Why isn't [1,2,3] equal to itself in Javascript? [duplicate]

Question

I was playing around with arrays in Javascript today and noticed this little gem:

alert([1, 2, 3] == [1, 2, 3]); //alerts false

It strikes me as rather odd that the array is not equal to itself.

But then I noticed this, which was even weirder:

alert([1, 2, 3] == "1,2,3");  //alerts true

?!?!?!?!!!?

Why in the world is [1, 2, 3] not == to itself but is == to the string?

I realize that == is not the same as ===. Even so, what evilness could cause Mr. Javascript do such weird things?

Answer 1

Ok, so first you need to understand how javascript treats values in your program. All of your variables that you create are going to merely be references to a location in memory where that object is stored. Therefore, when you do this:

alert( [1,2,3] == [1,2,3] );

...it does three things:

1. Place an array ([1,2,3]) onto the heap
2. Place another array ([1,2,3]) onto the heap (notice it will have a different memory location)
3. Compare the two references. They point to different objects in different locations in memory, thus it is considered not equal.

You can check for some sane behavior by running this code:

var a = [1,2,3];
var b = a;
alert (a == b)   // Result is true. Both point to the same object.

Now for your question about the string

When you use the == operator tries to convert the two operands to the same type (evil behavior...I know...)

When it does this, it decides to convert both to a string before it does the compare (thus the result is really "1,2,3" === "1,2,3", which evaluates to true.

I can't give you a complete picture, as there are few people who understand every nuance of the madness that is JavaScript, but hopefully this clears some of the fog.

Answer 2

== operator

[..] If either operand is a string, the other operand is converted to a string if possible. [..] If both operands are objects, then JavaScript compares internal references which are equal when operands refer to the same object in memory.

https://developer.mozilla.org/en/JavaScript/Reference/Operators/Comparison_Operators

That is to say, [1, 2, 3] converted to a string equals "1,2,3". One array object does not equal another array object though.