How Embedded Server Pages Works
Embedded Server Pages are HTML pages with embedded JavaScript logic. When a client requests an ESP page, the web page is compiled into pure JavaScript that is then interpreted at run-time. Although slower than pure C code, the ESP JavaScript calls C routines for many functions so the overall result is fast page generation and response to client requests.
Example ESP Web Page
So what does an ESP HTML web page look like? ESP pages are standard HTML pages with embedded Javascript code containing scripting logic. You can use all your normal HTML tags. ESP tags delimit the JavaScript using the delimiters <% and %> to bracket the scripting code.
Normal HTML header
<% Script Code Here %>
This script code is executed and replaced by the ESP interpreter with the output from the script code. This all occurs at the server before any of the HTML is sent to the client. The client never sees the script logic and this greatly enhances the security of your application or device. Don't confuse ESP with client side JavaScript. A single page may have both server side and client side JavaScript. ESP JavaScript is placed between <% and %> tags and is replaced by the web server before it is sent to the client. Once the page reaches the clients browser, it will execute any client side JavaScript which is typically between tags.
The following example demonstrates just a few of the constructs available using ESP.
Simple ESP Test Page The HTML query string for this page is @@QUERY_STRING
| Line | @@i |
<% displayCurrentSecurityStats("myDb", 1); %>
<%
// Inside JavaScript blocks, we can put any server-side
// JavaScript we like
i = 2;
write("i is " + i);
%>
As you can see, the page is standard HTML with JavaScript logic inside ESP tags. What makes this especially interesting, is that you can easily create new Embedded JavaScript procedures in your application to suite your needs. These can then be called from the ESP web pages. This close nexus between the HTML page and your application logic is what makes AppWeb such an easy platform to use to create dynamic web pages.
ESP pages typically have a ".esp" extension although they can in some web servers such as AppWeb be configured to match by URL prefix. They are processed by the ESP web server handler.
Accessing Variables
There are three methods to access JavaScript variables within ESP scripts. You can use the ESP write procedure to output the value of a variable back to the client. For example, assuming you have the current temperature in a JavaScript variable called temp.Today's temperature is <% write(temp); %>
As this kind of variable access is a very common occurrence, a shorter form may be more convenient. Because the JavaScript assignment operators sets the result, it can be used to return a value to the client. For example:
Today's temperature is <% =temp; %>
Even easier is to use the @@ directive which does not require any <% %> enclosing tags. You use this by prepending the required variable with @@. For example:
Today's temperatore is @@temperature
Scripted HTML
Because the ESP web page is compiled into JavaScript, you can script normal HTML tags. If a section of HTML is enclosed by a JavaScript for loop, you can output the HTML block each time round the loop. This is an easy way to generate tables. In the example below, three rows of a table are output.<% for (i = 0; i < 3; i++) { %>
Line @@i
<% } %>
The AppWeb ESP implementation also supports server side includes. To include another document use the directive:
<% include fileName.esp %>
This will replace the ESP script with the content of the nominated filename The included file is assumed to be an ESP page that will be parsed at the point of the include directive. It may contain further ESP tags and include directives.
You can also include pure Embedded JavaScript code using the JavaScript include procedure:
<% include("myLib.js"); %>
In this case, the included file must contain pure JavaScript and must not contain ESP directives.
Embedded JavaScript (EJS) is a subset of the ECMA-262 standard which describes ECMAScript. EJS implements the key lanaguage features and conforms to the ECMAScript language syntax specifications. A strict subset of ECMAScript is implemented, sufficient to enable simple and powerful scripting by Embedded Server Pages, yet small enough to ensure a small memory footprint.
For those unfamiliar with JavaScript, the initial syntax closely resembles the C language but variables and function arguments are typeless.
JavaScript also supports objects, object methods, associative arrays, and a for/in iterator statement. These features make it much closer to Perl or other scripting languages than C.Embedded JavaScript is multithreaded and will support multiple simultaneous instances. When used by ESP, each JavaScript session has its own local variable store and instances can share data via the session[] and application[] data store.
Embedded JavaScript implements the following JavaScript language elements.
- Comments (C and C++ style)
- Arrays and associative arrays
- Objects
- Identifiers
- Data types including booleans, integers, 64-bit integers, floating point and strings
- Expressions
- If/else
- for and for/in
- JavaScript functions
- return
- C/C++ bound functions
The supported operators are:
- < <= == != > >=
- + - / %
- << >>
- ++ --
The supported conditional operators are:
- && ||
- !
The following language elements are not implemented in ESP 2.0:
- Exceptions
- Labeled statements
- These control flow statements: break, case, continue, default, do/while, export, import, switch, var, while, with
- Regular expressions
ESP web pages can embed JavaScript directly in the web page or they can include libraries of Embedded JavaScript (EJS) code. EJS is a powerful language, however, it is normally best to keep scripting to a minimum and put complex code in a language such as C or C++.
See the Embedded Server Pages JavaScript API Reference for more detailed JavaScript documentation.
You may use your favorite HTML editor to create Embedded Server Pages. Dreamweaver is perhaps one of the best, but there are other fine choices. If your HTML editor supports PHP/ASP script editing, you may be able to use this feature as ASP uses the same <% %> delimiters as ESP. Otherwise, create your page using the page layout tool and then switch to the HTML code view to insert the ESP scripting at the relevant locations.
AppWeb is configured by default to use ESP, so you should have to do nothing to use ESP. The AppWeb configuration file has the following directives to load the ESP module and to add the ESP handler:
LoadModule esp libespModule
AddHandler espHandler .esp .asp
These configure AppWeb to use the ESP handler for all requests that have a ".esp" or ".asp" extension.
AppWeb also configures a Location block for ESP. This instructs AppWeb to send all requests that begin with "/esp/" to the ESP handler:
SetHandler espHandler
Lastly, the directory to store uploaded files is configured with the FileUploadDir directive.
FileUploadDir /tmp
When using ESP with WebServer, make sure you have a version later than version 2.5 downloaded from https://mbedthis.com/webServer. ESP is configured via the config.h header file.