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

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.

I tested this in Firefox with Firebug using code like this:

console.time("testEquality");
var n = 0;
while (true) {
  n++;
  if (n == 100000)
    break;
}
console.timeEnd("testEquality");

and

console.time("testTypeEquality");
var n = 0;
while (true) {
  n++;
  if (n === 100000)
    break;
}
console.timeEnd("testTypeEquality");

My results (tested five times each and averaged):

==: 115.2
===: 114.4

So I'd say that the miniscule difference (this is over 100000 iterations, remember) is negligible. Performance isn't a reason to do ===. Type safety (well, as safe as you're going to get in JavaScript), and code quality is.

The === operator is called a strict comparison operator, it does differ from the == operator.

Lets take 2 vars a and b.

For "a == b" to evaluate to true a and b need to be the same value.

In the case of "a === b" a and b must be the same value and also the same type for it to evaluate to true.

Take the following example

var a = 1;
var b = "1";

if (a == b) //evaluates to true as a and b are both 1
{
    alert("a == b");
}

if (a === b) //evaluates to false as a is not the same type as b
{
    alert("a === b");
}

In summary; using the == operator might evaluate to true in situations where you do not want it to so using the === operator would be safer.

In the 90% usage scenario it won't matter which one you use, but it is handy to know the difference when you get some unexpected behaviour one day.

In a typical script there will be no performance difference. More important may be the fact that thousand "===" is 1 KB heavier than thousand "==" :) JavaScript profilers can tell you if there is a performance difference in your case.

But personally I would do what JSLint suggests. This recommendation is there not because of performance issues, but because type coercion means ('\t\r\n' == 0) is true.

There is unlikely to be any performance difference between the two operations in your usage. There is no type-conversion to be done because both parameters are already the same type. Both operations will have a type comparison followed by a value comparison.

The problem is that you might easily get into trouble since JavaScript have a lot of implicit conversions meaning...

var x = 0;
var isTrue = x == null;
var isFalse = x === null;

Which pretty soon becomes a problem. The best sample of why implicit conversion is "evil" can be taken from this code in MFC / C++ which actually will compile due to an implicit conversion from CString to HANDLE which is a pointer typedef type...

CString x;
delete x;

Which obviously during runtime does very undefined things...

Google for implicit conversions in C++ and STL to get some of the arguments against it...

Strict equality is for the most part better

The fact that Javascript is a loosely typed language needs to be in the front of your mind constantly as you work with it. As long as the data structure is the same there really is no reason as to not use strict equality, with regular equality you often have an implicit conversion of values that happens automatically, this can have far-reaching effects on your code. It is very easy to have problems with this conversion seeing as they happen automatically.

With strict equality there is no automatic implicit conversion as the values must already be of the correct data structure.

The dilemma of "Should I use == or === in JavaScript comparison" is equal or analogous to a question of: "Should I use a 'spoon' or a 'fork' for eating.

The only reasonable answer to this question is that

1. You should use Dynamic Type comparison e.g.:== for loose Type comparisons.
2. You should use Static Type comparison e.g.:=== for strong Type comparisons.

That's because they are not the same. They don't have the same purpose and are not meant to be used for the same purpose.

Of course both 'forks' and 'spoons' are meant for 'eating', but you will chose to use them accordingly to what you've been served to eat.

Meaning: you'll resolve to using a 'spoon' i.e.: == for having a 'soup', and / or the 'fork' i.e.: === for picking.

Asking if it is better to use a "fork" or a "spoon" for "eating" - is equall to asking if it is better to use a static [===] versus dynamic [==] eq., op. in JS. Both questions are equally wrong and reflect a very narrow or shallow understanding of the subject in question.