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Pas2js : What is it ?


Pas2js is an open source 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 Node.js (target "nodejs"). It is available in 5 forms:

  • as a library
  • as a command-line program
  • as a webserver
  • as a node.js program
  • as a program running in the browser.

It transpiles from actual Pascal source, it has no intermediate .ppu files. 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 Node.js, basic support for the nodejs runtime environment is available.
  • An import unit for jQuery is available (libjquery)

As a non commercial open source project we are always searching for helping hands. If you want to contribute see here.

This project is not related to a similar named project on github.


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)
  • typinfo
  • objpas
  • browserconsole (support writeln)
  • dateutils
  • browserapp
  • nodejsapp

Where to get it

The pas2js compiler and RTL are - naturally - open source and can be downloaded and used freely.


The releases contain binaries for Windows (32 and 64bit), Linux (64 bit) and macOS.

Installation procedure:

1. Download pas2js from:

Every version has a directory with the version number.
A list of changes can be found on the changelog page Pas2JS Version Changes

2. Unpack it in folder of your choice. The example below uses C:\lazarus\pas2js\. The release contains three folders:

  • bin - contains the compiler as executable (pas2js or pas2js.exe), a pas2js.cfg, a library and some utilities.
  • demo - lots of examples
  • packages - the Pascal units of the RTL and other packages.
  • tools - html2form - HTML to pascal code converter program
  • utils - A script to create a pas2js.cfg: createconfig.pp

Pas2js Folder

3. To use pas2js into Lazarus IDE see:


The sources are available on gitlab, and a read-only mirror exists on github.

The makefile expects the sources of FPC to be present under the compiler directory. You can copy/clone the FPC sources there or set enviroment variable FPCDIR. Note that a symlink to the FPC sources will not work.

Cloning pas2js and fpc git repos:

git clone pas2js
cd pas2js
git config --local pull.rebase true
git clone compiler
cd compiler
git config --local pull.rebase true

Updating local git repos:

cd pas2js
git pull
cd compiler
git pull

Switching to the fixes branch:

cd pas2js
git checkout fixes_3_0
cd compiler
git checkout pas2js/fixes_3_0

Building on Linux/macOS

Change to the directory and build it with:

make clean all

This creates bin/$(TargetCPU)-$(TargetOS)/pas2js (Windows: pas2js.exe). For example on Linux 64bit it creates bin/x86_64-linux/pas2js, while under Windows 64bit it creates bin\x86_64-win\pas2js.exe. And a basic config file bin/x86_64-linux/pas2js.cfg.

Building on Windows

Make sure that you use the make.exe from fpc, not the one from Delphi by setting the PATH: For example if you installed the 32-bit version of fpc in C:\YourPathOfFPC\3.2.2:

set PATH=C:\YourPathOfFPC\3.2.2\bin\i386-win32;%PATH%

If you installed the 64-bit version of fpc in C:\YourPathOfFPC\3.2.2 use

set PATH=C:\YourPathOfFPC\3.2.2\bin\x86-64-win64;%PATH%

Then build with

make clean all

If you see "Error makefile ... Command syntax error" your "set PATH" was not correct. When make all has succesfully finished it created with a 32-bit fpc the executable bin/i386-win32/pas2js.exe and a basic config file bin/i386-win32/pas2js.cfg.


When you built via make, it creates a pas2js.cfg in bin/$(TargetCPU)-$(TargetOS)/.

You can create a basic pas2js.cfg yourself by either using tools/createconfig/createconfig or manually:

For more about pas2js.cfg see here: pas2js.cfg

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. Note that the snapshots and svn version already contains a default pas2js.cfg with unit search paths (-Fu) for the rtl and fcl. See here how for details about the pas2js.cfg.

Basically, the command is the same as any FPC command line. The only thing that is different is the target: -Tbrowser or -Tnodeejs

Here is the complete list of command line arguments.

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

for modules

Pas2js supports compiling modules:

pas2js -Tmodule mymodule.pas

When compiled succesfully, the code can be used in a web page using the following tag:

<script type="module" src="mymodule.js"><script>

More info can be found on the Modules page

Supported syntax elements

A significant amount of Object Pascal syntax is supported, including RTTI. A more detailed list can be found in the Pas2js Transpiler file in the sources.

  • Delphi and ObjFPC mode
  • Program, Units, namespaces
  • Library since 2.1.0
  • unit initialization, but not finalization
  • Var, Const, Type
  • string (unicodestring), char (widechar), Boolean, Double, Byte, Shortint, Word, Smallint, longword, Longint, nativeint(int53), nativeuint(int52), currency
  • resourcestrings
  • Pointer (as a reference to a class, array, record, pointer of record, interface, but no pointer arithmetic)
  • Record (but no variant records), advanced records (since 1.3)
  • Functions, Procedures, nested, anonymous functions (since 1.1)
  • function types, of object, reference to (closures)
  • function arguments: default, const, var, out
  • If-then-else
  • For-do
  • Repeat-until
  • While-do
  • With-do
  • try-finally
  • try-except
  • enums
  • sets
  • arrays static, dynamic, open, multi dimensionals, array of const
  • String like array operations: a:=[1,2,3]+[1,1];
  • class type, visibility, virtual, override, abstract, overload, properties, class properties, class var, class const, constructor, destructor, class constructor (since 1.5)
  • class-of
  • nested classes
  • interfaces: CORBA, COM, delegations, method resolution, reference counting, TVirtualInterface
  • external classes, vars, const
  • class helpers, record helpers, type helpers (since 1.3)
  • Enumeration
  • Type alias, e.g. type TTranslateString = type string;
  • RTTI
  • asm block for embedding JavaScript directly
  • compiler directives (e.g. $ifdef, $if, $define, $modeswitch, $R+)
  • compile time and run time range checking. Overflow checking since 1.5.
  • attributes (since 1.5)
  • $linklib directive to link modules.
  • generics (since 1.5)
  • Resources (since 1.5)

There are some constructs that are naturally not supported and will never be supported:

  • Anything involving memory pointers and pointer arithmetic.
  • Variant records

And there are some extra constructs unique to pas2js:

  • JSValue - a base type representing a JavaScript value.
  • function Str(X[,Y,Z...]): string - same as procedure Str, except as a function
  • async/await

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:

  • Extended RTTI
  • Operator Overloading
  • anonymous records, e.g. var a: array of record w: word; end;
  • Packages

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

Other unimplemented features

  • Array of interface
  • Enums with custom values
  • Finalization sections, class destructors
  • Futures
  • Global properties
  • Inline
  • Library
  • Objects
  • Pointer arithmetic
  • Record field interface
  • Variants

Lazarus integration of pas2js

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

Since Lazarus 1.9 the IDE can use pas2js.exe as a normal compiler.

The integration is described here: lazarus pas2js integration. It is still under construction, but deep 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.

Here is a simple example:

  TJSFunction = class external name 'Function'(TJSObject)
    Flength: NativeInt external name 'length';
    Fprototyp: TJSFunction external name 'prototyp';
    name: String;
    property prototyp: TJSFunction read Fprototyp;
    property length: NativeInt read Flength;
    function apply(thisArg: TJSObject; const ArgArray: TJSValueDynArray): JSValue; varargs;
    function bind(thisArg: TJSObject): JSValue; varargs;
    function call(thisArg: TJSObject): JSValue; varargs;

This declares the TJSFunction object : in Javascript, functions are objects.

  • The "external name 'Function'" means that you declare a Javascript class where the Javascript name of the class is 'Function'.
  • The (TJSObject) means it descends from TJSObject also an external class. There does not need to be an ancestor type.
  • Fields are declared just as in Pascal.
  • To declare read-only fields, a trick can be used: declare the field using an external name "thename" modifier, and declare a read-only property with the same name.
    (see the length declaration)
  • Varargs can be used to indicate that a function accepts any number of arguments.
  • JSValue can be used to indicate an unknown type.
    It is more or less equivalent to a Variant.

Create simple JS objects with the new function

Some JS-framework functions expect an JS object as parameter. Here is how to do that in Pascal using the new function from unit JS:

// JavaScript:
DoIt({name:"Fred", id:3, size:4.3});
// Pascal;
DoIt(new(['name','Fred', 'id',3, 'size',4.3]));

You can nest it to create sub objects:

// JavaScript:
DoIt({name:"Fred", size:{width:3,height:2}});
// Pascal;
DoIt(new(['name','Fred', 'size',new(['width',3, 'height',2])]));

You can use TJSArray._of to create JS arrays on the fly:

// JavaScript:
// Pascal;

Resource strings

The pas2js transpiler can generate a JSON file (extension .jrs) with all the resource strings in your program.

This is a quite simple file. A JSON object exists for every unit, with each json property a resource string.

  "trs2" : {
    "ResUsed" : "This resourcestring is used",
    "ResUnUsed" : "This resourcestring is not used",
    "ImplResUsed" : "This implementation resourcestring is used"
  "trs1" : {
    "MyString" : "The very nice string we will need to translate"

This file can be translated, and the translation file can be loaded using the rstranslate unit, part of the rtl. There are demo programs which demonstrate the use of this feature.

The generating of this file is controlled by the -Jr option. It can take 3 possible arguments:

  1. none This is the default, no file is generated.
  2. unit one file per compiled unit will be generated. This file will contain all resource strings of the unit.
  3. program one file is generated for the main file. This fill will contain all used resource strings for the main file and all the units it uses.

If you compile a program, then the program option will generate a file with all the used resource strings in your program.

The above example was generated using the command:

pas2js -Jrprogram trs1.pp -B

Note that the format is different from the format used by FPC:

  • Identifiers in the file are case sensitive: the names must be typed as they appear in the source file.
  • The strings are grouped per unit, this allows to load them fasters
  • The hash and bytes parts are missing, they make little sense in a Javascript context.


Exceptions are translated to actual Javascript exceptions. The rtl.js has several mechanisms to deal with uncaught exceptions. The basic mechanism is setting the showUncaughtExceptions to true before calling in your html file:

<script type="application/javascript">

the browser will then use a window.alert() to show uncaught exceptions.

More explanations can be found in pas2js_exceptions


The generated Javascript source code is of course visible and debuggable in the browser.

Moreover, the transpiler can generate a source map, which means that you will be able to see and debug the Pascal code in the browser. (not everything will work, but many things do. This depends on the browser too.)

A source map can be generated using the command-line parameter:


The easiest is to include the Pascal sources in the source map:


By default all source filenames are relative to You can tell the compiler to store all file names relative to a specific local base directory:


And you can store an URL in the map, so the browser will use URL/above-relative-file-name to get the source:


Porting from FPC/Delphi

See here for tips and traps porting code from FPC and Delphi.

Delphi cannot parse some of the constructs that exist in pas2js (namely: external classes). You can create stub declarations suitable for the Delphi parser with the stub creator.


Please report bugs in the FPC Bug Tracker with category pas2js.


Lazarus Widgetset

The ultimate goal is of course to have the LCL running in the web. Discussions on this topic are delegated to a separate page. pas2js_widgetsets


Why is a simple hello world program so big?

This is mainly due to the used rtl.js. The rtl.js contains code for Pascal modules, classes, RTTI, sets, range checks, etc and is written with big WebApps in mind, not for scripts with a few lines of code.

  1. You can use a Javascript minifier to reduce the created Javascript
  2. You can create your own minified rtl.js by removing all functions you don't need. Eventually this will be done automatically by pas2js.

Why are asm blocks bad?

Asm blocks are useful for things you cannot do with pas2js. But there are some downsides: pas2js does not parse the JS. Neither does it check the syntax, nor does it know what Pascal identifiers the code is referencing. That means any identifier only accessed by the asm block will be removed by the pas2js' optimizer.

Therefore always try to do it in Pascal. Remember you can typecast values to JSValue, objects to TJSObject, arrays to TJSArray, strings to TJSString, etc to use almost all JS features.

Why not parse asm blocks?

Any compiletime JS parser can only do a syntax check and parse only simple JS. But since simple JS can be better written in Pascal, it is somewhat pointless and has therefore low priority.

What about optimization X?

See here for Pas2js optimizations

See also