JavaScript

JavaScript (JS) is the dominant client-side scripting language of the Web. JS is a dynamic (or interpreted: Executes somethings at run-time instead of at compiling), weakly-typed (or loosely-typed: A variable may change its data type), prototype-based (object oriented but without classes but instead uses and possibly modifies instances or prototypes) programming language with first-class functions (functions can be first-class objects which allows their creation and modification at run-time).

JavaScript was developed by Brendan Eich at Netscape Communications Corporation, first as Mocha, then as LiveScript, then as JavaScript as part of the Netscape browser v2.0B3 in 1995-12. JavaScript has not relation to Java other than they both use a syntax similar to the c language and that Java was the hot new language at the time. JavaScript became widely popular and Microsoft implemented it as JScript in Internet Explorer 3.0 in 1996-08. Netscaped submitted JavaScript to Ecma International for standardization, which resulted in ECMA-262 or ECMAScript in 1997-06. Since then ECMAScript is the standard and it has various dialects, some with their own extensions, and some (EG: Flash's ActionScript) that are quite different. The main dialects includes Netscape's/Mozilla's JavaScript and Microsoft's JScript and JScript .NET.

Here are some miscellaneous items regarding JavaScript.

• Special characters can be inserted into literal strings, including the following that correspond with the escpaed characters in Regular Expressions: \b\t\n\v\r\e\"\'\cx\on\xn\un.
• JS uses the try ... catch ... finally and throw statements for error handling.VBS uses On Error and Next for error handling.
• JS is case sensitive. VBS is not.
• JS allows implicit or explicit variable declarations.
  • JS has no way of requiring explicit decarations like the Option Explicit feature of VBS.
  • If a variable is used that has not been explicitly declared with var) then JS assumes implicit variable declaration and that the variable is the largest scope applicable.
  • The delete operator can be used on an implicitly declared variable but not an explicitly declared one or those part of the JS language.
• JS is available for client-side scripting on most browsers, whereas VBS is usually available only for Microsoft Internet Explorer.
• JS probably has a larger body of resources and code out there than VBS because JS has been around longer.
• There are two types of JavaScript comments:

  //This is a single line comment.

  /* This is the start of a multi-line comment. 
  This is the end of a multi-line comment. */

• An HTML <a> tag can call a JS function directly with this syntax: <a href="javascript:functionName()">.
• Script within a <script> tag is often enclosed in an SGML comment declaration so that non-script using browsers can ignore the script. The close of the SGML comment is often commented out with a JS comment. EG:

  <script type="text/JavaScript">
  <!-- //Hide script from no-script browsers.
      Multiple lines of script may go here.
  //End with a JS comment. -->
  </script>

• JS has 3 top-level properties that are always available:
  • Infinity. A numeric value representing infinity. Initially equivalent to Number.POSITIVE_INFINITY.
  • NaN. A value representing Not-A-Number. In pseudo-code: NaN <> NaN <> Number.NaN, so use isNaN() for comparison instead.
  • undefined. A variable that has not been assigned a value is of type undefined.
• JS is weakly-typed (loosely-typed): A variable may change its data type. This is powerful but can be a common source of errors so a clear understanding of this is needed and developers must be keenly aware of any automatic type-conversions (type-castings).
  • Here are only 6 types in JS: boolean, function, number, object (includes null), string, and undefined.
  • For evaluating expressions as a boolean in JS: If expression is omitted or evaluates to 0, -0, null (not 'null'), false (not 'false'), NaN (not 'NaN'), undefined (not 'undefined'), or an empty string ("" or ''), then the object has an initial value of false. All other values will evaluate to true. EG:

    null==undefined;   //auto type-conversion, so true
    null===undefined;  //strict or no type-conversion, so false

  • For evaluating expressions as numbers in JS: Literal integers (EGs: 13, 07 (octal), 0xF7 (hexadecimal)) and floating-point literals (EGs: 3, 3.1, -3.2e-7) have the value stated, but others depend on context. undefined variables get treated as NaN. null variables get treated as 0. Empty string can be 0 (EG: Number("")) or can auto type-convert its neighbors (EG: 8+""+8 yields 88 not 16).
  • A mixture of types can only occur for binary operators. If a given binary operator only supports one type, then both pieces of data for that operator must either be of that type, or convertible to that type, otherwise an error results.
    • For + operators, if either argument is a String or converts preferentially to one, then it does string concatenation, not numeric addition. Otherwise, if neither argument prefers to be a string, then it does numeric addition. (Short version: "+ is either string")
    • For >, >=, <, <= operators, if neither argument can be converted to a number, then it does string comparison, not numeric comparison. Otherwise, if at least one argument can be converted to a number, then it does numeric comparison. (Short version: "compare is neither number")
    • For ==, != operators, first convert any Objects to their preferred primitive types. If the types are still different, convert any Boolean to a Number. If the types are still different, convert the non-Number argument to Number and compare as Numbers. Exception: null==undefined without any conversion defined.
  • Force the conversion if mixed types must be compared. EG:

    x = ( a+'' == b+'')  //String comparison forced  
    x = ( a -0 == b -0)  //Number comparison forced  
    x = ( !a   == !b  )  //Boolean comparison forced

  • Avoid comparing things with null, instead trust that the conversion to the Boolean type will work. EG:

    if ( a != null) { ...}   //may be buggy
    if ( a        ) { ...}   //better

  • On the other hand, the variable you get may have been explicitly converted into a string at some point, in which case then you can check for literal "null", "undefined", "true", "false", etc.
• JS can only handle event procedures by explicitly linking an object with a function. EG: <img .... onmouseover="RunMe()">. VBS on the other hand can also handle event procedures by making procedures whose name follows this syntax: Object_Event(). Multiple JS statements can be included in the explicit event link.

2008-03-17 22:30:42Z