JavaScript: The Definitive Guide

D.9 Miscellaneous Differences

Other differences between Navigator and Internet Explorer 3.0 are small details about the way values are computed and printed:

• The for/in statement in IE 3.0 does not always enumerate the same object properties that Navigator does. It does enumerate all user-defined properties, which is its primary function. But predefined properties of built-in objects are not always listed.

• The && and || operators behave somewhat differently in Navigator and Internet Explorer, although, since JavaScript is an untyped langauge, the difference is usually irrelevant. When the first operand of the && operator evaluates to true, then the operator returns the value of the second operand in Navigator. In Internet Explorer, this second operand is first converted to a Boolean value, and that value is returned. Thus the expression

  true && 10
  

  evaluates to 10 in Navigator but to true in Internet Explorer. This may seem like a major difference, but because JavaScript is an untyped langauge, it rarely matters. The && operator is almost always used in a Boolean context, such as the expression of an if statement, so even when Navigator returns a value like 10, that value will be immediately converted to the Boolean value true within that context. The same evaluation difference occurs when the first operand of the || operator evaluates to false.

• In Internet Explorer 3.0, Boolean values implicitly are converted to strings differently than they are in Navigator. The value true is converted to the string -1, and the value false is converted to the string 0. If you actually want them to be converted to the strings "true" and "false", you must convert them explicitly by adding them to the empty string.

• User-defined function values are also converted to strings differently in IE 3.0. In Navigator, functions are converted to a string that includes the complete body of the function. In fact, you can even use eval() function to define the function in some other window. This does not work in Internet Explorer, which omits the function body from its string representation of functions.