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ESLint is a static code analysis tool for identifying problematic patterns found in JavaScript code. It was created by Nicholas C. Zakas in 2013. Rules in ESLint are configurable, and customized rules can be defined and loaded. ESLint covers both code quality and coding style issues.

• A linter - tool for identifying and reporting programmatic, stylistic errors.
• ESLint does static error checking before running the program

• JSLint
• ESLint
• JSHint

• install Node.js
• install VSCode (as we will use VSCode)

• VS Code, Download ->https://code.visualstudio.com/download
• Node-js, Download-> https://nodejs.org/en/download

1. Create package.json: npm init

2. Install and configure eslint as dev dependency: npm init @eslint/config

3. Initialize eslint: eslint --init and follow steps:

   • How would you like to use ESLint? To check syntax and find problems
   • How would you like to use ESLint? None of these
   • How would you like to use ESLint? None of these
   • Does your project use TypeScript? › No
   • Where does your code run? Browser

4. Go to .eslintrc.json file and make add your necessary adjustments Example

   following codes will activate the recommended rules which can be changed inside the rules object.
     {
      "extends": "eslint:recommended"
     }
   
    <!-- first value is error level, it can have 3 values: off/0, warn/1, error/2 -->
    Example of ESLint rules
    "rules": {
        "quotes": ["error", "double"]
    }
   
    More Examples of ESLint rules
   "rules": {
       "no-var": "error",
       "eqeqeq": "error",
       "no-unused-vars": "error",
       "default-case": "error",
       "no-console": "error",
       "camelcase": "error",
       "consistent-return": "error",
       "func-style": "error",
       "max-depth": ["error", 2],
       "max-lines": ["error", 25],
       "prefer-arrow-callback": "error",
       "prefer-const": "error",
       "no-useless-return": "error",
       "no-plusplus": "error",
       "quotes": ["error", "single"]
     }
   

5. add some codes in a js file for testing the eslint. I have created a file called app.js and added the following codes for testing purpose

   var username = "anisul Islam";
   var password = "12345";
   var occupation = "full stack web developer";
   const user_presentaddress = "lala";
   
   var a = 6;
   a++;
   
   console.log(occupation);
   
   const validateUser = (pwd) => {
     if (password == pwd) {
       return true;
     }
   };
   
   console.log(validateUser(12345));
   
   switch (foo) {
     case 1:
       console.log(foo);
       break;
   
     case 2:
       console.log(foo);
       break;
   }
   
   const foo = true;
   if (foo) {
     if (foo) {
       if (foo) {
         console.log("hi");
       }
       console.log("hi");
     }
     console.log("hi");
   }
   
   setTimeout(function () {}, 1000);

6. make some changes in setting.json

   format on save and validate js codes: add the following codes in settings.json

    "editor.codeActionsOnSave": {
    "source.fixAll.eslint": true
    },
    "eslint.validate": ["javascript"]
   

7. Run the eslint: eslint fileName.js and check the erros

8. Fix automatically fixable errors: eslint fileName.js --fix

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