Coding Guidelines

Rob Moran edited this page Aug 3, 2018 · 16 revisions


Use 4 spaces per indentation


  • Use PascalCase for type names
  • Use PascalCase for enum values
  • Use camelCase for function and method names
  • Use camelCase for property names and local variables
  • Use whole words in names when possible
  • Use lower case, dash-separated file names (e.g. document-provider.ts)
  • Name files after the main Type it exports
  • Add architectural types to the file name separated by a dot. (e.g. file-navigator.plugin.ts)
  • Do not use "_" as a prefix for private properties


  • Do not export types or functions unless you need to share it across multiple components, see as well
  • Do not introduce new types or values to the global namespace


  • Do not use I prefix for interfaces.
  • Use Impl suffix for implementation of interfaces with the same name.
  • See #624 for the discussion on this.


  • Use JSDoc style comments for functions, interfaces, enums, and classes


null and undefined

Use undefined, do not use null.


  • Use arrow functions => over anonymous function expressions
  • Only surround arrow function parameters when necessary. For example, (x) => x + x is wrong but the following are correct:
x => x + x
(x,y) => x + y
<T>(x: T, y: T) => x === y
  • Always surround loop and conditional bodies with curly braces
  • Open curly braces always go on the same line as whatever necessitates them
  • Parenthesized constructs should have no surrounding whitespace. A single space follows commas, colons, and semicolons in those constructs. For example:
for (var i = 0, n = str.length; i < 10; i++) { }
if (x < 10) { }
function f(x: number, y: string): void { }
  • Use a single declaration per variable statement
    (i.e. use var x = 1; var y = 2; over var x = 1, y = 2;).
  • else goes on the line of the closing curly brace.

Dependency Injection

  • 1. Use the property injection over the construction injection. Adding new dependencies via the construction injection is a breaking change.
  • 2. Use postConstruct to initialize an object, for example to register event listeners.
export class MyComponent {

    protected readonly shell: ApplicationShell;

    protected init(): void { => this.doSomething());


  • 3. Make sure to add inSingletonScope for singleton instances, otherwise a new instance will be created on each injection request.
// bad

// good

  • 4. Don't export functions, convert them into class methods. Functions cannot be overridden to change the behaviour or workaround a bug.
// bad
export function createWebSocket(url: string): WebSocket {

// good
export class WebSocketProvider {
   protected createWebSocket(url: string): WebSocket {

export class MyWebSocketProvider extends WebSocketProvider {
   protected createWebSocket(url: string): WebSocket {
      // create a web socket with custom options


  • 4.1 Convenient functions which are based on the stable API can be exported in the corresponding namespace.

In this case clients:

  • can customize behaviour via exchanging the API implementation
  • have a choice to use convenient functions or an API directly
export namespace MonacoEditor {
    // convenient function to get a Monaco editor based on the editor manager API
    export function getCurrent(manager: EditorManager): MonacoEditor | undefined {
        return get(manager.currentEditor);

  • 4.2 The special case of 4.1 is functions on a JSON type.

JSON types are not supposed to be implementable, but only instantiable. They cannot have functions to avoid serialization issues.

export interface CompositeTreeNode extends TreeNode {
    children: ReadonlyArray<TreeNode>;

    // bad - JSON types should not have functions
    getFirstChild(): TreeNode | undefined;

// good - JSON types can have corresponding namespaces with functions
export namespace CompositeTreeNode {
    export function getFirstChild(parent: CompositeTreeNode): TreeNode | undefined {
        return parent.children[0];

// bad - JSON types should not be implemented
export class MyCompositeTreeNode implements CompositeTreeNode {

// good - JSON types can be extended
export interface MyCompositeTreeNode extends CompositeTreeNode {

  • 4.3 Auxiliary functions which are called from the customizable context can be exported in the corresponding namespace.
export class DirtyDiffModel {
    // this method can be overridden, subclasses have an access to `DirtyDiffModel.documentContentLines`
    protected handleDocumentChanged(document: TextEditorDocument): void {
        this.currentContent = DirtyDiffModel.documentContentLines(document);
export namespace DirtyDiffModel {
    // the auxiliary function
    export function documentContentLines(document: TextEditorDocument): ContentLines {


  • Use the lower-case-with-dashes format.
  • Prefix classes with theia when used as global classes.
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