How to Test Null Values ​​in JavaScript

Question:

I found the next code, and I got the impression it didn't make much sense.

function (data) {
    if (data != null && data !== undefined) {
        // codigo
    }
}

From here, three different scenarios can happen:

• The function is called with no arguments, making data an undeclared variable (and resulting in an error evaluating data != null ).

• The function was called specifically with null , or undefined , in which case the data != null already protects the internal code, making the second condition irrelevant.

• The function was called with a non-null argument, and if it passes the test, both conditions are true.

Is there any scenario where it makes sense to keep the second condition?

Answer:

You're right, it doesn't make much sense. About each of the points you raised:

The function is called with no arguments, making data an undeclared variable (and resulting in an error evaluating data != null).

There is actually no such scenario. data is always declared inside the function as it is a named argument. If you call the function without passing anything, it gets the value undefined .

The function was called specifically with a null argument, or undefined, in which case the data != null condition already protects the internal code, making the second condition irrelevant.

True, data != null returns false if data is null or undefined (and only then).

The function was called with a non-null argument, and if it passes the test, both conditions are true.

Right.

Is there any scenario where it makes sense to keep the second condition?

No 🙂 Unless you use strict equality operator === , which considers types. With == , any comparison with null or undefined is true if the other side is also null or undefined . That is, they are interchangeable with == . Therefore:

if (data !== null && data !== undefined)

it is the same as:

if (data != null)

it is the same as:

if (data != undefined)