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   
        function addProperties<T>(object: T): IPropertiesToAdd<T> {/*implmentions code*/};

        interface IPropertiesToAdd<T> extend T{
            on(): void;
            off(): void;
        }

//usage example
var str = new String('Hello')
addProperties(str)
str.charAt(3)
str.on() 

For the above code typescript compiler return the error that an interface can only add a class or interface, how I can express this in typescript.

Answer 1

You can create a new type alias which will allow your object to inherit the features of another object type. I found this bit of code here.

type IPropertiesToAdd<T extends {}> = T & {    // '{}' can be replaced with 'any'
    on(): void
    off(): void
};

interface ISomething {
    someValue: number
}

var extendedType: IPropertiesToAdd<ISomething> = {
    on(): void {
        console.log("switched on");
    },
    off(): void {
        console.log("switched off");
    },
    someValue: 1234,
};

I've tested this, and it seems that 'T' can be an interface, class, and an array type. I couldn't get union types to work.

This only works on anonymous objects, it can't be used for actual inheritance purposes.

Hope this helps.