As a C# developer you're accustomed to working with interfaces, classes, and subclasses.
C# has the 'as' keyword to cast an object from one type to another. For example:
private void SomeEventHandler(object sender, EventArgs e){ var someThing = sender as SomeClass; // ...}Furthermore, in C#, the as type-cast expression returns null if the object and target type are incompatible.
using System;public class Program{ public interface IThing {} public class A : IThing {} public class B : IThing {} public static void Main() { IThing thing = new A(); var a = thing as A; var b = thing as B; Console.WriteLine(a == null); // False Console.WriteLine(b == null); // True }}TypeScript
TypeScript also has an as keyword -- but beware that it behaves differently than in C#!
When you work with TypeScript, the as keyword is only telling the type checker to pretend something has a different type. You can think of the type annotations as a separate, meta layer on top of the actual code.
All the type-checking information is discarded when the TypeScript is compiled into JavaScript.
So if you were accustomed to C# and had the following TypeScript code, you might be surprised to find that b is not null.
type IThing = {};class A implements IThing {}class B implements IThing {}const thing: IThing = new A();const a = thing as A;const b = thing as B;console.log(a == null); // falseconsole.log(b == null); // false !!!The TypeScript compiler produces this ES5 JavaScript:
var A = (function () { function A() { } return A;}());var B = (function () { function B() { } return B;}());var thing = new A();var a = thing;var b = thing;console.log(a == null); // falseconsole.log(b == null); // false !!!
Remember, as is only for telling the type checker to treat something as a different type.
More in the TypeScript vs. C# Series
- LINQ
- more to come soon...