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Matrix JavaScript/ECMAScript Style Guide

The intention of this guide is to make Matrix's JavaScript codebase clean, consistent with other popular JavaScript styles and consistent with the rest of the Matrix codebase. For reference, the Matrix Python style guide can be found at https://github.com/matrix-org/synapse/blob/master/docs/code_style.md

This document reflects how we would like Matrix JavaScript code to look, with acknowledgement that a significant amount of code is written to older standards.

Write applications in modern ECMAScript and use a transpiler where necessary to target older platforms. When writing library code, consider carefully whether to write in ES5 to allow all JavaScript application to use the code directly or writing in modern ECMAScript and using a transpile step to generate the file that applications can then include. There are significant benefits in being able to use modern ECMAScript, although the tooling for doing so can be awkward for library code, especially with regard to translating source maps and line number through from the original code to the final application.

General Style

4 spaces to indent, for consistency with Matrix Python.
120 columns per line, but try to keep JavaScript code around the 90 column mark. Inline JSX in particular can be nicer with more columns per line.
No trailing whitespace at end of lines.
Don't indent empty lines.
One newline at the end of the file.
Unix newlines, never \r
Indent similar to our python code: break up long lines at logical boundaries, more than one argument on a line is OK
Use semicolons, for consistency with node.
UpperCamelCase for class and type names
lowerCamelCase for functions and variables.
Single line ternary operators are fine.
UPPER_SNAKE_CASE for constants
Use parentheses or ` instead of \ for line continuation where ever possible

Open braces on the same line (consistent with Node):

if (x) {
    console.log("I am a fish"); // Good
}

if (x)
{
    console.log("I am a fish"); // Bad
}

Spaces after if, for, else etc, no space around the condition:

if (x) {
    console.log("I am a fish"); // Good
}

if(x) {
    console.log("I am a fish"); // Bad
}

if ( x ) {
    console.log("I am a fish"); // Bad
}

No new line before else, catch, finally, etc:

if (x) {
    console.log("I am a fish");
} else {
    console.log("I am a chimp"); // Good
}

if (x) {
    console.log("I am a fish");
}
else {
    console.log("I am a chimp"); // Bad
}

Declare one variable per var statement (consistent with Node). Unless they are simple and closely related. If you put the next declaration on a new line, treat yourself to another var:

const key = "foo",
    comparator = function(x, y) {
        return x - y;
    }; // Bad

const key = "foo";
const comparator = function(x, y) {
    return x - y;
}; // Good

let x = 0, y = 0; // Fine

let x = 0;
let y = 0; // Also fine

A single line if is fine, all others have braces. This prevents errors when adding to the code.:

if (x) return true; // Fine

if (x) {
    return true; // Also fine
}

if (x)
    return true; // Not fine

Terminate all multi-line lists, object literals, imports and ideally function calls with commas (if using a transpiler). Note that trailing function commas require explicit configuration in babel at time of writing:

var mascots = [
    "Patrick",
    "Shirley",
    "Colin",
    "Susan",
    "Sir Arthur David" // Bad
];

var mascots = [
    "Patrick",
    "Shirley",
    "Colin",
    "Susan",
    "Sir Arthur David", // Good
];

Use null, undefined etc consistently with node: Boolean variables and functions should always be either true or false. Don't set it to 0 unless it's supposed to be a number. When something is intentionally missing or removed, set it to null. If returning a boolean, type coerce:
```
function hasThings() {
    return !!length; // bad
    return new Boolean(length); // REALLY bad
    return Boolean(length); // good
}
```
Don't set things to undefined. Reserve that value to mean "not yet set to anything." Boolean objects are verboten.
Use JSDoc
Use switch-case statements where there are 5 or more branches running against the same variable.

ECMAScript

Use const unless you need a re-assignable variable. This ensures things you don't want to be re-assigned can't be.
Be careful migrating files to newer syntax.
- Don't mix require and import in the same file. Either stick to the old style or change them all.
- Likewise, don't mix things like class properties and MyClass.prototype.MY_CONSTANT = 42;
- Be careful mixing arrow functions and regular functions, eg. if one function in a promise chain is an arrow function, they probably all should be.
Apart from that, newer ES features should be used whenever the author deems them to be appropriate.
While flow annotations are welcome and encouraged consider to migrate the JS file to TypeScript (see next section).

TypeScript

TypeScript is preferred over the use of JavaScript
Its desirable to convert existing JavaScript files to TypeScript. TypeScript conversions should be done in small chunks without functional changes to ease the review process.
Use full type definitions for function parameters and return values.
Avoid any types and any casts

React

Pull out functions in props to the class, generally as specific event handlers:

<Foo onClick={function(ev) {doStuff();}}> // Bad
<Foo onClick={(ev) => {doStuff();}}> // Equally bad
<Foo onClick={this.doStuff}> // Better
<Foo onClick={this.onFooClick}> // Best, if onFooClick would do anything other than directly calling doStuff

Prefer classes that extend React.Component (or React.PureComponent) instead of React.createClass

You can avoid the need to bind handler functions by using property initializers:

class Widget extends React.Component
    onFooClick = () => {
        ...
    }
}

To define propTypes, use a static property:

class Widget extends React.Component
    static propTypes = {
        ...
    }
}

If you need to specify initial component state, assign it to this.state in the constructor:

constructor(props) {
  super(props);
  // Don't call this.setState() here!
  this.state = { counter: 0 };
}

Prefer class components over function components and hooks (not a strict rule though)
Think about whether your component really needs state: are you duplicating information in component state that could be derived from the model?
Avoid things marked as Legacy or Deprecated in React 16 (e.g string refs and legacy contexts)