Question

I want to build an object obj1 with property obj2, which is another object. To avoid redeclaring obj1 and obj2, I use the following code:

if (!obj1) obj1 = {};
if (!obj1.obj2) obj1.obj2 = {};
// code to use obj1

Assume, obj1 and obj1.obj2 aren’t defined yet, the code causes the browser to report the error “obj1 is not defined”.

If I change the code to:

if (typeof obj1==='undefined') obj1 = {};
if (!obj1.obj2) obj1.obj2 = {};
// code to use obj1

Then there’s no error, while I think it should report “obj2 is not defined”. I’m puzzled as to why the JavaScript treats the short-hand falsy check of a reference and a property differently. Can anyone shed a light on that?

Answer 1

If you would do:

if (!window.obj1) window.obj1 = {};
if (!obj1.obj2) obj1.obj2 = {};

You will find the code works as you expect.

obj1 isn’t even a reference when you check it’s existance; it’s nothing. It doesn’t exist because you haven’t declared it (neither have you initialized it).

var obj1;

if (!obj1) obj1 = {};
if (!obj1.obj2) obj1.obj2 = {};

This will also work because you’ve declared the existance of obj1; you just haven’t initialized it.

All properties of an object that haven’t been set hold the value undefined; which is why it responds to your short hand !obj1.obj2

var obj1 = {};
obj1.a === undefined // true;

Variables however, must be defined before you can access them.