Possible Duplicate:
Why [] == [] is false in javascript?
I would like to ask about strange thing, i.e.:
var x = "pl";
var y = ["pl"];
[x] == y; // false - why?
x == y; // true - how ?
x === y; // false - okay
Can some one explain it?
Thanks in advance.
The first one is false because you're comparing two arrays (which are objects) - a comparison which will always be false unless the objects are actually the same object, or if the objects are coerced to a different type of value like in the second comparison.
In the second comparison, y is coerced to be a string value, and then found to be equal to "pl".
For instance, this code:
["pl"] + "foo" → "plfoo"
Incidentally, this is why you should always use === instead of == - it doesn't result in any surprising coercions. That's why the third comparison is false.
Array to Array (abstract equality comparison)
[x] == y; // false - why?
[x] and y do not refer to the same object. Arrays are objects and the == operator tests that they are the same object, not simply two objects having identical values for all properties. In order to determine object-equality in that way, you'll have to manually enumerate the properties of each object and test each value.
According to The Abstract Equality Comparison Algorithm used by ==:
Return true if x and y refer to the same object. Otherwise, return false.
String to Array (abstract equality comparison)
x == y; // true - how ? oO
y, an array, is coerced into a string because you used == when comparing it to x, a string.
According to The Abstract Equality Comparison Algorithm used by ==:
If Type(x) is either String or Number and Type(y) is Object, return the result of the comparison x == ToPrimitive(y).
String to Array (strict equality comparison)
x === y; // fasle - okey
===, unlike ==, will not coerce y into a string... so, you're comparing a string to an object.
According to The Strict Equality Comparison Algorithm used by ===:
If Type(x) is different from Type(y), return false.
[x] == y;
['pl'] == ['p1'] - comparing refs on 2 different arrays in memory
x == y;
The same as "pl" == ["p1"].toString(). JS convert the second argument to the string because the first one is also string
