Mastering Union and Intersection Type Multiple Line Formatting with ESLint

Are you tired of dealing with messy and hard-to-read code? Do you struggle to keep your types in check? Look no further! In this article, we’ll dive into the world of union and intersection types, and show you how to format them beautifully using multiple lines, all while keeping ESLint happy.

What are Union and Intersection Types?

Before we dive into the formatting, let’s take a quick look at what union and intersection types are. In TypeScript, a union type is a type that can be one of multiple types. For example:

type STRING_OR_NUMBER = string | number;

This type can be either a string or a number. On the other hand, an intersection type is a type that is a combination of multiple types. For example:

type STRING_AND_NUMBER = string & { num: number };

This type is both a string and an object with a num property that is a number.

The Problem with Multiple Line Formatting

When it comes to formatting union and intersection types, things can get a bit tricky. By default, TypeScript will try to keep the type on a single line, which can lead to some pretty ugly code:

type LONG_UNION_TYPE = 
  | string 
  | number 
  | boolean 
  | { foo: string } 
  | { bar: number } 
  | { baz: boolean };

Ouch! That’s a lot to take in on a single line. Fortunately, we can use ESLint to help us format these types across multiple lines.

Configuring ESLint for Multiple Line Formatting

To get started, you’ll need to install ESLint and the TypeScript parser:

npm install eslint @typescript-eslint/parser

Next, create a new file called `.eslintrc.json` in the root of your project with the following configuration:

{
  "root": true,
  "env": {
    "node": true
  },
  "extends": [
    "eslint:recommended",
    "plugin:@typescript-eslint/recommended"
  ],
  "parser": "@typescript-eslint/parser",
  "rules": {
    "type-annotation-spacing": ["error", {
      "before": false,
      "after": true,
      "overrides": {
        " intersectionType": {
          "before": true
        }
      }
    }],
    "multiline-type-annotation": ["error", {
      "before": true
    }]
  }
}

This configuration tells ESLint to use the TypeScript parser and enables the `type-annotation-spacing` and `multiline-type-annotation` rules. These rules will help us format our union and intersection types across multiple lines.

Formatting Union Types

Now that we have ESLint configured, let’s take a look at how to format union types across multiple lines. Here’s an example:

type UNION_TYPE =
  | string
  | number
  | boolean
  | { foo: string }
  | { bar: number }
  | { baz: boolean };

Notice how each type is on a new line, making the code much easier to read. To achieve this, we need to add a line break after the `=` symbol and indent each type with two spaces.

Nesting Union Types

But what happens when we need to nest union types? For example:

type NESTED_UNION_TYPE =
  | string
  | number
  | boolean
  | ({ foo: string } | { bar: number } | { baz: boolean });

In this case, we need to add an extra level of indentation to the nested union type:

type NESTED_UNION_TYPE =
  | string
  | number
  | boolean
  | (
    | { foo: string }
    | { bar: number }
    | { baz: boolean }
  );

By adding an extra level of indentation, we make it clear that the nested union type is a separate entity from the outer union type.

Formatting Intersection Types

Formatting intersection types is similar to formatting union types, but with a few key differences. Here’s an example:

type INTERSECTION_TYPE =
  & string
  & { num: number }
  & { foo: string };

Notice how each intersection type is on a new line, making the code easier to read. However, unlike union types, we need to add an ampersand (`&`) symbol at the beginning of each line to indicate that it’s an intersection type.

Nesting Intersection Types

When it comes to nesting intersection types, things can get a bit more complex. For example:

type NESTED_INTERSECTION_TYPE =
  & string
  & { num: number }
  & ({ foo: string } & { bar: number });

In this case, we need to add an extra level of indentation to the nested intersection type, just like with union types:

type NESTED_INTERSECTION_TYPE =
  & string
  & { num: number }
  & (
    & { foo: string }
    & { bar: number }
  );

By adding an extra level of indentation, we make it clear that the nested intersection type is a separate entity from the outer intersection type.

Best Practices for Multiple Line Formatting

When formatting union and intersection types across multiple lines, here are some best practices to keep in mind:

• Use consistent spacing and indentation to make the code easy to read.
• Keep each type on a new line to avoid clutter and improve readability.
• Use parentheses to group complex types and improve clarity.
• Use ESLint to enforce consistent formatting and catch errors.

Conclusion

In conclusion, formatting union and intersection types across multiple lines can be a bit tricky, but with the right configuration and best practices, you can write clean and readable code that’s easy to maintain. By following the guidelines outlined in this article, you’ll be well on your way to becoming a master of multiple line formatting with ESLint.

Rule	Description
type-annotation-spacing	Ensures consistent spacing between the type annotation and the assignment.
multiline-type-annotation	Allows type annotations to be formatted across multiple lines.

Remember, formatting is all about readability and maintainability. By taking the time to format your code properly, you’ll be able to write better code that’s easier to understand and maintain.

// Happy coding!

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