JavaScript object detection

Question

I am currently practicing javascript and I'm currently having trouble with object detection. I want to create an object and detect whether it exists. Below is some example code I am currently using.

The code sample below is how I am creating my object.

var obj = obj || {};

Should I be using this?

if (typeof obj !== "undefined") {
   // code
}

Or this?

if (obj !== null) {

}

Answer 1

The value undefined means that the variable has not been assigned a value. The value null indicates that it has been assigned the value null. The === operator (and its negation, !==) prevent type coercion.

Which is right? It all depends on what you are trying to accomplish. Both undefined and null are "falsy" values, meaning that each one evaluates to false in a boolean context (as do false, 0, and the empty string).

Note that if obj is null, then typeof obj is not "undefined".

Answer 2

The usual practice is to use:

var obj = obj || {};

When that code runs, you can be certain that obj has been defined (because of the var declaration), though it may not have been assigned a value.

If the value of obj resolves to a "falsey" value, then you will assign a reference to a new object.

Possible outcomes are:

1. If obj has not been assigned a value, a new object reference is assigned.

2. If obj has previously been assigned a "falsey" value, it will be replaced by a new object reference (which may mess with whatever code assigned it the falsey value).

3. If obj has previously been assigned a "truethy" primitive value, the following assignments of properties to obj will throw an error and script execution will stop.

4. if obj is already a reference to an object other than null, then subsequent assignments of properties will be to that object. Depending on the type of object, things may be ok. Or not. Presumably something outside your control created the object and so may modify or destroy it without your knowledge. What will you do?

It is not possible in javascript to deal, in general, with cases 2 and 3, or 4 where it's not your obj you're dealiing with. So use a name that is unlikely to clash with any other and hope you get #1 or #4 and things go well.

Incidentally, code added after yours may still result (more or less) in #4, even if you had #1 at the point the code is executed.

There is no additional benefit to tests like:

if (typeof obj == 'undefined')

because the outcome is essentially the same.