The version of JavaScript implemented in Appweb is a subset, suitable for use in embedded
systems. As JavaScript 1.2 is now quite a large language, its size prohibits its use in most
embedded devices. Embedded JavaScript is designed to solve this dilemma. It is a strict subset
of JavaScript that implements the essential elements of the language and is extensible by the
user to support new functions.
JavaScript may be extended via several mechanisms:
JavaScript functions may be created using the
function statement. JavaScript
functions, object definitions and variables can be grouped into
JavaScript
libraries that can be re-used via the include function.
To extend JavaScript for use with Embedded Server Pages, please read the
Extending Embedded Server Pages for details.
JavaScript Libraries
Libraries of
JavaScript functions, object definitions and constructors and variable declarations may be
created and then re-used by JavaScripts. The include function will include the named file as
though it were cut and pasted into the current file. For example:
<% include("inc.js"); %>
Creating JavaScript Functions from C Functions
JavaScript functions can most
easily be created using the JavaScript function statement. However, there are times when you
need to create a JavaScript function using the C language. This may be because you need the
highest level of performance or you need to call C APIs as part of the function. Embedded
JavaScript allows you to easily create JavaScript functions in C.
To create an EJS function in C, you create a function to execute when the JavaScript function
name is invoked. This function is passed the EJS instance handle and the actual arguments
passed to the JavaScript function at run-time.
You can create two kinds of EJS C functions. The simplest, shown below, automatically converts
all arguments to strings before calling the C function. These are called String EJS Functions
and are created via the ejsDefineStringCFunction API call. This method
of function definition is ideal when the arguments and function result will always be strings.
The other kind of function definition does not convert the arguments to strings. Arguments are
passed in an array of MprVar variables. These variables may be strings, boolean, integer,
floating point or object variables. This style of function definition is best when any type of
argument may be passed into the function.
For example, the following code fragment creates a String EJS function that will be invoked
when a script calls the myProc function.
#include "ejs.h"
static int myProc(EjsHandle eid, int argc, char **argv)
{
ejsSetReturnString(eid, "sunny day");
}
// Somewhere in the main program
espDefineStringCFunction(0, "myProc", myProc, 0);
Rebuilding From Source
Embedded JavaScript has several compile time
constants that tailor the its operation. EJS permits you to select the default number type to
be either a 32-bit integer, 64-bit integer or floating point. Depending on whether your system
supports floating point or if you intend to use floating point, setting the default number type
to be an integer will result in a smaller faster JavaScript interpreter. The default EJS build
defines numbers as 32-bit integers. Standard ECMAScript defines numbers to be floating point
numbers.
You may also select to disable floating point and 64-bit integer support. This further shrinks
the size of the interpreter.
