Class type check in TypeScript

Question

In ActionScript  it is possible to check the type at run-time using the is operator   var mySprite Sprite   new Sprite     trace mySprite is Sprite      true  trace mySprite is DisplayObject     true  trace mySprite is IEventDispatcher      true   Is it possible to detect if a variable  extends or  is a certain class or interface with TypeScript   I couldn t find anything about it in the language specs  It should be there when working with classes interfaces

User · Answer

TypeScript have a way of validating the type of a variable in runtime. You can add a validating function that returns a type predicate. So you can call this function inside an if statement, and be sure that all the code inside that block is safe to use as the type you think it is.

Example from the TypeScript docs:

function isFish(pet: Fish | Bird): pet is Fish {
   return (<Fish>pet).swim !== undefined;
}

// Both calls to 'swim' and 'fly' are now okay.
if (isFish(pet)) {
  pet.swim();
}
else {
  pet.fly();
}

See more at: https://www.typescriptlang.org/docs/handbook/advanced-types.html

User · Answer

You can use the instanceof operator for this  From MDN      The instanceof operator tests whether the prototype property of a   constructor appears anywhere in the prototype chain of an object    If you don t know what prototypes and prototype chains are I highly recommend looking it up  Also here is a JS  TS works similar in this respect  example which might clarify the concept    x000D   x000D      class Animal   x000D          name  x000D       x000D          constructor name    x000D              this name   name  x000D            x000D        x000D       x000D      const animal   new Animal  fluffy    x000D       x000D         true because Animal in on the prototype chain of animal x000D      console log animal instanceof Animal      true x000D         Proof that Animal is on the prototype chain x000D      console log Object getPrototypeOf animal      Animal prototype      true x000D       x000D         true because Object in on the prototype chain of animal x000D      console log animal instanceof Object    x000D         Proof that Object is on the prototype chain x000D      console log Object getPrototypeOf Animal prototype      Object prototype      true x000D       x000D      console log animal instanceof Function      false  Function not on prototype chain x000D       x000D       x000D   x000D   x000D    The prototype chain in this example is   animal   Animal prototype   Object prototype

User · Answer

4 19 4 The instanceof operator The instanceof operator requires the left operand to be of type Any  an object type  or a type parameter type  and the right operand to be of type Any or a subtype of the  Function  interface type  The result is always of the Boolean primitive type   So you could use mySprite instanceof Sprite   Note that this operator is also in ActionScript but it shouldn t be used there anymore   The is operator  which is new for ActionScript 3 0  allows you to test whether a variable or expression is a member of a given data type  In previous versions of ActionScript  the instanceof operator provided this functionality  but in ActionScript 3 0 the instanceof operator should not be used to test for data type membership  The is operator should be used instead of the instanceof operator for manual type checking  because the expression x instanceof y merely checks the prototype chain of x for the existence of y  and in ActionScript 3 0  the prototype chain does not provide a complete picture of the inheritance hierarchy    TypeScript s instanceof shares the same problems  As it is a language which is still in its development I recommend you to state a proposal of such facility  See also   MDN  instanceof

[typescript] Class type check in TypeScript

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