commas.js

/**
* Reasons to avoid comma separated lists in Javascript
* ----------------------------------------------------
* TL;DR use:
* 
*     var a = 1;
*     var b = 2;
* 
* instead of:
* 
*     var a = 1,
*         b = 2;
*/

/**
* 1. Increased coupling when adding variables
*/
var a = 1,
    b = 2;
    
// To add a variable to the end of the above list, I have to modify b = 2; to become b = 2,
var a = 1,
    b = 2, // I had to modify this line
    c = 3;
    
/**
* 2. Increased coupling when removing variables
*/
var a = 1,
    b = 2;
    
// Same thing here. If I want to remove b = 2, I have to also modify a = 1
var a = 1; // Had to modify this ending to be a semicolon

/**
* 3. Higher chance of git conflicts
*/
var a = 1,
    b = 2;
    
// If committer A adds a new variable, and committer B adds a different variable, there
// is a high chance the new lines will conflict

// Comitter A change:
var a = 1,
    b = 2,
    committerA = 3;

// Comitter B change:
var a = 1,
    b = 2,
    comitterB = 4;

// Git will most likely conflict trying to merge these two lines, and cannot resolve the
// conflict automatically, because one of the lines will need the semicolon replaced with comma

var a = 1,
    b = 2,
    committerA = 3, // needs to change from original commit
    comitterB = 4;

/**
* 4. Indenting is harder to manage and understand
*/

// With comma lists indenting becomes an issue
var a = 1,

    b = function() { // Indenting already starts with an extra tab
        // Should I be indented this much?
    };
    
// This is not an issue with individual var lines
var a = 1;
var b = function() {
  // I have only one level of indentation
};
}

There is another scenario you're forgetting... I use this one all the time:

var
  a = 1,
  b = 2,
  c = 3;

@MattMcFarland that has all the same problems as listed above. To remove line c you still have to modify line b and so on.