JavaScript null check

Question

I\'ve come across the following code:

function test(data) {
    if (data != null && data !== undefined) {
        // some code here
    }
}


        

Answer 1

In your case use data==null (which is true ONLY for null and undefined - on second picture focus on rows/columns null-undefined)

function test(data) {
    if (data != null) {
        console.log('Data: ', data);
    }
}

test();          // the data=undefined
test(null);      // the data=null
test(undefined); // the data=undefined

test(0); 
test(false); 
test('something');

Here you have all (src):

if

== (its negation !=)

=== (its negation !==)

Answer 2

An “undefined variable” is different from the value undefined.

An undefined variable:

var a;
alert(b); // ReferenceError: b is not defined

A variable with the value undefined:

var a;
alert(a); // Alerts “undefined”

When a function takes an argument, that argument is always declared even if its value is undefined, and so there won’t be any error. You are right about != null followed by !== undefined being useless, though.