TypeScript or JavaScript type casting

Question

How does one handle type casting in TypeScript or Javascript     Say I have the following TypeScript code   module Symbology         export class SymbolFactory             createStyle  symbolInfo   SymbolInfo    any                if  symbolInfo    null                                 return null                             if  symbolInfo symbolShapeType      marker                               how to cast to MarkerSymbolInfo                           return this createMarkerStyle  MarkerSymbolInfo  symbolInfo                                                                      createMarkerStyle markerSymbol   MarkerSymbolInfo    any                throw  createMarkerStyle not implemented                                      where SymbolInfo is a base class   How do I handle typecasting from SymbolInfo to MarkerSymbolInfo in TypeScript or Javascript

User · Answer

In typescript it is possible to do an instanceof check in an if statement and you will have access to the same variable with the Typed properties  So let s say MarkerSymbolInfo has a property on it called marker  You can do the following  if  symbolInfo instanceof MarkerSymbol           access  marker here     const marker   symbolInfo marker    It s a nice little trick to get the instance of a variable using the same variable without needing to reassign it to a different variable name  Check out these two resources for more information  TypeScript instanceof  amp  JavaScript instanceof

User · Answer

You can cast like this     return this createMarkerStyle  lt MarkerSymbolInfo gt  symbolInfo     Or like this if you want to be compatible with tsx mode   return this createMarkerStyle symbolInfo as MarkerSymbolInfo     Just remember that this is a compile-time cast  and not a runtime cast

User · Answer

This is called type assertion in TypeScript  and since TypeScript 1 6  there are two ways to express this     Original syntax var markerSymbolInfo    lt MarkerSymbolInfo gt  symbolInfo      Newer additional syntax var markerSymbolInfo   symbolInfo as MarkerSymbolInfo   Both alternatives are functionally identical  The reason for introducing the as-syntax is that the original syntax conflicted with JSX  see the design discussion here  If you are in a position to choose  just use the syntax that you feel more comfortable with  I personally prefer the as-syntax as it feels more fluent to read and write

[casting] TypeScript or JavaScript type casting

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