[Frontend Engineering]: Apply Eslint on Vanilla JavaScript code

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Abstract

This note tracked why we need eslint and how to use eslint in Valina JavaScript and TypeScript projects.

Background

Why?: People may have different habits and preferences on how to format code and name a variable. While this is not necessarily a problem, team collaboration, codebase maintainability, and readability would be annoying issues when working with other people on a large project.

So here comes a question, how can I enforce a consistent style across team members?

JSLint is the first trial of the above problem. Eslint, a successor, is adopted widely by developers. After typescript adoptions, typescript-eslint project emerged to satisfy the needs of force code style on typescript projects.

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Core concepts in ESlint

To leverage eslint, understanding these core concepts in eslint is critical.

• configuration file
  • What is it? The eslint configuration file is a guide to tell ESLint your expectations about what you want from it.
  • how to generate it if I'm a newbie of it? To install and configure ESlint, run the following command on your terminal

npm init @eslint/config

After a series of guiding steps from the above command, you will get a basic eslint configuration file .eslintrc.cjs under your root folder.

• Understand ESlint configuration core concepts

  • parser: An ESLint parser converts code into an abstract syntax tree that ESLint can evaluate. For example, example, @typescript-eslint/parser is a custom parser included in the typescript-eslint project that lets ESLint parse TypeScript code.

  module.exports = {
     parser: "@typescript-eslint/parser",
  }
  

  • plugins: An ESLint plugin is an npm module that can contain a set of ESLint rules, configurations, processors, and environments.
  • env: The JavaScript ecosystem has a variety of runtimes, this env lets you configure language options specific to the JavaScript used in your project.
  • rules:
  • extends: extending configuration files

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Some frequently used eslint rules

1. "multiline-ternary": ["error", "always"],: force breaking ternary into multiple lines
2. padding-line-between-statements : supported natively by eslint
3. "typescript-sort-keys": force sort typescript keys in an alphabetized. order

(to be continuted...)

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Format code automatically before commiting code

Though we add eslint configuration files for good intentions, sometimes , people might push code without eslint styling awareness due to eslint is not enabled on vscode.

What if we can reject a git commit if it violates our configured rules? Luckly, git provides a series of Git Hooks (opens new window):

• pre-commit
• pre-push
• pre-merge
• pre-rebase

These hooks give us some zone on committing code, let a tool to format our code automatically before commiting code.

Here, we have two tools to implement the desired feature we mentioned above.

• husky a git hook solution for js projects.
• lint-staged: Run linters against staged git files and don't let 💩 slip into your code base!

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@todo: sort class members https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/member-ordering.md#default-configuration

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References

1. ESlint official doc
2. Core concepts in ESLint
3. what's the difference between ESlint extends and plugins
4. Husky x LintStaged