Library types for TypeScript declaration merging

Is there a way to "extend" interfaces through declaration merging that were originally declared in a TypeScript library file?

Specifically, I am trying to extend the HTMLCanvasElement interface from the built-in TypeScript library lib.dom. While I understand how declaration merging works, I am having trouble with the correct imports for the HTMLCanvasElement interface.

import {HTMLCanvasElement} from '<what.to.use.here>';

declare module '<what.to.use.here>'{
    interface HTMLCanvasElement{
        //add my stuff
    }
}

Any help would be appreciated. Thank you :)

typescripttype-declaration

Answer №1

These types are found in the global namespace. If you are working in a script file (i.e., not a module), you can simply declare it again. However, if you are inside a module, you will need to declare it within the global scope without requiring an import statement.

declare global{
    interface HTMLCanvasElement{
        my:number
    }
}

export var x = 1;// just for demonstration purposes
let c: HTMLCanvasElement;
c.animate // usual functionality 
c.my //my custom functionality

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