Difference between revisions of "Porting from FPC/Delphi to pas2js"
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| − | This page | + | This page is for Delphians and FPC porting code to pas2js. It contains useful tips and common traps. |
=Numbers= | =Numbers= | ||
| − | * Internally all numbers are ''double'' | + | * 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'' | + | * There is '''no''' ''Int64'' and no ''QWord''. |
* All '''bitwise operators are limited to 32bit''', including the ''mod'' operator, which is limited to signed 32bit. | * 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 ''{$R+}'' is enabled ''i:=300'' will raise an ''ERangeError''. | * '''Integers overflows''' at runtime differ from Delphi/FPC. For example adding ''var i: byte = 200; ... i:=i+100;''' will result in ''i=300'' instead of ''i=44'' as in Delphi/FPC. When range checking ''{$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; | ||
=Strings= | =Strings= | ||
| Line 16: | Line 20: | ||
JavaScript enforces an absolute async manner of programming: | JavaScript enforces an absolute async manner of programming: | ||
| − | *Many calls are asynchronous and return immediately | + | *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''' 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 | + | *There is no multithreading, no shared memory. Many browsers/JS engines support webworkers, but that is more like processes than threads. |
| + | |||
| + | =Details= | ||
| + | |||
| + | A more detailed list can be found in the {{gitlab|repository|FPC|3.3.1/utils/pas2js/docs/translation.html|translation.html}}.file in the sources. | ||
| + | [[Category:Pas2js]] | ||
Latest revision as of 21:41, 8 March 2023
This page is for Delphians and FPC porting code to pas2js. It contains useful tips and common traps.
Numbers
- 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;
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. 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.
Details
A more detailed list can be found in the translation.html.file in the sources.
