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JavaScript gotchas

JavaScript, as you are well aware at this point, is unlike other programming languages in many ways.

Therefore, it helps to keep in mind some of the

gotchas

of the language.

Avoid string concatenation

Earlier in the book, you learned about the hazards of string concatenation using the plus (

) operator. To

work around this problem, you learned how to create a

StringBuffer

object to encapsulate string con-

catenations using an

Array

and the

join()

method.

Whenever you are doing more than five string concatenations in a row, it’s best to use the

StringBuffer

object.

Use built-in methods first

Whenever possible, use built-in methods of JavaScript objects before making your own. The built-in

methods are compiled in C++ and as such, run much faster than JavaScript that must be interpreted on

the fly. For example, you could write a function that calculates the value of a number when raised to the

power of

in this way:

function power(iNum, n) {

var iResult = iNum;

for (var i=1; i < n; i++) {

iResult *= iNum;

}

return iResult;

}

Although this function works perfectly well, JavaScript already provides a way to calculate the power of

a number by using

Math.pow()

alert(Math.pow(3, 4)); //raise 3 to the 4th power

The big difference is that

Math.pow()

is part of the browser, written and compiled in C++, and it is

much faster than the custom

power()

function defined previously.

Numerous built-in methods are provided specifically to take care of these common tasks. It’s always bet-

ter for execution time to use built-in methods instead of functions you define.

Store commonly used values

Whenever you use the same value more than once, store it in a local variable for easy access. This is

especially true for values that are normally accessed through a property of an object. Consider the fol-

lowing code:

oDiv1.style.left = document.body.clientWidth;

oDiv2.style.left = document.body.clientWidth;

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