to assign a variable depending on one of the previously assigned variables.

  // bad
  let i,
  	len,
  	dragonball,
  	items = getItems(),
  	goSportsTeam = true;
   
  // bad
  let i;
  const items = getItems();
  let dragonball;
  const goSportsTeam = true;
  let len;
   
  // good
  const goSportsTeam = true;
  const items = getItems();
  let dragonball;
  let i;
  let length;

• Assign variables where you need them, but place them in a reasonable place. let and const are block scoped and not function scoped.

  // bad - unnecessary function call
  function checkName(hasName) {
  	const name = getName();
   
  	if (hasName === "test") {
  		return false;
  	}
   
  	if (name === "test") {
  		this.setName("");
  		return false;
  	}
   
  	return name;
  }
   
  // good
  function checkName(hasName) {
  	if (hasName === "test") {
  		return false;
  	}
   
  	const name = getName();
   
  	if (name === "test") {
  		this.setName("");
  		return false;
  	}
   
  	return name;
  }

• Don’t chain variable assignments. Chaining variable assignments creates implicit global variables.

  // bad
  (function example() {
  	// JavaScript interprets this as
  	// let a = ( b = ( c = 1 ) );
  	// The let keyword only applies to variable a; variables b and c become
  	// global variables.
  	let a = (b = c = 1);
  })();
   
  console.log(a); // throws ReferenceError
  console.log(b); // 1
  console.log(c); // 1
   
  // good
  (function example() {
  	let a = 1;
  	let b = a;
  	let c = a;
  })();
   
  console.log(a); // throws ReferenceError
  console.log(b); // throws ReferenceError
  console.log(c); // throws ReferenceError
   
  // the same applies for `const`

• Avoid using unary increments and decrements (++, --). As per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like num += 1 instead of num++ or num ++. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.

  // bad
   
  const array = [1, 2, 3];
  let num = 1;
  num++;
  --num;
   
  let sum = 0;
  let truthyCount = 0;
  for (let i = 0; i < array.length; i++) {
  	let value = array[i];
  	sum += value;
  	if (value) {
  		truthyCount++;
  	}
  }
   
  // good
   
  const array = [1, 2, 3];
  let num = 1;
  num += 1;
  num -= 1;
   
  const sum = array.reduce((a, b) => a + b, 0);
  const truthyCount = array.filter(Boolean).length;

• Avoid linebreaks before or after = in an assignment.

  // bad
  const foo = superLongLongLongLongLongLongLongLongFunctionName();
   
  // bad
  const foo = "superLongLongLongLongLongLongLongLongString";
   
  // good
  const foo = superLongLongLongLongLongLongLongLongFunctionName();
   
  // good
  const foo = "superLongLongLongLongLongLongLongLongString";

• Disallow unused variables. Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.

  // bad
   
  const some_unused_var = 42;
   
  // Write-only variables are not considered as used.
  let y = 10;
  y = 5;
   
  // A read for a modification of itself is not considered as used.
  let z = 0;
  z = z + 1;
   
  // Unused function arguments.
  function getX(x, y) {
  	return x;
  }
   
  // good
   
  function getXPlusY(x, y) {
  	return x + y;
  }
   
  const x = 1;
  const y = a + 2;
   
  alert(getXPlusY(x, y));
   
  // 'type' is ignored even if unused because it has a rest property sibling.
  // This is a form of extracting an object that omits the specified keys.
  const { type, ...coords } = data;
  // 'coords' is now the 'data' object without its 'type' property.

Hoisting

• var declarations get hoisted to the top of their closest enclosing function scope, their assignment does not. const and let declarations are blessed with a new concept called Temporal Dead Zones (TDZ) (opens in a new tab). It’s important to know why typeof is no longer safe (opens in a new tab).

  // we know this wouldn’t work (assuming there
  // is no notDefined global variable)
  function example() {
  	console.log(notDefined); // => throws a ReferenceError
  }
   
  // creating a variable declaration after you
  // reference the variable will work due to
  // variable hoisting. Note: the assignment
  // value of `true` is not hoisted.
  function example() {
  	console.log(declaredButNotAssigned); // => undefined
  	var declaredButNotAssigned = true;
  }
   
  // the interpreter is hoisting the variable
  // declaration to the top of the scope,
  // which means our example could be rewritten as:
  function example() {
  	let declaredButNotAssigned;
  	console.log(declaredButNotAssigned); // => undefined
  	declaredButNotAssigned = true;
  }
   
  // using const and let
  function example() {
  	console.log(declaredButNotAssigned); // => throws a ReferenceError
  	console.log(typeof declaredButNotAssigned); // => throws a ReferenceError
  	const declaredButNotAssigned = true;
  }

• Anonymous function expressions hoist their variable name, but not the function assignment.

  function example() {
  	console.log(anonymous); // => undefined
   
  	anonymous(); // => TypeError anonymous is not a function
   
  	var anonymous = function () {
  		console.log("anonymous function expression");
  	};
  }

• Named function expressions hoist the variable name, not the function name or the function body.

  function example() {
  	console.log(named); // => undefined
   
  	named(); // => TypeError named is not a function
   
  	superPower(); // => ReferenceError superPower is not defined
   
  	var named = function superPower() {
  		console.log("Flying");
  	};
  }
   
  // the same is true when the function name
  // is the same as the variable name.
  function example() {
  	console.log(named); // => undefined
   
  	named(); // => TypeError named is not a function
   
  	var named = function named() {
  		console.log("named");
  	};
  }

• Function declarations hoist their name and the function body.

  function example() {
  	superPower(); // => Flying
   
  	function superPower() {
  		console.log("Flying");
  	}
  }

• Variables, classes, and functions should be defined before they can be used. When variables, classes, or functions are declared after being used, it can harm readability since a reader won't know what a thing that's referenced is. It's much clearer for a reader to first encounter the source of a thing (whether imported from another module, or defined in the file) before encountering a use of the thing.

  // bad
   
  // Variable a is being used before it is being defined.
  console.log(a); // this will be undefined, since while the declaration is hoisted, the initialization is not
  var a = 10;
   
  // Function fun is being called before being defined.
  fun();
  function fun() {}
   
  // Class A is being used before being defined.
  new A(); // ReferenceError: Cannot access 'A' before initialization
  class A {}
   
  // `let` and `const` are hoisted, but they don't have a default initialization.
  // The variables 'a' and 'b' are in a Temporal Dead Zone where JavaScript
  // knows they exist (declaration is hoisted) but they are not accessible
  // (as they are not yet initialized).
   
  console.log(a); // ReferenceError: Cannot access 'a' before initialization
  console.log(b); // ReferenceError: Cannot access 'b' before initialization
  let a = 10;
  const b = 5;
   
  // good
   
  var a = 10;
  console.log(a); // 10
   
  function fun() {}
  fun();
   
  class A {}
  new A();
   
  let a = 10;
  const b = 5;
  console.log(a); // 10
  console.log(b); // 5

Comparison Operators & Equality

• Use === and !== over == and !=.

• Conditional statements such as the if statement evaluate their expression using coercion with the ToBoolean abstract method and always follow these simple rules:

  • Objects evaluate to true
  • Undefined evaluates to false
  • Null evaluates to false
  • Booleans evaluate to the value of the boolean
  • Numbers evaluate to false if +0, -0, or NaN, otherwise true
  • Strings evaluate to false if an empty string '', otherwise true

  if ([0] && []) {
  	// true
  	// an array (even an empty one) is an object, objects will evaluate to true
  }

• Use shortcuts for booleans, but explicit comparisons for strings and numbers.

  // bad
  if (isValid === true) {
  	// ...
  }
   
  // good
  if (isValid) {
  	// ...
  }
   
  // bad
  if (name) {
  	// ...
  }
   
  // good
  if (name !== "") {
  	// ...
  }
   
  // bad
  if (collection.length) {
  	// ...
  }
   
  // good
  if (collection.length > 0) {
  	// ...
  }

• Use braces to create blocks in case and default clauses that contain lexical declarations (e.g. let, const, function, and class). Lexical declarations are visible in the entire switch block but only get initialized when assigned, which only happens when its case is reached. This causes problems when multiple case clauses attempt to define the same thing.

  // bad
  switch (foo) {
  	case 1:
  		let x = 1;
  		break;
  	case 2:
  		const y = 2;
  		break;
  	case 3:
  		function f() {
  			// ...
  		}
  		break;
  	default:
  		class C {}
  }
   
  // good
  switch (foo) {
  	case 1: {
  		let x = 1;
  		break;
  	}
  	case 2: {
  		const y = 2;
  		break;
  	}
  	case 3: {
  		function f() {
  			// ...
  		}
  		break;
  	}
  	case 4:
  		bar();
  		break;
  	default: {
  		class C {}
  	}
  }

• Ternaries should not be nested and generally be single line expressions.

  // bad
  const foo = maybe1 > maybe2 ? "bar" : value1 > value2 ? "baz" : null;
   
  // split into 2 separated ternary expressions
  const maybeNull = value1 > value2 ? "baz" : null;
   
  // better
  const foo = maybe1 > maybe2 ? "bar" : maybeNull;
   
  // best
  const foo = maybe1 > maybe2 ? "bar" : maybeNull;

• Avoid unneeded ternary statements.

  // bad
  const foo = a ? a : b;
  const bar = c ? true : false;
  const baz = c ? false : true;
  const quux = a != null ? a : b;
   
  // good
  const foo = a || b;
  const bar = !!c;
  const baz = !c;
  const quux = a ?? b;

• When mixing operators, enclose them in parentheses. The only exception is the standard arithmetic operators: +, -, and ** since their precedence is broadly understood. We recommend enclosing / and * in parentheses because their precedence can be ambiguous when they are mixed. This improves readability and clarifies the developer’s intention.

  // bad
  const foo = (a && b < 0) || c > 0 || d + 1 === 0;
   
  // bad
  const bar = a ** b - (5 % d);
   
  // bad
  // one may be confused into thinking (a || b) && c
  if (a || (b && c)) {
  	return d;
  }
   
  // bad
  const bar = a + (b / c) * d;
   
  // good
  const foo = (a && b < 0) || c > 0 || d + 1 === 0;
   
  // good
  const bar = a ** b - (5 % d);
   
  // good
  if (a || (b && c)) {
  	return d;
  }
   
  // good
  const bar = a + (b / c) * d;

• The nullish coalescing operator (??) is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined. Otherwise, it returns the left-hand side operand. It provides precision by distinguishing null/undefined from other falsy values, enhancing code clarity and predictability.

  // bad
  const value = 0 ?? "default";
  // returns 0, not 'default'
   
  // bad
  const value = "" ?? "default";
  // returns '', not 'default'
   
  // good
  const value = null ?? "default";
  // returns 'default'
   
  // good
  const user = {
  	name: "John",
  	age: null,
  };
  const age = user.age ?? 18;
  // returns 18

Blocks

• Use braces with all multiline blocks.

  // bad
  if (test) return false;
   
  // good
  if (test) return false;
   
  // good
  if (test) {
  	return false;
  }
   
  // bad
  function foo() {
  	return false;
  }
   
  // good
  function bar() {
  	return false;
  }

• If you’re using multiline blocks with if and else, put else on the same line as your if block’s closing brace.

  // bad
  if (test) {
  	thing1();
  	thing2();
  } else {
  	thing3();
  }
   
  // good
  if (test) {
  	thing1();
  	thing2();
  } else {
  	thing3();
  }

• If an if block always executes a return statement, the subsequent else block is unnecessary. A return in an else if block following an if block that contains a return can be separated into multiple if blocks.

  // bad
  function foo() {
  	if (x) {
  		return x;
  	} else {
  		return y;
  	}
  }
   
  // bad
  function cats() {
  	if (x) {
  		return x;
  	} else if (y) {
  		return y;
  	}
  }
   
  // bad
  function dogs() {
  	if (x) {
  		return x;
  	} else {
  		if (y) {
  			return y;
  		}
  	}
  }
   
  // good
  function foo() {
  	if (x) {
  		return x;
  	}
   
  	return y;
  }
   
  // good
  function cats() {
  	if (x) {
  		return x;
  	}
   
  	if (y) {
  		return y;
  	}
  }
   
  // good
  function dogs(x) {
  	if (x) {
  		if (z) {
  			return y;
  		}
  	} else {
  		return z;
  	}
  }

Control Statements

• In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should begin the line. Requiring operators at the beginning of the line keeps the operators aligned and follows a pattern similar to method chaining. This also improves readability by making it easier to visually follow complex logic.

  // bad
  if ((foo === 123 || bar === "abc") && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) {
  	thing1();
  }
   
  // bad
  if (foo === 123 && bar === "abc") {
  	thing1();
  }
   
  // bad
  if (foo === 123 && bar === "abc") {
  	thing1();
  }
   
  // bad
  if (foo === 123 && bar === "abc") {
  	thing1();
  }
   
  // good
  if (foo === 123 && bar === "abc") {
  	thing1();
  }
   
  // good
  if ((foo === 123 || bar === "abc") && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) {
  	thing1();
  }
   
  // good
  if (foo === 123 && bar === "abc") {
  	thing1();
  }

• Don't use selection operators in place of control statements.

  // bad
  !isRunning && startRunning();
   
  // good
  if (!isRunning) {
  	startRunning();
  }

Comments

• Use /** ... */ for multiline comments.

  // bad
  // make() returns a new element
  // based on the passed in tag name
  //
  // @param {String} tag
  // @return {Element} element
  function make(tag) {
  	// ...
   
  	return element;
  }
   
  // good
  /**
   * make() returns a new element
   * based on the passed-in tag name
   */
  function make(tag) {
  	// ...
   
  	return element;
  }

• Use // for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it’s on the first line of a block.

  // bad
  const active = true; // is current tab
   
  // good
  // is current tab
  const active = true;
   
  // bad
  function getType() {
  	console.log("fetching type...");
  	// set the default type to 'no type'
  	const type = this.type || "no type";
   
  	return type;
  }
   
  // good
  function getType() {
  	console.log("fetching type...");
   
  	// set the default type to 'no type'
  	const type = this.type || "no type";
   
  	return type;
  }
   
  // also good
  function getType() {
  	// set the default type to 'no type'
  	const type = this.type || "no type";
   
  	return type;
  }

• Start all comments with a space to make it easier to read.

  // bad
  //is current tab
  const active = true;
   
  // good
  // is current tab
  const active = true;
   
  // bad
  /**
   *make() returns a new element
   *based on the passed-in tag name
   */
  function make(tag) {
  	// ...
   
  	return element;
  }
   
  // good
  /**
   * make() returns a new element
   * based on the passed-in tag name
   */
  function make(tag) {
  	// ...
   
  	return element;
  }

• Prefixing your comments with FIXME or TODO helps other developers quickly understand if you’re pointing out a problem that needs to be revisited, or if you’re suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME: -- need to figure this out or TODO: -- need to implement.

• Use // FIXME: to annotate problems.

  class Calculator extends Abacus {
  	constructor() {
  		super();
   
  		// FIXME: shouldn’t use a global here
  		total = 0;
  	}
  }

• Use // TODO: to annotate solutions to problems.

  class Calculator extends Abacus {
  	constructor() {
  		super();
   
  		// TODO: total should be configurable by an options param
  		this.total = 0;
  	}
  }