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Revision as of 16:52, 16 December 2017 by Michael (talk | contribs) (for the browser)

Pas2js : What is it ?


Pas2js is a pascal to javascript transpiler. It parses Object Pascal and emits Javascript. The javascript is currently of level ECMAScript 5 and should run in any browser or in nodejs. it is available in 3 forms:

  • as a library
  • as a command-line program
  • as a webserver

It transpiles from actual pascal source, it has no intermediate .ppu file. That means all sources must always be available.

Through external class definitions, the compiler can use Javascript classes:

  • All classes available in the Javascript runtime, and in the browser are available
    through import units (comparable to the windows or unix units for the native compiler).
  • For nodejs, basic support for the nodejs runtime environment is available.
  • An import unit for jQuery is available (libjquery)


For the generated code to work, a small javascript file is needed: rtl.js. It defines an object rtl. This object will start the Object Pascal code if you include a call to in the html page.

<script type="application/javascript">

pas2js can automatically include this file in the generated output, like this:

pas2js -Jc -Jirtl.js -Tbrowser hello.pas

For nodejs, the compiler will insert the call to automatically at the end of the generated javascript file.

There is a basic Object Pascal RTL, several units from the FPC Packages are also available

  • system
  • sysutils
  • Math
  • strutils
  • rtlconst
  • classes
  • contnrs
  • DB (yes, TDataset)
  • fpcunit testsuite
  • custapp
  • restconnection
  • js (javascript system objects)
  • web (browser provided objects)
  • libjquery (jquery is available too)
  • nodejs (basic node runtime environment)
  • typeinfo
  • objpas
  • browserconsole (support writeln)
  • dateutils
  • browserapp
  • nodejsapp

Where to get it

The pas2js compiler is - naturally - open source and can be downloaded and used freely. The sources are currently the primary means to get it.


  • The Pas2js compiler is part of Free Pascal. As such, the compiler is present in the utils/pas2js directory of the

FPC SVN repo


  • The pas2js RTL is still in flux. To enable faster releases, the RTL and demos are in a separate repository in SVN:

svn co pas2js


Till pas2js is integrated in the main FPC distribution, snapshots will be made available as time permits. They will contain binaries for Windows, Linux (64 bit) and Macos.

A beta snapshot can be found for the following versions:

How to use pas2js

The command-line arguments are kept mostly the same as the FPC command-line arguments. Error messages are also in the same format.

The compiler needs access to all sources, and so you need to specify the path to the sources of all used units.

As for the FPC compiler, a configuration file is supported, which has the same syntax as the FPC config file.

Basically, the command is the same as any FPC command line. The only thing that is different is the target: browser or nodeejs

for the browser

Consider the classical:

program hello;

  Writeln('Hello, world!');

Yes, writeln is supported. Here is how to compile it:

pas2js -Jc -jirtl.js -Tbrowser hello.pas

When compiled succesfully, the code can be run in the browser by opening a html file in the browser with the following content:

    <meta charset="utf-8"/>
    <script type="application/javascript" src="hello.js"></script>
    <script type="application/javascript">;

The files that are needed are:

  • hello.html
  • hello.js

Whether hello.html is opened by double-clicking it in the explorer or put on a server and opened with an URL, is not relevant for the functioning.

The output is visible in the browser's web developer console. By including the browserconsole unit, it will be visible in the browser page:

program hello;

uses browserconsole;

  Writeln('Hello, world!');

for NodeJS

pas2js -Tnodejs hello.pas

When compiled succesfully, the code can be run in node using the following command.

nodejs hello.js

Supported syntax elements

Basically, Delphi 7 syntax minus interfaces is supported. This includes RTTI. A more detailed list can be found in the translation.html file in the sources.

There are some constructs that are naturally not supported and will never be supported: Anything involving memory pointers.

Planned language features

Basically, the idea is to get the pas2js transpiler up to the same level as FPC or Delphi. That means the following needs to be added:

  • More runtime/compile time checks
  • Interfaces
  • Generics
  • Type helpers
  • Anonymous functions

Needless to say, anything requiring direct memory access is not going to be supported.

Lazarus integration of pas2js

Lazarus understands the concept of external classes as used by pas2js, so code completion will work.

pas2js comes with several lazarus packages that 'import' the pas2js rtl units: by doing so, the IDE will know which units to use.

  • pas2js_rtl : the basic RTL
  • fcl_base_pas2js: The custapp and other units
  • fpcunit_pas2js: the fpcunit units
  • pas2js_fcldb: the database units

A new project wizard is under construction, but deeper integration with lazarus is planned.

Importing Javascript classes

To import a javascript class, one writes a normal class definition that mimics the Javascript class. It is possible to use properties. Many examples can be found in the JS, web, nodejs and libjquery units.