There are places in any programming language where you can get caught if you are not careful, and every language has specific surprises in it. Take, for example, the null value: The one in JScript behaves differently than the Null value in the C or C++ languages.
Here are some of the trouble areas that you may run into as you write JScript scripts.
Because syntax is much more rigid in programming languages than in natural languages, it is important to pay strict attention to detail when you write scripts. If, for example, you mean for a particular parameter to be a string, you will run into trouble if you forget to enclose it in quotation marks when you type it.
Order of Script Interpretation
JScript interpretation is part of the your Web browser's HTML parsing process. So, if you place a script inside the <HEAD> tag in a document, it is interpreted before any of the <BODY> tag is examined. If you have objects that are created in the <BODY> tag, they do not exist at the time the <HEAD> is being parsed, and cannot be manipulated by the script.
This behavior is specific to Internet Explorer. ASP and WSH have different execution models (as would other hosts).
Automatic Type Coercion
JScript is a loosely typed language with automatic coercion. Consequently, despite the fact that values having different types are not equal, the expressions in the following example evaluate to true.
"100" == 100; false == 0;
To check that both the type and value are the same, use the strict equality operator, ===. the following both evaluate to false:
"100" === 100; false === 0;
When a particular operation is performed during the evaluation of an expression has more to do with operator precedence than with the location of the expression. Thus, in the following example, multiplication is performed before subtraction, even though the subtraction appears first in the expression.
theRadius = aPerimeterPoint - theCenterpoint * theCorrectionFactor;
Using for...in Loops with Objects
When you step through the properties of an object with a for...in loop, you cannot necessarily predict or control the order in which the fields of the object are assigned to the loop counter variable. Moreover, the order may be different in different implementations of the language.
The with statement is convenient for addressing properties that already exist in a specified object, but cannot be used to add properties to an object. To create new properties in an object, you must refer to the object specifically.
Although you use the this keyword inside the definition of an object, to refer to the object itself, you cannot ordinarily use this or similar keywords to refer to the currently executing function when that function is not an object definition. You can, if the function is to be assigned to an object as a method, use the this keyword within the function, to refer to the object.
Writing a Script That Writes a Script in Internet Explorer
The </SCRIPT> tag terminates the current script if the interpreter encounters it. To display "</SCRIPT>" itself, rewrite this as at least two strings, for example, "</SCR" and "IPT>", which you can then concatenate together in the statement that writes them out.
Implicit Window References in Internet Explorer
Because more than one window can be open at a time, any window reference that is implicit is taken to point to the current window. For other windows, you must use an explicit reference.