Question

I have a simple json parsed object that sometimes has a variable tt defined and sometimes doesn’t.

For some reason jsonobject.tt == null returns correctly 1 or 0 based on whether tt is defined. jasonobject.tt === null just returns 0 regardless. I thought === was the thing to use to avoid issues.

What’s going on here?

Answer 1

=== is the strict equality operator, it compares type as well as value. The value null is the Null Type that has exactly one value – null.

Undefined is the Undefined Type, which also has only one value – ‘undefined’.

When using the strict equality operator, null !== undefined because they are different types (see step 1 of the Strict Equality Comparison Algorithm, ECMA-262 § 11.9.6).

== is the equality operator. Comparisons using == use the Abstract Equality Comparison Algorithm (ECMA-262 § 11.9.3), which includes:

1. If Type(x) is the same as Type(y), then …
2. If x is null and y is undefined, return true.
3. If x is undefined and y is null, return true.

So null == undefined returns true by definition. Strictly, testing for the presence of a property (regardless of its value), should use hasOwnProperty:

if (jsonobject.hasOwnProperty('tt')) {
  // property exists
}

however in practice there isn’t much difference to a strict test for undefined:

if (jsonobject.tt === undefined) 

because whether the property exists and has a value of undefined or hasn’t been defined at all is usually equivalent. Using === also means that the above will return false if tt exists but has been assigned a value of null.