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Postfix increment/decrement

var iNum = 10;

--iNum;

alert(iNum); //outputs “9”

alert(--iNum); //outputs “8”

alert(iNum); //outputs “8”

The second line decrements

num

, and the third line displays the result

(“9”

). The fourth line displays

num once again, but this time the prefix decrement is applied in the same statement, which results in

the number

“8”

being displayed. To prove that all decrements are complete, the fifth line once again

outputs

“8”

The prefix increment and decrement are equal in terms of order of precedence when evaluating a mathe-

matical expression and, therefore, are evaluated left to right. For instance:

var iNum1 = 2;

var iNum2 = 20;

var iNum3 = --iNum1 + ++iNum2; //equals 22

var iNum4 = iNum1 + iNum2; //equals 22

In the previous code,

iNum3

is equal to

because the expression evaluates to 1 + 21. The variable

iNum4

is also equal to

and also adds 1 + 21.

Postfix increment/decrement

Two operators, also taken directly from C (and Java), are the postfix increment and postfix decrement.

They also add one to a number value, as indicated by the two plus signs (++) placed

after

a variable:

var iNum = 10;

iNum++

As you might expect, postfix decrement subtracts one from a value and is indicated by two minus signs

(– –) placed after the variable:

var iNum = 10;

iNum--;

The second line of code decreases the value of

iNum

to 9.

Unlike the prefix operators, postfix operators increment or decrement

after

the containing expression is

evaluated. Consider the following example:

var iNum = 10;

iNum--;

alert(iNum); //outputs “9”

alert(iNum--); //outputs “9”

alert(iNum); //outputs “8”

Just as in the prefix example, the second line decrements

iNum

, and the third line displays the result (9).

The fourth line displays num once again, but this time the postfix decrement is applied in the same state-

ment. However, because the decrement doesn’t happen until after the expression is evaluated, this alert

also displays the number

. When the fifth line is executed, the alert displays

, because the postfix

decrement was executed after line 4 but before line 5.

ECMAScript Basics

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