JavaScript: The Definitive Guide, 6th Edition

Miscellaneous Operators

JavaScript supports a number of other miscellaneous operators, described in the following sections.

The Conditional Operator (?:)

The conditional operator is the only ternary operator (three operands) in JavaScript and is sometimes actually called the ternary operator. This operator is sometimes written ?:, although it does not appear quite that way in code. Because this operator has three operands, the first goes before the ?, the second goes between the ? and the :, and the third goes after the :. It is used like this:

x > 0 ? x : -x     // The absolute value of x

The operands of the conditional operator may be of any type. The first operand is evaluated and interpreted as a boolean. If the value of the first operand is truthy, then the second operand is evaluated, and its value is returned. Otherwise, if the first operand is falsy, then the third operand is evaluated and its value is returned. Only one of the second and third operands is evaluated, never both.

While you can achieve similar results using the if statement (if), the ?: operator often provides a handy shortcut. Here is a typical usage, which checks to be sure that a variable is defined (and has a meaningful, truthy value) and uses it if so or provides a default value if not:

greeting = "hello " + (username ? username : "there");

This is equivalent to, but more compact than, the following if statement:

greeting = "hello ";
if (username)
    greeting += username;
else
    greeting += "there";

The typeof Operator

typeof is a unary operator that is placed before its single operand, which can be of any type. Its value is a string that specifies the type of the operand. The following table specifies the value of the typeof operator for any JavaScript value:

x	typeof x
undefined	"undefined"
null	"object"
true or false	"boolean"
any number or NaN	"number"
any string	"string"
any function	"function"
any nonfunction native object	"object"
any host object	An implementation-defined string, but not “undefined”, “boolean”, “number”, or “string”.

You might use the typeof operator in an expression like this:

(typeof value == "string") ? "'" + value + "'" : value

The typeof operator is also useful when used with the switch statement (switch). Note that you can place parentheses around the operand to typeof, which makes typeof look like the name of a function rather than an operator keyword:

typeof(i)

Note that typeof returns “object” if the operand value is null. If you want to distinguish null from objects, you’ll have to explicitly test for this special-case value. typeof may return a string other than “object” for host objects. In practice, however, most host objects in client-side JavaScript have a type of “object”.

Because typeof evaluates to “object” for all object and array values other than functions, it is useful only to distinguish objects from other, primitive types. In order to distinguish one class of object from another, you must use other techniques, such as the instanceof operator (see The instanceof Operator), the class attribute (see The class Attribute), or the constructor property (see The prototype Attribute and The constructor Property).

Although functions in JavaScript are a kind of object, the typeof operator considers functions to be sufficiently different that they have their own return value. JavaScript makes a subtle distinction between functions and “callable objects.” All functions are callable, but it is possible to have a callable object—that can be invoked just like a function—that is not a true function. The ECMAScript 3 spec says that the typeof operator returns “function” for all native object that are callable. The ECMAScript 5 specification extends this to require that typeof return “function” for all callable objects, whether native objects or host objects. Most browser vendors use native JavaScript function objects for the methods of their host objects. Microsoft, however, has always used non-native callable objects for their client-side methods, and before IE 9 the typeof operator returns “object” for them, even though they behave like functions. In IE9 these client-side methods are now true native function objects. See Callable Objects for more on the distinction between true functions and callable objects.

The delete Operator

delete is an unary operator that attempts to delete the object property or array element specified as its operand.^[3] Like the assignment, increment, and decrement operators, delete is typically used for its property deletion side effect, and not for the value it returns. Some examples:

var o = { x: 1, y: 2}; // Start with an object
delete o.x;            // Delete one of its properties
"x" in o               // => false: the property does not exist anymore

var a = [1,2,3];       // Start with an array
delete a[2];           // Delete the last element of the array
2 in a                 // => false: array element 2 doesn't exist anymore
a.length               // => 3: note that array length doesn't change, though

Note that a deleted property or array element is not merely set to the undefined value. When a property is deleted, the property ceases to exist. Attempting to read a nonexistent property returns undefined, but you can test for the actual existence of a property with the in operator (The in Operator). Deleting an array element leaves a “hole” in the array and does not change the array’s length. The resulting array is sparse.

delete expects its operand to be an lvalue. If it is not an lvalue, the operator takes no action and returns true. Otherwise, delete attempts to delete the specified lvalue. delete returns true if it successfully deletes the specified lvalue. Not all properties can be deleted, however: some built-in core and client-side properties are immune from deletion, and user-defined variables declared with the var statement cannot be deleted. Functions defined with the function statement and declared function parameters cannot be deleted either.

In ECMAScript 5 strict mode, delete raises a SyntaxError if its operand is an unqualified identifier such as a variable, function, or function parameter: it only works when the operand is a property access expression (Property Access Expressions). Strict mode also specifies that delete raises a TypeError if asked to delete any nonconfigurable property (see Property Attributes). Outside of strict mode, no exception occurs in these cases and delete simply returns false to indicate that the operand could not be deleted.

Here are some example uses of the delete operator:

var o = {x:1, y:2};  // Define a variable; initialize it to an object
delete o.x;          // Delete one of the object properties; returns true
typeof o.x;          // Property does not exist; returns "undefined"
delete o.x;          // Delete a nonexistent property; returns true
delete o;            // Can't delete a declared variable; returns false.
                     // Would raise an exception in strict mode.
delete 1;            // Argument is not an lvalue: returns true
this.x = 1;          // Define a property of the a global object without var
delete x;            // Try to delete it: returns true in non-strict mode
                     // Exception in strict mode. Use 'delete this.x' instead
x;                   // Runtime error: x is not defined

We’ll see the delete operator again in Deleting Properties.

The void Operator

void is a unary operator that appears before its single operand, which may be of any type. This operator is unusual and infrequently used: it evaluates its operand, then discards the value and returns undefined. Since the operand value is discarded, using the void operator makes sense only if the operand has side effects.

The most common use for this operator is in a client-side javascript: URL, where it allows you to evaluate an expression for its side effects without the browser displaying the value of the evaluated expression. For example, you might use the void operator in an HTML <a> tag as follows:

<a href="javascript:void window.open();">Open New Window</a>

This HTML could be more cleanly written using an onclick event handler rather than a javascript: URL, of course, and the void operator would not be necessary in that case.

The Comma Operator (,)

The comma operator is a binary operator whose operands may be of any type. It evaluates its left operand, evaluates its right operand, and then returns the value of the right operand. Thus, the following line:

i=0, j=1, k=2;

evaluates to 2 and is basically equivalent to:

i = 0; j = 1; k = 2;

The left-hand expression is always evaluated, but its value is discarded, which means that it only makes sense to use the comma operator when the left-hand expression has side effects. The only situation in which the comma operator is commonly used is with a for loop (for) that has multiple loop variables:

// The first comma below is part of the syntax of the var statement
// The second comma is the comma operator: it lets us squeeze 2
// expressions (i++ and j--) into a statement (the for loop) that expects 1.
for(var i=0,j=10; i < j; i++,j--)
    console.log(i+j);

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^[3] If you are a C++ programmer, note that the delete keyword in JavaScript is nothing like the delete keyword in C++. In JavaScript, memory deallocation is handled automatically by garbage collection, and you never have to worry about explicitly freeing up memory. Thus, there is no need for a C++-style delete to delete entire objects.