This page contains useful tips for Delphians and FPC users on how to port code to pas2js.

Numbers

Internally all numbers are double
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.

Strings

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.

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 allow to start workers, but that is more like processes than threads.

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