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.

The identity operator is standardized by ECMAScript v3 and implemented in JavaScript 1.3 and later. With the introduction of the identity operator, JavaScript supports = , == , and === operators. Be sure you understand the differences between the assignment, equality, and identity operators, and be careful to use the right one when coding! Although it is tempting to call all three operators "equals," it may help to reduce confusion if you read "gets or is assigned" for = , "is equal to" for == , and "is identical to" for === .

In JavaScript, numbers, strings, and boolean values are compared by value . In this case, two separate values are involved, and the == and === operators check that these two values are identical. This means that two variables are equal or identical only if they contain the same value. For example, two strings are equal only if they each contain exactly the same characters.

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.)

The following rules are used to determine whether two values are identical according to the === operator:

• If the two values have different types, they are not identical.

• If both values are numbers and have the same value, they are identical, unless either or both values are NaN , in which case they are not identical. The NaN value is never identical to any other value, including itself! To check whether a value is NaN , use the global isNaN( ) function.

• If both values are strings and contain exactly the same characters in the same positions, they are identical. If the strings differ in length or content, they are not identical. Note that in some cases, the Unicode standard allows more than one way to encode the same string. For efficiency, however, JavaScript string comparison compares strictly on a character-by-character basis, and it assumes that all strings have been converted to a "normalized form" before they are compared. See the "String.localeCompare( )" reference page in the core reference section of this book for another way to compare strings.