JavaScript Coding Standards

For almost all of our code style rules, refer to the Airbnb JavaScript Style Guide.

When writing ES6 or React code, please also refer to the Airbnb React/JSX Style Guide.

We use Prettier to automatically style our code.

• You can run Prettier to fix the style on all files with npm run prettier
• You can run Prettier in watch mode to fix the styles when they are saved with npm run prettier-watch

There are a few things that we have customized for our tastes which will take precedence over Airbnb's guide.

Naming Conventions

Event Handlers

• When you have an event handler, do not prefix it with "on" or "handle". The method should be named for what it does, not what it handles. This promotes code reuse by minimizing assumptions that a method must be called in a certain fashion (eg. only as an event handler).

• One exception for allowing the prefix of "on" is when it is used for callback props of a React component. Using it in this way helps to distinguish callbacks from public component methods.

  // Bad
  const onSubmitClick = () => {
      // Validate form items and submit form
  };
  
  // Good
  const validateAndSubmit = () => {
      // Validate form items and submit form
  };

Boolean variables and props

• Boolean props or variables must be prefixed with should or is to make it clear that they are Boolean. Use should when we are enabling or disabling some features and is in most other cases.

// Bad
<SomeComponent showIcon />

// Good
<SomeComponent shouldShowIcon />

// Bad
const valid = props.something && props.somethingElse;

// Good
const isValid = props.something && props.somethingElse;

Functions

Any function declared in a library module should use the function myFunction keyword rather than const myFunction.

// Bad
const myFunction = () => {...};

export {
    myFunction,
}

// Good
function myFunction() {
    ...
}

export {
    myFunction,
}

Using arrow functions is the preferred way to write an anonymous function such as a callback method.

// Bad
_.map(someArray, function (item) {...});

// Good
_.map(someArray, (item) => {...});

Empty functions (noop) should be declare as arrow functions with no whitespace inside. Avoid _.noop()

// Bad
const callback = _.noop;
const callback = () => { };

// Good
const callback = () => {};

var, const and let

• Never use var
• Use const when you are not reassigning a variable
• Try to write your code in a way where the variable reassignment isn't necessary
• Use let only if there are no other options

// Bad
let array = [];

if (someCondition) {
    array = ['addValue1'];
}

// Good
const array = [];

if (someCondition) {
    array.push('addValue1');
}

Object / Array Methods

We have standardized on using underscore.js methods for objects and collections instead of the native Array instance methods. This is mostly to maintain consistency, but there are some type safety features and conveniences that underscore methods provide us e.g. the ability to iterate over an object and the lack of a TypeError thrown if a variable is undefined.

// Bad
myArray.forEach(item => doSomething(item));
// Good
_.each(myArray, item => doSomething(item));

// Bad
const myArray = Object.keys(someObject).map(key => doSomething(someObject[key]));
// Good
const myArray = _.map(someObject, (value, key) => doSomething(value));

// Bad
myCollection.includes('item');
// Good
_.contains(myCollection, 'item');

// Bad
const modifiedArray = someArray.filter(filterFunc).map(mapFunc);
// Good
const modifiedArray = _.chain(someArray)
    .filter(filterFunc)
    .map(mapFunc)
    .value();

Accessing Object Properties and Default Values

Use lodashGet() to safely access object properties and || to short circuit null or undefined values that are not guaranteed to exist in a consistent way throughout the codebase. In the rare case that you want to consider a falsy value as usable and the || operator prevents this then be explicit about this in your code and check for the type using an underscore method e.g. _.isBoolean(value) or _.isEqual(0).

// Bad
const value = somePossiblyNullThing ?? 'default';
// Good
const value = somePossiblyNullThing || 'default';
// Bad
const value = someObject.possiblyUndefinedProperty?.nestedProperty || 'default';
// Bad
const value = (someObject && someObject.possiblyUndefinedProperty && someObject.possiblyUndefinedProperty.nestedProperty) || 'default';
// Good
const value = lodashGet(someObject, 'possiblyUndefinedProperty.nestedProperty', 'default');

JSDocs

• Always document parameters and return values.

• Optional parameters should be enclosed by [] e.g. @param {String} [optionalText].

• Document object parameters with separate lines e.g. @param {Object} parameters followed by @param {String} parameters.field.

• If a parameter accepts more than one type use * to denote there is no single type.

• Use uppercase when referring to JS primitive values (e.g. Boolean not bool, Number not int, etc).

• When specifying a return value use @returns instead of @return. If there is no return value do not include one in the doc.

• Avoid descriptions that don't add any additional information. Method descriptions should only be added when it's behavior is unclear.

• Do not use block tags other than @param and @returns (e.g. @memberof, @constructor, etc).

• Do not document default parameters. They are already documented by adding them to a declared function's arguments.

• Do not use record types e.g. {Object.<string, number>}.

• Do not create @typedef to use in JSDocs.

• Do not use type unions e.g. {(number|boolean)}.

// Bad
/**
 * Populates the shortcut modal
 * @param {bool} shouldShowAdvancedShortcuts whether to show advanced shortcuts
 * @return {*}
 */
function populateShortcutModal(shouldShowAdvancedShortcuts) {
}

// Good
/**
 * @param {Boolean} shouldShowAdvancedShortcuts
 * @returns {Boolean}
 */
function populateShortcutModal(shouldShowAdvancedShortcuts) {
}

Destructuring

We should avoid using object destructuring in situations where it reduces code clarity. Here are some general guidelines on destructuring.

General Guidelines

• Avoid object destructuring for a single variable that you only use once. It's clearer to use dot notation for accessing a single variable.

// Bad
const {data} = event.data;

// Good
const {name, accountID, email} = data;

React Components

Always use destructuring to get prop values. Destructuring is necessary to assign default values to props.

// Bad
function UserInfo(props) {
    return (
        <View>
            <Text>Name: {props.name}</Text>
            <Text>Email: {props.email}</Text>
        </View>
}

UserInfo.defaultProps = {
    name: 'anonymous';
}

// Good
function UserInfo({ name = 'anonymous', email }) {
    return (
        <View>
            <Text>Name: {name}</Text>
            <Text>Email: {email}</Text>
        </View>
    );
}

Named vs Default Exports in ES6 - When to use what?

ES6 provides two ways to export a module from a file: named export and default export. Which variation to use depends on how the module will be used.

• If a file exports a single JS object (e.g. a React component, or an IIFE), then use export default
• Files with multiple exports should always use named exports
• Files with a single method or variable export are OK to use named exports
• Mixing default and named exports in a single file is OK (e.g. in a self contained module), but should rarely be used
• All exports (both default and named) should happen at the bottom of the file
• Do not export individual features inline.

// Bad
export const something = 'nope';
export const somethingElse = 'stop';

// Good
const something = 'yep';
const somethingElse = 'go';

export {
    something,
    somethingElse,
};

Classes and constructors

Class syntax

Using the class syntax is preferred wherever appropriate. Airbnb has clear guidelines in their JS style guide which promotes using the class syntax. Don't manipulate the prototype directly. The class syntax is generally considered more concise and easier to understand.

Constructor

Classes have a default constructor if one is not specified. No need to write a constructor function that is empty or just delegates to a parent class.

// Bad
class Jedi {
    constructor() {}

    getName() {
        return this.name;
    }
}

// Bad
class Rey extends Jedi {
    constructor(...args) {
        super(...args);
    }
}

// Good
class Rey extends Jedi {
    constructor(...args) {
        super(...args);
        this.name = 'Rey';
    }
}

ESNext: Are we allowed to use [insert new language feature]? Why or why not?

JavaScript is always changing. We are excited whenever it does! However, we tend to take our time considering whether to adopt the latest and greatest language features. The main reason for this is consistency. We have a style guide so that we don't have to have endless conversations about how our code looks and can focus on how it runs.

So, if a new language feature isn't something we have agreed to support it's off the table. Sticking to just one way to do things reduces cognitive load in reviews and also makes sure our knowledge of language features progresses at the same pace. If a new language feature will cause considerable effort for everyone to adapt to or we're just not quite sold on the value of it yet we won't support it.

Here are a couple of things we would ask that you avoid to help maintain consistency in our codebase:

• Async/Await - Use the native Promise instead
• Optional Chaining - Use lodashGet() to fetch a nested value instead
• Null Coalescing Operator - Use lodashGet() or || to set a default value for a possibly undefined or null variable

React Coding Standards

React specific styles

Method Naming and Code Documentation

• Prop callbacks should be named for what has happened, not for what is going to happen. Components should never assume anything about how they will be used (that's the job of whatever is implementing it).

// Bad
const propTypes = {
    /** A callback to call when we want to save the form */
    onSaveForm: PropTypes.func.isRequired,
};

// Good
const propTypes = {
    /** A callback to call when the form has been submitted */
    onFormSubmitted: PropTypes.func.isRequired,
};

• Do not use underscores when naming private methods.
• Add descriptions to all propTypes using a block comment above the definition. No need to document the types (that's what propTypes is doing already), but add some context for each property so that other developers understand the intended use.

// Bad
const propTypes = {
    currency: PropTypes.string.isRequired,
    amount: PropTypes.number.isRequired,
    isIgnored: PropTypes.bool.isRequired
};

// Bad
const propTypes = {
    // The currency that the reward is in
    currency: React.PropTypes.string.isRequired,

    // The amount of reward
    amount: React.PropTypes.number.isRequired,

    // If the reward has been ignored or not
    isIgnored: React.PropTypes.bool.isRequired
}

// Good
const propTypes = {
    /** The currency that the reward is in */
    currency: React.PropTypes.string.isRequired,

    /** The amount of the reward */
    amount: React.PropTypes.number.isRequired,

    /** If the reward has not been ignored yet */
    isIgnored: React.PropTypes.bool.isRequired
}

All propTypes and defaultProps must be defined at the top of the file in variables called propTypes and defaultProps. These variables should then be assigned to the component at the bottom of the file.

MyComponent.propTypes = propTypes;
MyComponent.defaultProps = defaultProps;
export default MyComponent;

Any nested propTypes e.g. that may appear in a PropTypes.shape({}) should also be documented.

// Bad
const propTypes = {
    /** Session data */
    session: PropTypes.shape({
        authToken: PropTypes.string,
        login: PropTypes.string,
    }),
}

// Good
const propTypes = {
    /** Session data */
    session: PropTypes.shape({

        /** Token used to authenticate the user */
        authToken: PropTypes.string,

        /** User email or phone number */
        login: PropTypes.string,
    }),
}

Inline Ternaries

• Use inline ternary statements when rendering optional pieces of templates. Notice the white space and formatting of the ternary.

// Bad
{
    const optionalTitle = props.title ? <div className="title">{props.title}</div> : null;
    return (
        <div>
            {optionalTitle}
            <div className="body">This is the body</div>
        </div>
    );
}

// Good
{
    return (
        <div>
            {props.title
                ? <div className="title">{props.title}</div>
                : null}
            <div className="body">This is the body</div>
        </div>
    );
}

// Good
{
    return (
        <div>
            {props.title
                ? <div className="title">{props.title}</div>
                : <div className="title">Default Title</div>
            }
            <div className="body">This is the body</body>
        </div>
    );
}

Important Note:

In React Native, one must not attempt to falsey-check a string for an inline ternary. Even if it's in curly braces, React Native will try to render it as a <Text> node and most likely throw an error about trying to render text outside of a <Text> component. Use _.isEmpty() instead.

// Bad! This will cause a breaking an error on native platforms
{
    return (
        <View>
            {props.title
                ? <View style={styles.title}>{props.title}</View>
                : null}
            <View style={styles.body}>This is the body</View>
        </View>
    );
}

// Good
{
    return (
        <View>
            {!_.isEmpty(props.title)
                ? <View style={styles.title}>{props.title}</View>
                : null}
            <View style={styles.body}>This is the body</View>
        </View>
    );
}

Function component style

When writing a function component you must ALWAYS add a displayName property and give it the same value as the name of the component (this is so it appears properly in the React dev tools)

    function Avatar(props) {...};

    Avatar.propTypes = propTypes;
    Avatar.defaultProps = defaultProps;
    Avatar.displayName = 'Avatar';

    export default Avatar;

Forwarding refs

When forwarding a ref define named component and pass it directly to the forwardRef. By doing this we remove potential extra layer in React tree in form of anonymous component.

    function FancyInput(props, ref) {
        ...
        return <input {...props} ref={ref} />
    }

    export default React.forwardRef(FancyInput)

Stateless components vs Pure Components vs Class based components vs Render Props - When to use what?

Class components are DEPRECATED. Use function components and React hooks.

https://react.dev/reference/react/Component#migrating-a-component-with-lifecycle-methods-from-a-class-to-a-function

Composition vs Inheritance

From React's documentation -

Props and composition give you all the flexibility you need to customize a component’s look and behavior in an explicit and safe way. Remember that components may accept arbitrary props, including primitive values, React elements, or functions. If you want to reuse non-UI functionality between components, we suggest extracting it into a separate JavaScript module. The components may import it and use that function, object, or a class, without extending it.

Use an HOC a.k.a. Higher order component if you find a use case where you need inheritance.

If several HOC need to be combined there is a compose() utility. But we should not use this utility when there is only one HOC.

// Bad
export default compose(
    withLocalize,
)(MyComponent);

// Good
export default compose(
    withLocalize,
    withWindowDimensions,
)(MyComponent);

// Good
export default withLocalize(MyComponent)

Note: If you find that none of these approaches work for you, please ask an Expensify engineer for guidance via Slack or GitHub.

Use Refs Appropriately

React's documentation explains refs in detail. It's important to understand when to use them and how to use them to avoid bugs and hard to maintain code.

A common mistake with refs is using them to pass data back to a parent component higher up the chain. In most cases, you can try lifting state up to solve this.

There are several ways to use and declare refs and we prefer the callback method.

Are we allowed to use [insert brand new React feature]? Why or why not?

We love React and learning about all the new features that are regularly being added to the API. However, we try to keep our organization's usage of React limited to the most stable set of features that React offers. We do this mainly for consistency and so our engineers don't have to spend extra time trying to figure out how everything is working. That said, if you aren't sure if we have adopted something please ask us first.

React Hooks: Frequently Asked Questions

Are Hooks a Replacement for HOCs or Render Props?

In most cases, a custom hook is a better pattern to use than an HOC or Render Prop. They are easier to create, understand, use and document. However, there might still be a case for a HOC e.g. if you have a component that abstracts some conditional rendering logic.

Should I wrap all my inline functions with useCallback() or move them out of the component if they have no dependencies?

The answer depends on whether you need a stable reference for the function. If there are no dependencies, you could move the function out of the component. If there are dependencies, you could use useCallback() to ensure the reference updates only when the dependencies change. However, it's important to note that using useCallback() may have a performance penalty, although the trade-off is still debated. You might choose to do nothing at all if there is no obvious performance downside to declaring a function inline. It's recommended to follow the guidance in the React documentation and add the optimization only if necessary. If it's not obvious why such an optimization (i.e. useCallback() or useMemo()) would be used, leave a code comment explaining the reasoning to aid reviewers and future contributors.

Why does useState() sometimes get initialized with a function?

React saves the initial state once and ignores it on the next renders. However, if you pass the result of a function to useState() or call a function directly e.g. useState(doExpensiveThings()) it will still run on every render. This can hurt performance depending on what work the function is doing. As an optimization, we can pass an initializer function instead of a value e.g. useState(doExpensiveThings) or useState(() => doExpensiveThings()).

Is there an equivalent to componentDidUpdate() when using hooks?

The short answer is no. A longer answer is that sometimes we need to check not only that a dependency has changed, but how it has changed in order to run a side effect. For example, a prop had a value of an empty string on a previous render, but now is non-empty. The generally accepted practice is to store the "previous" value in a ref so the comparison can be made in a useEffect() call.

Are useCallback() and useMemo() basically the same thing?

No! It is easy to confuse useCallback() with a memoization helper like _.memoize() or useMemo() but they are really not the same at all. useCallback() will return a cached function definition and will not save us any computational cost of running that function. So, if you are wrapping something in a useCallback() and then calling it in the render then it is better to use useMemo() to cache the actual result of calling that function and use it directly in the render.

What is the exhaustive-deps lint rule? Can I ignore it?

A useEffect() that does not include referenced props or state in its dependency array is usually a mistake as often we want effects to re-run when those dependencies change. However, there are some cases where we might actually only want to re-run the effect when only some of those dependencies change. We determined the best practice here should be to allow disabling the “next line” with a comment //eslint-disable-next-line react-hooks/exhaustive-deps and an additional comment explanation so the next developer can understand why the rule was not used.

Should I declare my components with arrow functions (const) or the function keyword?

There are pros and cons of each, but ultimately we have standardized on using the function keyword to align things more with modern React conventions. There are also some minor cognitive overhead benefits in that you don't need to think about adding and removing brackets when encountering an implicit return. The function syntax also has the benefit of being able to be hoisted where arrow functions do not.

How do I auto-focus a TextInput using useFocusEffect()?

const focusTimeoutRef = useRef(null);

useFocusEffect(useCallback(() => {
    focusTimeoutRef.current = setTimeout(() => textInputRef.current.focus(), CONST.ANIMATED_TRANSITION);
    return () => {
        if (!focusTimeoutRef.current) {
            return;
        }
        clearTimeout(focusTimeoutRef.current);
    };
}, []));

This works better than using onTransitionEnd because -

1. onTransitionEnd is only fired for the top card in the stack, and therefore does not fire on the new top card when popping a card off the stack. For example - pressing the back button to go from the workspace invite page to the workspace members list.
2. Using InteractionsManager.runAfterInteractions with useFocusEffect will interrupt an in-progress transition animation.

Note - This is a solution from this PR. You can find detailed discussion in comments.

Onyx Best Practices

Onyx Documentation

Collection Keys

Our potentially larger collections of data (reports, policies, etc) are typically stored under collection keys. Collection keys let us group together individual keys vs. storing arrays with multiple objects. In general, do not add a new collection key if it can be avoided. There is most likely a more logical place to put the state. And failing to associate a state property with it's logical owner is something we consider to be an anti-pattern (unnecessary data structure adds complexity for no value).

For example, if you are storing a boolean value that could be associated with a report object under a new collection key it is better to associate this information with the report itself and not create a new collection key.

Exception: There are some gotchas when working with complex nested array values in Onyx. So, this could be another valid reason to break a property off of it's parent object (e.g. reportActions are easier to work with as a separate collection).

If you're not sure whether something should have a collection key reach out in #expensify-open-source for additional feedback.