Examples

Declaring and initializing two variables

var a = 0, b = 0;


Assigning two variables with single string value

var a = 'A';
var b = a;

This is equivalent to:

var a, b = a = 'A';

Be mindful of the order:

var x = y, y = 'A';
console.log(x + y); // undefinedA

Here, x and y are declared before any code is executed, but the assignments occur later. At the time x = y is evaluated, y exists so no ReferenceError is thrown and its value is undefined. So, x is assigned the undefined value. Then, y is assigned the value 'A'. Consequently, after the first line, x === undefined && y === 'A', hence the result.


Initialization of several variables

var x = 0;
function f() {
  var x = y = 1; // Declares x locally; declares y globally.
}
f();

console.log(x, y); // 0 1

// In non-strict mode:
// x is the global one as expected;
// y is leaked outside of the function, though!

The same example as above but with a strict mode:

'use strict';

var x = 0;
function f() {
  var x = y = 1; // Throws a ReferenceError in strict mode.
}
f();

console.log(x, y);


Implicit globals and outer function scope

Variables that appear to be implicit globals may be references to variables in an outer function scope: 

var x = 0; // Declares x within file scope, then assigns it a value of 0.

console.log(typeof z); // "undefined", since z doesn't exist yet

function a() {
  var y = 2; // Declares y within scope of function a, then assigns it a value of 2.

  console.log(x, y); // 0 2

  function b() {
    x = 3; // Assigns 3 to existing file scoped x.
    y = 4; // Assigns 4 to existing outer y.
    z = 5; // Creates a new global variable z, and assigns it a value of 5.
           // (Throws a ReferenceError in strict mode.)
  }

  b(); // Creates z as a global variable.
  console.log(x, y, z); // 3 4 5
}

a(); // Also calls b.
console.log(x, z);     // 3 5
console.log(typeof y); // "undefined", as y is local to function a