逗号

• 19.1 行首逗号：不需要。

  // bad
  const story = [
      once
    , upon
    , aTime
  ];
  // good
  const story = [
    once,
    upon,
    aTime,
  ];
  // bad
  const hero = {
      firstName: 'Ada'
    , lastName: 'Lovelace'
    , birthYear: 1815
    , superPower: 'computers'
  };
  // good
  const hero = {
    firstName: 'Ada',
    lastName: 'Lovelace',
    birthYear: 1815,
    superPower: 'computers',
  };

• 19.2 增加结尾的逗号: 需要。

  为什么? 这会让 git diffs 更干净。另外，像 babel 这样的转译器会移除结尾多余的逗号，也就是说你不必担心老旧浏览器的尾逗号问题。

  // bad - git diff without trailing comma
  const hero = {
       firstName: 'Florence',
  -    lastName: 'Nightingale'
  +    lastName: 'Nightingale',
  +    inventorOf: ['coxcomb graph', 'modern nursing']
  }
  // good - git diff with trailing comma
  const hero = {
       firstName: 'Florence',
       lastName: 'Nightingale',
  +    inventorOf: ['coxcomb chart', 'modern nursing'],
  }
  // bad
  const hero = {
    firstName: 'Dana',
    lastName: 'Scully'
  };
  const heroes = [
    'Batman',
    'Superman'
  ];
  // good
  const hero = {
    firstName: 'Dana',
    lastName: 'Scully',
  };
  const heroes = [
    'Batman',
    'Superman',
  ];