http://*.com/questions/6429225/javascript-null-or-undefined

var x;

x == null            // true

x == undefined       // true

x === null           // false

x === undefined      // true

var y = null;

y == null            // true

y == undefined       // true

y === null           // true

y === undefined      // false

typeof x             // 'undefined'

typeof y             // 'object'

var z = {abc: null};

z.abc == null        // true

z.abc == undefined   // true

z.abc === null       // true

z.abc === undefined  // false

z.xyz == null        // true

z.xyz == undefined   // true

z.xyz === null       // false

z.xyz === undefined  // true

null = 1;            // throws error: invalid left hand assignment

undefined = 1;       // works fine: this can cause some problems