JavaScript: The Definitive Guide, 6th Edition

Relational Expressions

This section describes JavaScript’s relational operators. These operators test for a relationship (such as “equals,” “less than,” or “property of”) between two values and return true or false depending on whether that relationship exists. Relational expressions always evaluate to a boolean value, and that value is often used to control the flow of program execution in if, while, and for statements (see Chapter 5). The subsections that follow document the equality and inequality operators, the comparison operators, and JavaScript’s other two relational operators, in and instanceof.

Equality and Inequality Operators

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 strict equality operator (or sometimes 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.

JavaScript supports =, ==, and === operators. Be sure you understand the differences between these assignment, equality, and strict equality operators, and be careful to use the correct one when coding! Although it is tempting to read all three operators “equals,” it may help to reduce confusion if you read “gets or is assigned” for =, “is equal to” for ==, and “is strictly equal to” for ===.

The != and !== operators test for the exact opposite of the == and === operators. The != inequality operator returns false if two values are equal to each other according to == and returns true otherwise. The !== operator returns false if two values are strictly equal to each other and returns true otherwise. As you’ll see in Logical Expressions, the ! operator computes the Boolean NOT operation. This makes it easy to remember that != and !== stand for “not equal to” and “not strictly equal to.”

As mentioned in Immutable Primitive Values and Mutable Object References, JavaScript objects are compared by reference, not by value. An object is equal to itself, but not to any other object. If two distinct objects have the same number of properties, with the same names and values, they are still not equal. Two arrays that have the same elements in the same order are not equal to each other.

The strict equality operator === evaluates its operands, and then compares the two values as follows, performing no type conversion:

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

• If both values are null or both values are undefined, they are equal.

• If both values are the boolean value true or both are the boolean value false, they are equal.

• If one or both values is NaN, they are not equal. The NaN value is never equal to any other value, including itself! To check whether a value x is NaN, use x !== x. NaN is the only value of x for which this expression will be true.

• If both values are numbers and have the same value, they are equal. If one value is 0 and the other is -0, they are also equal.

• If both values are strings and contain exactly the same 16-bit values (see the sidebar in Text) in the same positions, they are equal. If the strings differ in length or content, they are not equal. Two strings may have the same meaning and the same visual appearance, but still be encoded using different sequences of 16-bit values. JavaScript performs no Unicode normalization, and a pair of strings like this are not considered equal to the === or to the == operators. See String.localeCompare() in Part III for another way to compare strings.

• If both values refer to the same object, array, or function, they are equal. If they refer to different objects they are not equal, even if both objects have identical properties.

The equality operator == is like the strict equality operator, but it is less strict. If the values of the two operands are not the same type, it attempts some type conversions and tries the comparison again:

• If the two values have the same type, test them for strict equality as described above. If they are strictly equal, they are equal. If they are not strictly equal, they are not equal.

• If the two values do not have the same type, the == operator may still consider them equal. Use the following rules and type conversions to check for equality:

  • If one value is null and the other is undefined, they are equal.

  • If one value is a number and the other is a string, convert the string to a number and try the comparison again, using the converted value.

  • If either value is true, convert it to 1 and try the comparison again. If either value is false, convert it to 0 and try the comparison again.

  • If one value is an object and the other is a number or string, convert the object to a primitive using the algorithm described in Object to Primitive Conversions and try the comparison again. An object is converted to a primitive value by either its toString() method or its valueOf() method. The built-in classes of core JavaScript attempt valueOf() conversion before toString() conversion, except for the Date class, which performs toString() conversion. Objects that are not part of core JavaScript may convert themselves to primitive values in an implementation-defined way.

  • Any other combinations of values are not equal.

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 values are now the same, the comparison returns true.

Comparison Operators

The comparison operators test the relative order (numerical or alphabetics) of their two operands:

Less than (<)

The < operator evaluates to true if its first operand is less than its second operand; otherwise it evaluates to false.

Greater than (>)

The > operator evaluates to true if its first operand is greater than its second operand; otherwise it evaluates to false.

Less than or equal (<=)

The <= operator evaluates to true if its first operand is less than or equal to its second operand; otherwise it evaluates to false.

Greater than or equal (>=)

The >= operator evaluates to true if its first operand is greater than or equal to its second operand; otherwise it evaluates to false.

The operands of these comparison operators may be of any type. Comparison can be performed only on numbers and strings, however, so operands that are not numbers or strings are converted. Comparison and conversion occur as follows:

• If either operand evaluates to an object, that object is converted to a primitive value as described at the end of Object to Primitive Conversions: if its valueOf() method returns a primitive value, that value is used. Otherwise, the return value of its toString() method is used.

• If, after any required object-to-primitive conversion, both operands are strings, the two strings are compared, using alphabetical order, where “alphabetical order” is defined by the numerical order of the 16-bit Unicode values that make up the strings.

• If, after object-to-primitive conversion, at least one operand is not a string, both operands are converted to numbers and compared numerically. 0 and -0 are considered equal. Infinity is larger than any number other than itself, and -Infinity is smaller than any number other than itself. If either operand is (or converts to) NaN, then the comparison operator always returns false.

Remember that JavaScript strings are sequences of 16-bit integer values, and that string comparison is just a numerical comparison of the values in the two strings. The numerical encoding order defined by Unicode may not match the traditional collation order used in any particular language or locale. Note in particular that string comparison is case-sensitive, and all capital ASCII letters are “less than” all lowercase ASCII letters. This rule can cause confusing results if you do not expect it. For example, according to the < operator, the string “Zoo” comes before the string “aardvark”.

For a more robust string-comparison algorithm, see the String.localeCompare() method, which also takes locale-specific definitions of alphabetical order into account. For case-insensitive comparisons, you must first convert the strings to all lowercase or all uppercase using String.toLowerCase() or String.toUpperCase().

Both the + operator and the comparison operators behave differently for numeric and string operands. + favors strings: it performs concatenation if either operand is a string. The comparison operators favor numbers and only perform string comparison if both operands are strings:

1 + 2        // Addition. Result is 3.
"1" + "2"    // Concatenation. Result is "12".
"1" + 2      // Concatenation. 2 is converted to "2". Result is "12".
11 < 3       // Numeric comparison. Result is false.
"11" < "3"   // String comparison. Result is true.
"11" < 3     // Numeric comparison. "11" converted to 11. Result is false.
"one" < 3    // Numeric comparison. "one" converted to NaN. Result is false.

Finally, note that the <= (less than or equal) and >= (greater than or equal) operators do not rely on the equality or strict equality operators for determining whether two values are “equal.” Instead, the less-than-or-equal operator is simply defined as “not greater than,” and the greater-than-or-equal operator is defined as “not less than.” The one exception occurs when either operand is (or converts to) NaN, in which case all four comparison operators return false.

The in Operator

The in operator expects a left-side operand that is or can be converted to a string. It expects a right-side operand that is an object. It evaluates to true if the left-side value is the name of a property of the right-side object. For example:

var point = { x:1, y:1 };  // Define an object
"x" in point               // => true: object has property named "x"
"z" in point               // => false: object has no "z" property.
"toString" in point        // => true: object inherits toString method

var data = [7,8,9];        // An array with elements (indices) 0, 1, and 2
"0" in data                // => true: array has an element "0"
1 in data                  // => true: numbers are converted to strings
3 in data                  // => false: no element 3

The instanceof Operator

The instanceof operator expects a left-side operand that is an object and a right-side operand that identifies a class of objects. The operator evaluates to true if the left-side object is an instance of the right-side class and evaluates to false otherwise. Chapter 9 explains that, in JavaScript, classes of objects are defined by the constructor function that initializes them. Thus, the right-side operand of instanceof should be a function. Here are examples:

var d = new Date();  // Create a new object with the Date() constructor
d instanceof Date    // Evaluates to true; d was created with Date()
d instanceof Object  // Evaluates to true; all objects are instances of Object
d instanceof Number  // Evaluates to false; d is not a Number object
var a = [1, 2, 3];   // Create an array with array literal syntax
a instanceof Array   // Evaluates to true; a is an array
a instanceof Object  // Evaluates to true; all arrays are objects
a instanceof RegExp  // Evaluates to false; arrays are not regular expressions

Note that all objects are instances of Object. instanceof considers the “superclasses” when deciding whether an object is an instance of a class. If the left-side operand of instanceof is not an object, instanceof returns false. If the right-hand side is not a class of objects, it throws a TypeError.

In order to understand how the instanceof operator works, you must understand the “prototype chain.” This is JavaScript’s inheritance mechanism, and it is described in Inheritance. To evaluate the expression o instanceof f, JavaScript evaluates f.prototype, and then looks for that value in the prototype chain of o. If it finds it, then o is an instance of f (or of a subclass of f) and the operator returns true. If f.prototype is not one of the values in the prototype chain of o, then o is not an instance of f and instanceof returns false.