- 1 Pas2js : What is it ?
- 2 Where to get it
- 3 How to use pas2js
- 4 Supported syntax elements
- 5 Planned language features
- 6 Other not implemented features
- 7 Lazarus integration of pas2js
- 9 Create simple JS objects with the new function
- 10 Debugging
- 11 Bugs
- 12 Examples
- 13 FAQ
Pas2js : What is it ?
- as a library
- as a command-line program
- as a webserver
It transpiles from actual Pascal source, it has no intermediate .ppu files. That means all sources must always be 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)
pas2js can automatically include this file in the generated output, like this:
pas2js -Jc -Jirtl.js -Tbrowser hello.pas
There is a basic Object Pascal RTL, several units from the FPC Packages are also available
- DB (yes, TDataset)
- fpcunit testsuite
- web (browser provided objects)
- libjquery (jquery is available too)
- nodejs (basic node runtime environment)
- browserconsole (support writeln)
Where to get it
The pas2js compiler and RTL are - naturally - open source and can be downloaded and used freely.
The snapshots contain binaries for Windows (32 and 64bit), Linux (64 bit) and Macos.
The snapshots are uploaded to
Every version has a directory with the version number. A list of changes can be found on the changelog page Pas2JS Version Changes
svn co https://svn.freepascal.org/svn/projects/pas2js/trunk pas2js
You need fpc 3.0.4 or better to compile it.
Change to the directory and build it with:
make clean all
This creates compiler/utils/pas2js/pas2js (Windows: compiler\utils\pas2js\pas2js.exe)
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
Here is the complete list of command line arguments.
for the browser
Consider the classical:
program hello; begin Writeln('Hello, world!'); end.
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:
The files that are needed are:
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; begin Writeln('Hello, world!'); end.
pas2js -Tnodejs hello.pas
When compiled succesfully, the code can be run in node using the following command.
Note: on MacOS it is "node hello.js"
Supported syntax elements
Basically, Delphi 7 syntax is supported. This includes RTTI. A more detailed list can be found in the translation.html.file in the sources.
- Delphi and ObjFPC mode
- Program, Units, namespaces
- 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
- Pointer (as a reference to a class, array, record, pointer of record, interface, but no pointer arithmetic)
- Record (but no variant records)
- Functions, Procedures, nested
- function types, of object, reference to (closures)
- function arguments: default, const, var, out
- arrays static, dynamic, open, multi dimensionals
- class type, visibility, virtual, override, abstract, overload, properties, class properties, class var, class const, constructor, destructor
- nested classes
- interfaces: CORBA, COM, delegations, method resolution, reference counting, TVirtualInterface
- Enumeration for..in..do
- Type alias, e.g. type TTranslateString = type string;
- compiler directives (e.g. $ifdef, $if, $define, $modeswitch, $R+)
- compile time and run time range checking
There are some constructs that are naturally not supported and will never be supported:
- Anything involving memory pointers and pointer arithmetic.
- Variant records
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:
- Anonymous functions
- Advanced records
- Runtime checks: Overflow -Co, $Q
- Type helpers
Needless to say, anything requiring direct memory access is not going to be supported.
Other not implemented features
- Array of const
- Enums with custom values
- Global properties
- Helpers for types, classes, records
- Operator overloading
- Pointer arithmetic
- RTTI extended, $RTTI
- Variant records
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.
Here is a simple example:
TJSFunction = class external name 'Function'(TJSObject) private Flength: NativeInt external name 'length'; Fprototyp: TJSFunction external name 'prototyp'; public 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; end;
This declares the
- The "
(TJSObject)means it descends from
TJSObjectalso 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)
Varargscan be used to indicate that a function accepts any number of arguments.
JSValuecan 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:
// Pascal; DoIt(new(['name','Fred', 'id',3, 'size',4.3]));
You can nest it to create sub objects:
// Pascal; DoIt(new(['name','Fred', 'size',new(['width',3, 'height',2])]));
You can use TJSArray._of to create JS arrays on the fly:
// Pascal; DoIt(new(['numbers',TJSArray._of(1,2,3)]));
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
You can tell the compiler to store all file names relative to a 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:
Please report bugs in the FPC bugtracker with category pas2js: http://bugs.freepascal.org
- Time Tracking Application: https://www.devstructor.com/demos/pas2js-time/source.zip (sources: https://www.devstructor.com/demos/pas2js-time/source.zip)
- Drawing and animation on canvas: http://ragnemalm.se/images/santa/santa.html (sources: http://ragnemalm.se/images/santa/)
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.
- You can create your own minified rtl.js by removing all functions you don't need. Eventually this will be done automatically by pas2js.