Switch from single var to one-var-per-variable

Frontenders at Airbnb at have been discussing this change for a bit, and we've come to favor one-var-per-variable over one-var-only declarations. Two things improve the maintainability of this style over one var for multiple variables: 1. You never have a diff of a line that's removing a `;` and inserting a `,`. 2. You can't accidentally declare global variables because you have a one-character mistake (semicolon instead of comma): ```javascript var foo = 1, bar = 2; baz = 3; // added later and (accidentally) declared globally ```

Author: Jake Teton-Landis

README.md

@@ -50,8 +50,8 @@
  + `undefined`
 
  ```javascript
- var foo = 1,
-   bar = foo;
+ var foo = 1;
+ var bar = foo;
 
  bar = 9;
 
@@ -64,8 +64,8 @@
  + `function`
 
  ```javascript
- var foo = [1, 2],
-   bar = foo;
+ var foo = [1, 2];
+ var bar = foo;
 
  bar[0] = 9;
 
@@ -151,9 +151,9 @@
  - When you need to copy an array use Array#slice. [jsPerf](http://jsperf.com/converting-arguments-to-an-array/7)
 
  ```javascript
- var len = items.length,
-  itemsCopy = [],
-   i;
+ var len = items.length;
+ var itemsCopy = [];
+ var i;
 
  // bad
  for (i = 0; i < len; i++) {
@@ -216,10 +216,10 @@
  - When programmatically building up a string, use Array#join instead of string concatenation. Mostly for IE: [jsPerf](http://jsperf.com/string-vs-array-concat/2).
 
  ```javascript
- var items,
-  messages,
-  length,
-   i;
+ var items;
+ var messages;
+ var length;
+ var i;
 
  messages = [{
  state: 'success',
@@ -366,17 +366,28 @@
  var superPower = new SuperPower();
  ```
 
- - Use one `var` declaration for multiple variables and declare each variable on a newline.
+ - Use one `var` declaration per variable.
 
  ```javascript
  // bad
+ var items = getItems(),
+ goSportsTeam = true,
+ dragonball = 'z';
+
+ // good
  var items = getItems();
  var goSportsTeam = true;
  var dragonball = 'z';
+ ```
 
- // good
+ It's easier to add new variable declarations this way, and you never have
+ to worry about swapping out a `;` for a `,` or introducing punctuation-only
+ diffs. Also, you can't make the following mistake:
+
+ ```javascript
+ // can you catch the error?
  var items = getItems(),
- goSportsTeam = true,
+ goSportsTeam = true;
  dragonball = 'z';
  ```
 
@@ -389,17 +400,18 @@
  goSportsTeam = true;
 
  // bad
- var i, items = getItems(),
- dragonball,
- goSportsTeam = true,
- len;
+ var i;
+ var items = getItems();
+ var dragonball;
+ var goSportsTeam = true;
+ var len;
 
  // good
- var items = getItems(),
-  goSportsTeam = true,
-  dragonball,
-  length,
-   i;
+ var items = getItems();
+ var goSportsTeam = true;
+ var dragonball;
+ var length;
+ var i;
  ```
 
  - Assign variables at the top of their scope. This helps avoid issues with variable declaration and assignment hoisting related issues.
@@ -843,14 +855,18 @@
 
  ```javascript
  // bad
- var once
+ var story = [
+ once
  , upon
- , aTime;
+ , aTime
+ ];
 
  // good
- var once,
- upon,
- aTime;
+ var story = [
+ once,
+ upon,
+ aTime
+ ];
 
  // bad
  var hero = {