fleet/frontend/README.md

# Fleet Front-End

The Fleet front-end is a Single Page Application using React with Typescript and Hooks.

## Table of Contents
- [Running the Fleet web app](#running-the-fleet-web-app)
- [Storybook](#storybook)
- [Directory Structure](#directory-structure)
- [Deprecated](#deprecated)
- [Patterns](#patterns)
  - [Typing](#typing)
  - [React Hooks (Functional Components)](#react-hooks-functional-components)
  - [React Context](#react-context)
  - [Fleet API Calls](#fleet-api-calls)
  - [Page Routing](#page-routing)
  - [Other](#other)

## Running the Fleet web app

For details instruction on building and serving the Fleet web application
consult the [Contributing documentation](../docs/Contributing/README.md).

## Storybook

[Storybook](https://storybook.js.org/) is a tool to document and visualize components, and we 
use it to capture our global components used across Fleet. Storybook is key when developing new 
features and testing components before release. It runs a separate server exposed on port `6006`.
To run this server, do the following:

- Go to your root fleet project directory
- Run `make deps`
- Run `yarn storybook`

The URL `localhost:6006` should automatically show in your browser. If not, visit it manually.

As mentioned, there are two key times Storybook should be used:

1. When building new features

As we create new features, we re-use Fleet components often. Running Storybook before implementing 
new UI elements can clarify if new components need to be created or already exist. This helps us 
avoid duplicating code.

2. Testing components

After creating a component, create a new file, `component.stories.tsx`, within its directory. Then, 
fill it with the appropriate Storybook code to create a new Storybook entry. You will be able to visualize 
the component within Storybook to determine if it looks and behaves as expected.

## Directory Structure

Component directories in the Fleet front-end application encapsulate the entire
component, including files for the component and its styles. The
typical directory structure for a component is as follows:

```
└── ComponentName
  ├── _styles.scss
  ├── ComponentName.tsx
  ├── index.ts
```

- `_styles.scss`: The component css styles
- `ComponentName.tsx`: The React component
- `index.ts`: Exports the React component
  - This file is helpful as it allows other components to import the component
    by it's directory name. Without this file the component name would have to
    be duplicated during imports (`components/ComponentName` vs. `components/ComponentName/ComponentName`).

### [app_constants](./app_constants)

The app_constants directory exports the constants used in the app. Examples
include the app's URL paths, settings, and http statuses. When building features
that require constants, the constants should be added here for accessibility
throughout the application.

### [components](./components)

The component directory contains the React components rendered by pages. They
are typically not connected to the redux state but receive props from their
parent components to render data and handle user interactions.

### [context](./context)

The context directory contains the React Context API pattern for various entities.
Only entities that are needed across the app has a global context. For example,
the [logged in user](./context/app.tsx) (`currentUser`) has multiple pages and components 
where its information is pulled.

### [interfaces](./interfaces)

Files in the interfaces directory are used to specify the Typescript interface for a reusable Fleet
entity. This is designed to DRY up the code and increase re-usability. These
interfaces are imported in to component files and implemented when defining the
component's props.

**Additionally, local interfaces are used for props of local components.**

### [layouts](https://github.com/fleetdm/fleet/tree/main/frontend/layouts)

The Fleet application has only 1 layout, the [Core Layout](./layouts/CoreLayout/CoreLayout.jsx).
The Layout is rendered from the [router](./router/index.tsx) and are used to set up the general 
app UI (header, sidebar) and render child components.
The child components rendered by the layout are typically page components.

### [pages](./pages)

Page components are React components typically rendered from the [router](./router).
React Router passed props to these pages in case they are needed. Examples include 
the `router`, `location`, and `params` objects.

### [router](./router)

The router directory is where the react router lives. The router decides which
component will render at a given URL. Components rendered from the router are
typically located in the [pages directory](./pages). The router directory also holds a `paths`
file which holds the application paths as string constants for reference
throughout the app. These paths are typically referenced from the [App
Constants](./app_constants) object.

### [services](./services)

CRUD functions for all Fleet entities (e.g. `query`) that link directly to the Fleet API.

### [styles](./styles)

The styles directory contains the general app style setup and variables. It
includes variables for the app color hex codes, fonts (families, weights and sizes), and padding.

### [templates](./templates)

The templates directory contains the HTML file that renders the React application via including the `bundle.js`
and `bundle.css` files. The HTML page also includes the HTML element in which the React application is mounted.

### [utilities](./utilities)

The utilities directory contains re-usable functions and constants for use throughout the
application. The functions include helpers to convert an array of objects to
CSV, debounce functions to prevent multiple form submissions, format API errors,
etc.

## Deprecated

These directories and files are still used (as of 9/14/21) but are being replaced by newer code:

- [fleet](./fleet); now using [services](./services)
- [redux](./redux); now using [services](./services), local states, and various entities directly (e.g. React Router)
- [Form.jsx Higher Order Component](./components/forms/README.md); now creating forms with local states with React Hooks (i.e. `useState`)

To view the deprecated documentation, [click here](./README_deprecated.md).

## Patterns

### Typing
All Javascript and React files use Typescript, meaning the extensions are `.ts` and `.tsx`. Here are the guidelines on how we type at Fleet:

- Use *[global entity interfaces](#interfaces)* when interfaces are used multiple times across the app
- Use *local interfaces* when typing entities limited to the specific page or component
- Local interfaces for page and component props

  ```typescript
  // page
  interface IPageProps {
    prop1: string;
    prop2: number;
    ...
  }

  // Note: Destructure props in page/component signature
  const PageOrComponent = ({
    prop1,
    prop2,
  }: IPageProps) => {
    
    return (
      // ...
    );
  };
  ```

- Local states 
```typescript 
const [item, setItem] = useState<string>("");
```

- Fetch function signatures (i.e. `react-query`)
```typescript
useQuery<IHostResponse, Error, IHost>(params)
```

- Custom functions, including callbacks
```typescript
const functionWithTableName = (tableName: string): boolean => {
  // do something
};
```

### React Hooks (Functional Components)

[Hooks](https://reactjs.org/docs/hooks-intro.html) are used to track state and use other features
of React. Hooks are only allowed in functional components, which are created like so:

```typescript
import React, { useState, useEffect } from "React";

const PageOrComponent = (props) => {
  const [item, setItem] = useState<string>("");

  // runs only on first mount (replaces componentDidMount)
  useEffect(() => {
    // do something
  }, []);

  // runs only when `item` changes (replaces componentDidUpdate) 
  useEffect(() => {
    // do something
  }, [item]);
  
  return (
    // ...
  );
};
```

**Note: Other hooks are available per [React's documentation](https://reactjs.org/docs/hooks-intro.html).**

### React Context

[React Context](https://reactjs.org/docs/context.html) is a store similar to Redux. It stores 
data that is desired and allows for retrieval of that data in whatever component is in need.
View currently working contexts in the [context directory](./context).

### Fleet API Calls

**Deprecated:** 

Redux was used to make API calls, along with the [fleet](./fleet) directory.

**Current:**

The [services](./services) directory stores all API calls and is to be used in two ways: 
- A direct `async/await` assignment
- Using `react-query` if requirements call for loading data right away or based on dependencies. 

Examples below:

*Direct assignment*
```typescript
// page
import ...
import queriesAPI from "services/entities/queries";

const PageOrComponent = (props) => {
  const doSomething = async () => {
    try {
      const response = await queriesAPI.load(param);
      // do something
    } catch(error) {
      console.error(error);
      // maybe trigger renderFlash
    }
  };
  
  return (
    // ...
  );
};
```

*React Query*

`react-query` ([docs here](https://react-query.tanstack.com/overview)) is a data-fetching library that
gives us the ability to fetch, cache, sync and update data with a myriad of options and properties.

```typescript
import ...
import { useQuery, useMutation } from "react-query";
import queriesAPI from "services/entities/queries";

const PageOrComponent = (props) => {
  // retrieve the query based on page/component load
  // and dependencies for when to refetch
  const {
    isLoading,
    data,
    error,
    ...otherProps,
  } = useQuery<IResponse, Error, IData>(
    "query",
    () => queriesAPI.load(param),
    {
      ...options
    }
  );

  // `props` is a bucket of properties that can be used when 
  // updating data. for example, if you need to know whether
  // a mutation is loading, there is a prop for that.
  const { ...props } = useMutation((formData: IForm) =>
    queriesAPI.create(formData)
  );
  
  return (
    // ...
  );
};
```

### Page Routing

**Deprecated:** 

Redux was used to manage redirecting to different pages of the app.

**Current:**

We use React Router directly to navigate between pages. For page components,
React Router (v3) supplies a `router` prop that can be easily accessed.
When needed, the `router` object contains a `push` function that redirects
a user to whatever page desired. For example:

```typescript
// page
import PATHS from "router/paths";
import { InjectedRouter } from "react-router/lib/Router";

interface IPageProps {
  router: InjectedRouter; // v3
}

const PageOrComponent = ({
  router,
}: IPageProps) => {
  const doSomething = () => {
    router.push(PATHS.HOME);
  };
  
  return (
    // ...
  );
};
```

### Other

**Local states**

Our first line of defense for state management is local states (i.e. `useState`). We
use local states to keep pages/components separate from one another and easy to 
maintain. If states need to be passed to direct children, then prop-drilling should 
suffice as long as we do not go more than two levels deep. Otherwise, if states need 
to be used across multiple unrelated components or 3+ levels from a parent, 
then the [app's context](#react-context) should be used. 

**File size**

The recommend line limit per page/component is 500 lines. This is only a recommendation.
Larger files are to be split into multiple files if possible.