10 Steps to Create a Custom ESLint Plugin
Tags: javascript, webdev, programming, coding
Published: 2024-06-28 14:02:45

**Ever get frustrated with confusing or inconsistent function names in your code?** 

In this article, we're going to solve that problem by creating a custom ESLint plugin to keep code neat and readable by enforcing consistent function names.

So, **what's ESLint?** Think of it as a helpful tool that checks your code for mistakes and makes sure you're following good coding practices. The cool part? You can create your own rules to fit your style!

### Let's get started: 

- **New Project:** Create a new project and install ESLint by running the following command in your terminal: 
```
mkdir myplugin
cd myplugin
npm init --y
npx eslint --init
```
---
- **Plugin Directory:** Next, create a directory called **eslint-plugin** and a file named **index.js** inside it. This is where our custom rules will live.

---
- **Plugin Structure:** In **index.js**, we'll set up our plugin. This file will have some properties like **meta** and **rules**.

  * **meta:** This holds the plugin name and version.
  * **rules:** This is where we'll define our custom rule to check function names.

```javascript
const plugin = {
  meta: {
    name: "func-prefix-matching",
    version: "0.0.1",
  },
  rules: {}
};
```
---

- **Rule Object:** Inside `rules`, we'll create our rule. The key is the rule ID, and the value is an object with a `create` function.

```javascript
const plugin = {
  meta: {
    name: "func-prefix-matching",
    version: "0.0.1",
  },
  rules: {
    prefix: {},
  },
};
```
---
- **Checking Function Names:** `create` function returns an object with a `Identifier` method. In the `Identifier` function, we check if the code is an arrow function. Then, we make sure the function names start with a specific prefix, like "on" or "get" etc.

```javascript
const rulePrefix = ["is", "pre", "on", "post", "get", "set"];

const isValidName = (name) => {
  const isValid = (prefix) => name.indexOf(prefix) === 0;
  return rulePrefix.some(isValid);
};

const plugin = {
  meta: {
    name: "func-prefix-matching",
    version: "0.0.1",
  },
  rules: {
    prefix: {
      create(context) {
        return {
          Identifier: (node) => {
            if (node.parent?.init?.type === "ArrowFunctionExpression") {
              const { name } = node;
              if (!isValidName(name)) {
                context.report(
                  node,
                  `${name} should start with ${rulePrefix.join(", ")}.`
                );
              }
            }
          },
        };
      },
    },
  },
};


```
---
- **Reporting Errors:** If a function name doesn't start with the right prefix, we use `context.report()` to flag it as an error.

```javascript
context.report(node, `${name} should start with ${rulePrefix.join(", ")}.`);
```
---
- **ESLint Configuration:** Now, we need to tell ESLint to use our plugin. We do this by adding it to the `plugins` object in our ESLint configuration file.
```javascript
import prefixMatcher from "./plugin/index.mjs";

export default [
  {
    files: ["src/**/*.js"],
    plugins: { prefixMatcher },
  },
];
```
---
- **Configuring the Rule:** Set up the rule in the `rules` section with your chosen namespace.
```javascript
import prefixMatcher from "./plugin/index.mjs";

export default [
  {
    files: ["src/**/*.js"],
    plugins: { prefixMatcher },
    rules: {
      "prefixMatcher/prefix": "warn",
    },
  },
];

```
---

- **Testing Code:** Create some code examples with both correct and incorrect function names in a folder called `src`.
```javascript
const someFunction = () => {
  console.log("Hello World");
};
someFunction();
```
---
- **Running ESLint:** Finally, run `npm run lint` in your terminal to see ESLint in action.
```shell
npm run lint
```
---
If everything's set up correctly, ESLint will check your code and show error messages for any function names that don't follow the rules. 
```shell
1:7  warning  someFunction should start with is, pre, on, post, get, set  prefixMatcher/prefix

✖ 1 problem (0 errors, 1 warning)
```

Thanks for reading! I hope this article helps you create your own ESLint plugin to enforce consistent function names in your code. I'm also attaching the code for the plugin **[here](https://stackblitz.com/edit/stackblitz-starters-d1f3hp)**.

---

##### Must Read If you haven't

{% link https://dev.to/devsmitra/how-to-create-the-app-using-jsx-without-react-k08 %}

{% link https://dev.to/devsmitra/reactjs-signal-for-state-mangement-21pf %}

{% link https://dev.to/devsmitra/simplify-javascripts-async-concepts-with-one-gif-3c6a %}

---

###### More content at [Dev.to](https://dev.to/devsmitra "Rahul Sharma(DevsMitra)").

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