Porting from FPC/Delphi to pas2js

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This page is for Delphians and FPC porting code to pas2js. It contains useful tips and common traps.


  • Internally all numbers are double. Therefore integers range from -9,007,199,254,740,991 to 9,007,199,254,740,991, so about 1/1024th of a 64bit value.
  • There is no Int64 and no QWord.
  • All bitwise operators are limited to 32bit, including the mod operator, which is limited to signed 32bit.
  • Integers overflows' at runtime differ from Delphi/FPC. For example adding var i: byte = 200; ... i:=i+100; will result in i=300 instead of i=44 as in Delphi/FPC. When range checking {$R+} is enabled i:=300 will raise an ERangeError.
  • Division by zero does not raise EDivByZero, instead it results in NaN.
  • Currency has only 54 bits. Currency is internally a double, multiplied by 10000 and truncated. The below values are the safe limits, within every step exists. Since currency is a double it can take much larger values, but the result may differ from Delphi/FPC:
    • MaxCurrency = 900719925474.0991; // fpc: 922337203685477.5807;
    • MinCurrency = -900719925474.0991; // fpc: -922337203685477.5808;


  • String is UnicodeString and there are no other string types.
  • Strings are immutable in JS. That means changing a single character creates a new string. That's why some fast Delphi/FPC string functions are much slower in pas2js.

Asynchronous vs Waiting

JavaScript enforces an absolute async manner of programming:

  • Many calls are asynchronous and return immediately. For example loading a resource.
  • There is no Application.ProcessMessages. You cannot wait till some event occurs. You must set an event. That's why anonymous functions are so frequently used in JS - they keep the local variables accessible.
  • There is no multithreading, no shared memory. Many browsers/JS engines support webworkers, but that is more like processes than threads.


A more detailed list can be found in the translation.html.file in the sources.