Porting from FPC/Delphi to pas2js
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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.
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.
Asynchronous vs Waiting
JavaScript enforces an absolute async manner of programming:
- Many calls are asynchronous and return immediately, like 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.
