The strict equality operator (===
) behaves identically to the abstract equality operator (==
) except no type conversion is done, and the types must be the same to be considered equal.
Reference: Javascript Tutorial: Comparison Operators
The ==
operator will compare for equality after doing any necessary type conversions. The ===
operator will not do the conversion, so if two values are not the same type ===
will simply return false
. Both are equally quick.
To quote Douglas Crockford's excellent JavaScript: The Good Parts,
JavaScript has two sets of equality operators: ===
and !==
, and their evil twins ==
and !=
. The good ones work the way you would expect. If the two operands are of the same type and have the same value, then ===
produces true
and !==
produces false
. The evil twins do the right thing when the operands are of the same type, but if they are of different types, they attempt to coerce the values. the rules by which they do that are complicated and unmemorable. These are some of the interesting cases:
'' == '0' // false
0 == '' // true
0 == '0' // true
false == 'false' // false
false == '0' // true
false == undefined // false
false == null // false
null == undefined // true
' \t\r\n ' == 0 // true
The lack of transitivity is alarming. My advice is to never use the evil twins. Instead, always use ===
and !==
. All of the comparisons just shown produce false
with the ===
operator.
Update:
A good point was brought up by @Casebash in the comments and in @Phillipe Laybaert's answer concerning objects. For objects, ==
and ===
act consistently with one another (except in a special case).
var a = [1,2,3];
var b = [1,2,3];
var c = { x: 1, y: 2 };
var d = { x: 1, y: 2 };
var e = "text";
var f = "te" + "xt";
a == b // false
a === b // false
c == d // false
c === d // false
e == f // true
e === f // true
The special case is when you compare a primitive with an object that evaluates to the same primitive, due to its toString
or valueOf
method. For example, consider the comparison of a string primitive with a string object created using the String
constructor.
"abc" == new String("abc") // true
"abc" === new String("abc") // false
Here the ==
operator is checking the values of the two objects and returning true
, but the ===
is seeing that they're not the same type and returning false
. Which one is correct? That really depends on what you're trying to compare. My advice is to bypass the question entirely and just don't use the String
constructor to create string objects from string literals.
Reference
http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3
Best Answer
The argument that if the compiler can provide a default copy constructor, it should be able to provide a similar default
operator==()
makes a certain amount of sense. I think that the reason for the decision not to provide a compiler-generated default for this operator can be guessed by what Stroustrup said about the default copy constructor in "The Design and Evolution of C++" (Section 11.4.1 - Control of Copying):So instead of "why doesn't C++ have a default
operator==()
?", the question should have been "why does C++ have a default assignment and copy constructor?", with the answer being those items were included reluctantly by Stroustrup for backwards compatibility with C (probably the cause of most of C++'s warts, but also probably the primary reason for C++'s popularity).For my own purposes, in my IDE the snippet I use for new classes contains declarations for a private assignment operator and copy constructor so that when I gen up a new class I get no default assignment and copy operations - I have to explicitly remove the declaration of those operations from the
private:
section if I want the compiler to be able to generate them for me.