A linter checks source files against rules for confusing code, unexpected behavior, and readability problems. It complements the TypeScript compiler by finding common problems the compiler does not detect. For TypeScript, typescript-eslint adapts ESLint’s JavaScript linting to TypeScript code. section 6.3 section 6.5
To lint a project in the tools folder:
- Install the development packages: `eslint@8.36.0`, `@typescript-eslint/parser@5.55.0`, and `@typescript-eslint/eslint-plugin@5.55.0`. section 6.3
- Create `tools/.eslintrc`. Set the TypeScript parser, point `parserOptions.project` at `./tsconfig.json`, and extend the recommended ESLint and typescript-eslint rule sets. Ignore `node_modules` and `dist`. section 6.3
- From `tools`, run `npx eslint .`. The final period tells ESLint to check the current directory. section 6.3
Listing 6.10 The contents of the .eslintrc file in the tools folder
{
"root": true,
"ignorePatterns": ["node_modules", "dist"],
"parser": "@typescript-eslint/parser",
"parserOptions": {
"project": "./tsconfig.json"
},
"plugins": [
"@typescript-eslint"
],
"extends": [
"eslint:recommended",
"plugin:@typescript-eslint/eslint-recommended",
"plugin:@typescript-eslint/recommended"
]
}
A complete `.eslintrc` shows how the parser, TypeScript project file, ignored folders, and recommended rules fit together. section 6.3
Reading a lint result
If a variable declared with `let` is never reassigned, `prefer-const` reports it. If code contains `debugger`, `no-debugger` reports that statement. The diagnostic output identifies the file, line, rule, and number of errors; it also indicates which errors may be fixable with `--fix`. section 6.3
C:\tools\src\index.ts
3:5 error 'printMessage' is never reassigned.
Use 'const' instead prefer-const
5:5 error 'message' is never reassigned.
Use 'const' instead prefer-const
8:1 error Unexpected 'debugger' statement no-debugger
10:5 error 'total' is never reassigned.
Use 'const' instead prefer-const
4 problems (4 errors, 0 warnings)
3 errors and 0 warnings potentially fixable with the `--fix` option.
The sample output shows rule names beside specific source locations and reports three potentially fixable errors. section 6.3
Keep the rules useful enough that important reports do not disappear in unwanted warnings. To disable `prefer-const` throughout the project, add `"rules": { "prefer-const": 0 }` to `.eslintrc`. To retain `no-debugger` generally while allowing a deliberate breakpoint, write `debugger; // eslint-disable-line no-debugger` in the source file. A project-wide rule change and a one-line exception solve different problems. section 6.3.1
