ESLintによるLintとPrettierによるフォーマット

lintとformatを使う理由

ESLint：問題を検出する

// Tiny "lint" demo: simple checks on a string of code
const sample = "const x=1; console.log(x)";

function lint(code) {
  const issues = [];
  if (code.includes("console.log")) {
    issues.push("no-console: avoid console.log in production code");
  }
  if (code.includes("=1;")) {
    issues.push("spacing: add spaces around = for readability");
  }
  if (!code.trim().endsWith(";")) {
    issues.push("semi: missing semicolon");
  }
  return issues;
}

console.log("lint issues:", lint(sample));

Prettier：レイアウトを統一する

// Tiny "format" demo: normalize spaces and add semicolon
function format(code) {
  // Very naive formatting just for demo
  const spaced = code.replaceAll("=1;", " = 1;");
  const withSemi = spaced.trim().endsWith(";") ? spaced.trim() : spaced.trim() + ";";
  return withSemi;
}

const ugly = "const x=1\nconsole.log( x )";
console.log("formatted:", format(ugly));

小さなESLint設定

// Example .eslintrc.json (printed lines)
const eslintrc = [
  "{",
  "  \"env\": { \"es2022\": true, \"node\": true },",
  "  \"extends\": [\"eslint:recommended\"],",
  "  \"rules\": {",
  "    \"no-console\": \"warn\",",
  "    \"semi\": [\"error\", \"always\"]",
  "  }",
  "}"
];

for (const l of eslintrc) console.log(l);

小さなPrettierとscripts

// Example .prettierrc (printed lines)
const prettierrc = [
  "{",
  "  \"semi\": true,",
  "  \"singleQuote\": true,",
  "  \"tabWidth\": 2",
  "}"
];

for (const l of prettierrc) console.log(l);

// Example npm scripts (printed)
const scripts = [
  "\"scripts\": {",
  "  \"lint\": \"eslint .\",",
  "  \"format\": \"prettier --write .\"",
  "}"
];

for (const l of scripts) console.log(l);

日常のワークフロー

PrettierとESLintのクイズ

まとめ