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Jarrod Brooks
5,071 Pointswhy is null === undefined ?
I am doing this code challenge and I watched the video several times. I tried doing this in the console several times and I do not understand why null could be === to undefined. It is my understanding that null and undefined share the same type which would be mean that null == undefined is true. However because null and undefined are not the SAME and same TYPE then: null === undefined should be false. What am I missing here?
6 Answers
David Smith
21,220 PointsI think the Instructions and the code should be worded differently to avoid confusion like this.
identical() is just a function name, not actually what is identical
Below is an explanation with double and triple equals:
myNullVariable == MyUndefinedVariable will equal true, because type conversion takes place, so the identical method will be called
myNullVariable === myUndefinedVariable will equal false, because null does NOT equal undefined, so the identical method will not called.
So the correct answer is to use the triple equals (===) to not call the identical() function. This is what the instructions are asking you to do
Adam Sackfield
Courses Plus Student 19,663 PointsThe question is saying that given the original code
var myUndefinedVariable;
var myNullVariable = null;
if(myNullVariable == myUndefinedVariable) {
identical();
}
That the identical method IS BEING CALLED but the variables are not identical, fix this. So in effect add the extra (=) t make it (===) therefore preventing the identical() function from being called as they are not equal.
Does that clear it up, it confused me at first but the question is correct. You are getting a pass as the output is false. Try putting all the code into the console
Adam Sackfield
Courses Plus Student 19,663 PointsThis is the console outputs I get from the two lines you are talking about
null == undefined
true
null === undefined
false
The identity (===) operator behaves identically to the equality (==) operator except no type conversion is done, and the types must be the same to be considered equal.
David Smith
21,220 PointsBoth behave the same, however type conversion takes place
Double equals (==) Does type conversion
Triple equals(===) Does NOT do type conversion. It matches the types as they are to do the comparison
A really good website to view a table of equality comparisons is here A more detailed explanation is in a stackover flow question here
Whenever I code JavaScript i always use the Triple equals to avoid subtle bugs from appering
Jarrod Brooks
5,071 PointsThat is what I get also, however when I use that logic I don't get a correct answer. This is the code challenge http://teamtreehouse.com/library/null-and-undefined-2
instructions: The identical() method is being called but the variables are not identical. See if you can fix the code.
<script>
var myUndefinedVariable;
var myNullVariable = null;
if(myNullVariable == myUndefinedVariable) {
identical();
}
</script>
To get a correct answer it wants me to say: if(myNullVariable === myUndefinedVariable) { identical(); }
However I don't understand why that is a correct answer because that would imply that null === undefined would be true. What am I missing?
Dino Paškvan
Courses Plus Student 44,108 PointsThe task is saying that the method identical() is being invoked even when the values are not identical (null and undefined are falsey values so through the use of the standard equality operator (==) they're getting converted to boolean false — type coercion, and they appear to be equal), and that shouldn't happen. The task is asking you to prevent that from happening, and you do that by using the strict equality operator (===).
Jarrod Brooks
5,071 PointsThank you for your replies. I understand it now. I was confused by the wording in the question. Once I looked at it differently I was able to grasp it.