5.4 Equality Operators
5.4.1 Equality (==) and Identity (===)
The == and === operators check whether two values are the same, using two different definitions of sameness. Both operators accept operands of any type, and both return true if their operands are the same and false if they are different. The === operator is known as the identity operator, and it checks whether its two operands are "identical" using a strict definition of sameness. The == operator is known as the equality operator; it checks whether its two operands are "equal" using a more relaxed definition of sameness that allows type conversions.
On the other hand, objects, arrays, and functions are compared by reference. This means that two variables are equal only if they refer to the same object. Two separate arrays are never equal or identical, even if they contain equal or identical elements. Two variables that contain references to objects, arrays, or functions are equal only if they refer to the same object, array, or function. If you want to test that two distinct objects contain the same properties or that two distinct arrays contain the same elements, you'll have to check the properties or elements individually for equality or identity. (And, if any of the properties or elements are themselves objects or arrays, you'll have to decide how deep you want the comparison to go.)
As an example of testing for equality, consider the comparison:
"1" == true
This expression evaluates to true, indicating that these very different-looking values are in fact equal. The boolean value true is first converted to the number 1, and the comparison is done again. Next, the string "1" is converted to the number 1. Since both numbers are now the same, the comparison returns true.
22.214.171.124 Equality and inequality in Netscape
5.4.2 Inequality (!=) and Non-Identity (!==)
As we'll see, the ! operator computes the Boolean NOT operation. This makes it easy to remember that != stands for "not equal to" and !== stands for "not identical to." See the previous section for details on how equality and identity are defined for different data types.