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JavaScript: Tutorial - A Guide to JavaScript - Page 05

Bitwise Operators

The bitwise operators are almost the same as they are in the C/C++ language.

Boolean Operators

Logical NOT

The logical NOT operator is an exclamation point (!). It first converts the operand to a Boolean value and then negates it. true is returned if the operand is undefined. true is returned if the operand is NaN. true is returned if the operand is null. false is returned if the operand is any number other than 0 (including Infinity). true is returned if the operand is the number 0. false is returned if the operand is a non-empty string. true is returned if the operand is an empty string. false is returned if the operand is an object.

One can simulate the behavior of the Boolean() casting function by using two NOT operators in a row.

alert(!!0);	 // false
alert(!!NaN);	 // false
alert(!!"");	// false
alert(!!15);	// true

Logical AND

The logical AND operator is a double ampersand (&&).

if(true && true) {
	alert("Good to go!");
}

Logical AND can be used with any type of operand, not just Boolean values. When any operand is not a primitive Boolean, logical AND does not always return a Boolean value. If the first operand is an object, the second operand is always returned. If the second operand is an object, the object is returned only if the first operand evaluates to true. If both operands are objects, the second operand is returned. If either operand is null, null is returned. If either operand is NaN, NaN is returned. If either operand is undefined, undefined is returned. If the first operand determines the result, the second operand is never evaluated. So, if the first operand is false, no matter what the value of the second operand, the expression can not be true.

Logical OR

The logical OR operator is a double pipe (||).

if(true || false) {
	alert("Good to go!");
}

Just like logical AND, if any operand is not a Boolean, logical OR will not always return a Boolean value. If the first operand is an object, the first operand is returned. If the first operand evaluates to false, the second operand is returned. If both operands are objects, the first operand is returned. If both operands are null, null is returned. If both operands are NaN, NaN is returned. If both operands are undefined, undefined is returned. if the first operand evaluates to true, the second operand is not evaluated. So, if the first operand is true, no matter what the value of the second operand, the expression can not be false.

This behavior can be used to avoid assigning a null or undefined value to a variable. Consider this:

var obj = preferredObject || backupObject;

The variable obj will be assigned one of two values. The preferredObject variable contains the value that is preferred if it is available, whereas the backupObject variable contains the backup value if the preferred one isn't available. If preferredObject is not null, then it is assigned to obj; if it is null, then backupObject is assigned to obj.

Additive Operators

The additive operators are add and subtract.

Add

The add operator is a plus sign (+).

var sum = 3 + 5;
alert(sum);

If the two operands are numbers then an add is performed. If either operand is NaN then the result is NaN. If Infinity is added to Infinity then the result is Infinity. If –Infinity is added to –Infinity then the result is –Infinity. If Infinity is added to –Infinity then the result is NaN. If +0 is added to +0 then the result is +0. If –0 is added to +0 then the result is +0. If –0 is added to –0 then the result is –0.

If one of the operands is a string then they are concatenated. If both operands are strings then the second string is concatenated to the first. If only one operand is a string then the other operand is converted to a string and the result is the concatenation of the two strings.

If either operand is an object, number, or Boolean, its toString() method is called to get a string value and then the previous rules regarding strings are applied. The String() function is called to retrieve the values "undefined" and "null" for undefined and null. This code demonstrates this:

var num1 = 5;
var num2 = 3;
var message = "The sum of 5 and 3 is " + num1 + num2;
alert(message); // "The sum of 5 and 3 is 53"

var num1 = 5;
var num2 = 3;
var message = "The sum of 5 and 3 is " + (num1 + num2);
alert(message); // "The sum of 5 and 3 is 8" (THIS IS THE DESIRED RESULT)

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