TypeScript Style Guide and Coding Conventions

Source: Typescript Book - Style Guide

Key Sections:

Variable and Function

  • Use camelCase for variable and function names

Reason: Conventional JavaScript

Bad

var FooVar;
function BarFunc() { }

Good

var fooVar;
function barFunc() { }

Class

  • Use PascalCase for class names.

Reason: This is actually fairly conventional in standard JavaScript.

Bad

class foo { }

Good

class Foo { }
  • Use camelCase of class members and methods

Reason: Naturally follows from variable and function naming convention.

Bad

class Foo {
    Bar: number;
    Baz() { }
}

Good

class Foo {
    bar: number;
    baz() { }
}

Interface

  • Use PascalCase for name.

Reason: Similar to class

  • Use camelCase for members.

Reason: Similar to class

  • Don't prefix with I

Reason: Unconventional. lib.d.ts defines important interfaces without an I (e.g. Window, Document etc).

Bad

interface IFoo {
}

Good

interface Foo {
}

Type

  • Use PascalCase for name.

Reason: Similar to class

  • Use camelCase for members.

Reason: Similar to class

Namespace

  • Use PascalCase for names

Reason: Convention followed by the TypeScript team. Namespaces are effectively just a class with static members. Class names are PascalCase => Namespace names are PascalCase

Bad

namespace foo {
}

Good

namespace Foo {
}

Enum

  • Use PascalCase for enum names

Reason: Similar to Class. Is a Type.

Bad

enum color {
}

Good

enum Color {
}
  • Use PascalCase for enum member

Reason: Convention followed by TypeScript team i.e. the language creators e.g SyntaxKind.StringLiteral. Also helps with translation (code generation) of other languages into TypeScript.

Bad

enum Color {
    red
}

Good

enum Color {
    Red
}

Null vs. Undefined

  • Prefer not to use either for explicit unavailability

Reason: these values are commonly used to keep a consistent structure between values. In TypeScript you use types to denote the structure

Bad

let foo = { x: 123, y: undefined };

Good

let foo: { x: number, y?: number } = { x:123 };
  • Use undefined in general (do consider returning an object like {valid:boolean, value?:Foo} instead)

Bad

return null;

Good

return undefined;
  • Use null where it's a part of the API or conventional

Reason: It is conventional in Node.js e.g. error is null for NodeBack style callbacks.

Bad

cb(undefined)

Good

cb(null)
  • Use truthy check for objects being null or undefined

Bad

if (error === null)

Good

if (error)
  • Use == null / != null (not === / !==) to check for null / undefined on primitives as it works for both null/undefined but not other falsy values (like '', 0, false) e.g.

Bad

if (error !== null) // does not rule out undefined

Good

if (error != null) // rules out both null and undefined

Formatting

The TypeScript compiler ships with a very nice formatting language service. Whatever output it gives by default is good enough to reduce the cognitive overload on the team.

Use tsfmt to automatically format your code on the command line. Also, your IDE (atom/vscode/vs/sublime) already has formatting support built-in.

Examples:

// Space before type i.e. foo:<space>string
const foo: string = "hello";

Quotes

  • Prefer single quotes (') unless escaping.

Reason: More JavaScript teams do this (e.g. airbnb, standard, npm, node, google/angular, facebook/react). It's easier to type (no shift needed on most keyboards). Prettier team recommends single quotes as well

Double quotes are not without merit: Allows easier copy paste of objects into JSON. Allows people to use other languages to work without changing their quote character. Allows you to use apostrophes e.g. He's not going.. But I'd rather not deviate from where the JS Community is fairly decided.

  • When you can't use double quotes, try using back ticks (`).

Reason: These generally represent the intent of complex enough strings.

Spaces

  • Use 2 spaces. Not tabs.

Reason: More JavaScript teams do this (e.g. airbnb, idiomatic, standard, npm, node, google/angular, facebook/react). The TypeScript/VSCode teams use 4 spaces but are definitely the exception in the ecosystem.

Semicolons

  • Use semicolons.

Reasons: Explicit semicolons helps language formatting tools give consistent results. Missing ASI (automatic semicolon insertion) can trip new devs e.g. foo() \n (function(){}) will be a single statement (not two). TC39 warning on this as well. Example teams: airbnb, idiomatic, google/angular, facebook/react, Microsoft/TypeScript.

Array

  • Annotate arrays as foos: Foo[] instead of foos: Array<Foo>.

Reasons: It's easier to read. It's used by the TypeScript team. Makes easier to know something is an array as the mind is trained to detect [].

Filename

Name files with camelCase. E.g. utils.ts, map.ts etc.

Reason: Conventional across many JS teams.

When the file exports a component and your framework (like React) wants component to be PascalCased, use pascal case file name to match e.g. Accordion.tsx, MyControl.tsx.

Reason: Helps with consistency (little overthought required) and its what the ecosystem is doing.

type vs. interface

  • Use type when you might need a union or intersection:
type Foo = number | { someProperty: number }
  • Use interface when you want extends or implements e.g.
interface Foo {
  foo: string;
}
interface FooBar extends Foo {
  bar: string;
}
class X implements FooBar {
  foo: string;
  bar: string;
}
  • Otherwise use whatever makes you happy that day. I use type

== or ===

Both are mostly safe for TypeScript users. I use === as that is what is used in the TypeScript codebase.