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5.4. Equality Operators

This section describes the JavaScript equality and inequality operators. These are operators that compare two values to determine whether they are the same or different and return a boolean value (true or false) depending on the result of the comparison. As we'll see in Chapter 6, they are most commonly used in things like if statements and for loops, to control the flow of program execution.

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:

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

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.

When the equality operator in JavaScript 1.1 attempted to convert a string to a number and failed, it displayed an error message noting that the string could not be converted, instead of converting the string to NaN and returning false as the result of the comparison. This bug has been fixed in JavaScript 1.2.



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