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Minimize statement count

In this example, the value

document.body.clientWidth

is used twice, but it’s retrieved using named

properties (an expensive operation compared to accessing a single variable). You can rewrite this code to

use a local variable instead of accessing

document.body.clientWidth

twice:

var iClientWidth = document.body.clientWidth;

oDiv1.style.left = iClientWidth;

oDiv2.style.left = iClientWidth;

Note that this example also reduces the algorithmic complexity of the previous code by cutting two calls

to linear algorithms down to one.

Minimize statement count

It stands to reason that the fewer statements a script has, the less time needed for it to execute. You can

minimize the number of statements in JavaScript code in numerous ways when defining variables, deal-

ing with iterating numbers, and using array and object literals.

Define multiple variables

As mentioned earlier in the book, you can use the

var

statement to define more than one variable at a

time. Further, you can define variables of different types using the same

var

statement. For instance, the

following code block uses four separate

var

statements to define four variables:

var iFive = 5;

var sColor = “blue”;

var aValues = [1,2,3];

var oDate = new Date();

These four statements can be rewritten to use a single

var

statement:

var iFive = 5 , sColor = “blue”, aValues = [1,2,3], oDate = new Date();

By eliminating three statements you make this code segment run faster.

Insert iterative values

Any time you are using an iterative value (that is, a value that is being incremented or decremented at

various locations), combine statements whenever possible. Consider the following code snippet:

var sName = aValues[i];

i++;

The two preceding statements each have a single purpose: The first retrieves a value from

aValues

and

stores it in

sName

; the second iterates the variable

. These can be combined into a single statement by

inserting the iterative value into the first statement:

var sName = aValues[i++];

This single statement accomplishes the same thing as the previous two statements. Because the incre-

ment operator is postfix, the value of

isn’t incremented until after the rest of the statement executes.

Whenever you have a similar situation, try to insert the iterative value into the last statement that uses it.

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