Which equals operator (== vs ===) should be used in JavaScript comparisons?
I'm using JSLint to go through JavaScript, and it's returning many suggestions to replace == (two equals signs) with === (three equals signs) when doing things like comparing idSele_UNVEHtype.value.length == 0 inside of an if statement.
Is there a performance benefit to replacing == with === ?
Any performance improvement would be welcomed as many comparison operators exist.
If no type conversion takes place, would there be a performance gain over == ?
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.
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
' trn ' == 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 reference types. For reference types == 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 literal with an object that evaluates to the same literal, due to its toString or valueOf method. For example, consider the comparison of a string literal with a string object created by 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.
Reference
http://www.ecma-international.org/ecma-262/5.1/#sec-11.9.3
Using the == operator (Equality)
true == 1; //true, because 'true' is converted to 1 and then compared
"2" == 2; //true, because "2" is converted to 2 and then compared
Using the === operator (Identity)
true === 1; //false
"2" === 2; //false
This is because the equality operator == does type coercion , meaning that the interpreter implicitly tries to convert the values before comparing.
On the other hand, the identity operator === does not do type coercion , and thus does not convert the values when comparing.
In the answers here, I didn't read anything about what equal means. Some will say that === means equal and of the same type , but that's not really true. It actually means that both operands reference the same object , or in case of value types, have the same value .
So, let's take the following code:
var a = [1,2,3];
var b = [1,2,3];
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
The same here:
var a = { x: 1, y: 2 };
var b = { x: 1, y: 2 };
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
Or even:
var a = { };
var b = { };
var c = a;
var ab_eq = (a === b); // false (even though a and b are the same type)
var ac_eq = (a === c); // true
This behavior is not always obvious. There's more to the story than being equal and being of the same type.
The rule is:
For value types (numbers):
a === b returns true if a and b have the same value and are of the same type
For reference types:
a === b returns true if a and b reference the exact same object
For strings:
a === b returns true if a and b are both strings and contain the exact same characters
Strings: the special case...
Strings are not value types, but in Javascript they behave like value types, so they will be "equal" when the characters in the string are the same and when they are of the same length (as explained in the third rule)
Now it becomes interesting:
var a = "12" + "3";
var b = "123";
alert(a === b); // returns true, because strings behave like value types
But how about this?:
var a = new String("123");
var b = "123";
alert(a === b); // returns false !! (but they are equal and of the same type)
I thought strings behave like value types? Well, it depends who you ask... In this case a and b are not the same type. a is of type Object , while b is of type string . Just remember that creating a string object using the String constructor creates something of type Object that behaves as a string most of the time.