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All of our typescript classes inherit (directly or indirectly) from:
export class WrObject { className:string; public instanceOf(name : String) : boolean { return this.className === name; } } We then declare a subclass as:
export class Section extends WrObject { public static CLASS_NAME = 'Section'; className = Section.CLASS_NAME; public instanceOf(name : String) : boolean { if (this.className === name) return true; return super.instanceOf(name); } } And you can then check with:
if (obj.instanceOf(Section.CLASS_NAME)) It all works great. However, I think it would be cleaner if we could do:
if (obj.instanceOf(Section)) Is there any way to do that? Or any suggestions as to a better approach?
thanks - dave
623
The instanceof operator can be used to help TypeScript narrow the type of a class object variable. It only works on class structures and not other TypeScript structures, such as interfaces.
The typeof and the instanceof operator is quite different. typeof returns a type of entity that it's operated on. instanceof of returns true if an object is created from a given constructor and false otherwise. All non-primitive objects are instances of Object , so that'll always return true .
The JavaScript instanceof operator is used to check the type of an object at the run time. It returns a boolean value(true or false). If the returned value is true, then it indicates that the object is an instance of a particular class and if the returned value is false then it is not.
The instanceof operator tests to see if the prototype property of a constructor appears anywhere in the prototype chain of an object. The return value is a boolean value. Its behavior can be customized with Symbol.
If you are willing to accept the prototypal nature of JavaScript you can just use instanceof which checks the prototype chain:
class Foo{} class Bar extends Foo{} class Bas{} var bar = new Bar(); console.log(bar instanceof Bar); // true console.log(bar instanceof Foo); // true console.log(bar instanceof Object); // true console.log(bar instanceof Bas); // false You can do it. Just replace your instanceOf implementation with this one:
public instanceOf(cls: { CLASS_NAME: string; }) { return cls.CLASS_NAME === this.className || super.instanceOf(cls); } If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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