Commas

Leading commas

• Nope.

// 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',
};

Trailing commas

• Yup.

tslint:

"trailing-comma": [
  true,
  {
    "singleline": false,
    "multiline": true
  }
]

Why? This leads to cleaner git diffs. Also, transpilers like Babel will remove the additional trailing comma in the transpiled code which means you don't have to worry about the trailing comma problem in legacy browsers.

// 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',
];