extending interface with generic in typescript

Question

I want to build an function which takes any object and return that object with few added properties. Something like:

    //this code doesn\'t work   
                


        

Answer 1

Interface IPropertiesToAdd defines a type variable T that is used to extend an interface named T. This is not possible. An interface can not be referred using a variable name; it must have a fixed name, e.g. Evnt:

interface Evnt {
  name: T;
}

interface IPropertiesToAdd extends Evnt {
  on(): void;
  off(): void;
}

I am not sure what you are trying to achieve in your case. I have extended the example a bit, so it compiles:

function addProperties(object: Evnt): IPropertiesToAdd {
  /* minimum implementation to comply with interface*/
  var ext:any = {};
  ext.name = object.name
  ext.on = function() {};
  ext.off = function() {};
  return ext;
};

interface Evnt {
  name: T;
}

interface IPropertiesToAdd extends Evnt {
  on(): void;
  off(): void;
}

//usage example
var str = {name: 'Hello'}
var evnt = addProperties(str)
evnt.charAt(3); // error because evnt is not of type 
                // `string` but `IPropertiesToAdd`
evnt.on()