Porting from FPC/Delphi to pas2js: what changed
Revision 0 to 1. Removed lines have a minus sign; added lines have a plus sign.
Title: Porting from FPC/Delphi to pas2js
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| 1 | + | This page is for Delphians and FPC porting code to pas2js. It contains useful tips and common traps. | |
| 2 | + | ||
| 3 | + | # Numbers | |
| 4 | + | ||
| 5 | + | - 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. | |
| 6 | + | - There is **no** *Int64* and no *QWord*. | |
| 7 | + | - All **bitwise operators are limited to 32bit**, including the *mod* operator, which is limited to signed 32bit. | |
| 8 | + | - **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*. | |
| 9 | + | - **Division by zero** does not raise *EDivByZero*, instead it results in *NaN*. | |
| 10 | + | - **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: | |
| 11 | + | - MaxCurrency = 900719925474.0991; // fpc: 922337203685477.5807; | |
| 12 | + | - MinCurrency = -900719925474.0991; // fpc: -922337203685477.5808; | |
| 13 | + | ||
| 14 | + | # Strings | |
| 15 | + | ||
| 16 | + | - String is UnicodeString and there are no other string types. | |
| 17 | + | - 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. | |
| 18 | + | ||
| 19 | + | # Asynchronous vs Waiting | |
| 20 | + | ||
| 21 | + | JavaScript enforces an absolute async manner of programming: | |
| 22 | + | ||
| 23 | + | - Many calls are asynchronous and return immediately. For example loading a resource. | |
| 24 | + | - 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. | |
| 25 | + | - There is no multithreading, no shared memory. Many browsers/JS engines support webworkers, but that is more like processes than threads. | |
| 26 | + | ||
| 27 | + | # Details | |
| 28 | + | ||
| 29 | + | A more detailed list can be found in the [translation.html](https://gitlab.com/freepascal.org/fpc/source/-/tree/3.3.1/utils/pas2js/docs/translation.html).file in the sources. | |
| 30 | + | ||
| 31 | + | --- | |
| 32 | + | *From the Free Pascal wiki, page "Porting from FPC/Delphi to pas2js", as it stood on 2023-03-08 after a change by Kai Burghardt, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).* | |
| 33 | + | Category: Pas2js |