Typescript: Overriding / extending Lib.d.ts with preexisting identifer

I need to get typescript to stop complaining about my code. It runs fine in the browser but fullscreen api are not official yet so typescript definitions aren't up to date.

I am calling document.documentElement.msRequestFullscreen. This causes type error:

Property 'msRequestFullscreen' does not exist on type 'HTMLElement'.

Upon looking at lib.d.ts, I find this:

documentElement: HTMLElement;

So documentElement is set to type HTMLElement. I tried adding a custom definition to override documentElement. My Custom definition:

// Extend Document Typings
interface Document {
   msExitFullscreen: any;
   mozCancelFullScreen: any;
   documentElement: {
      msRequestFullscreen: any;
      mozRequestFullScreen: any;
   }
}

I tried extending the interface for Document but it gives error

Error is:

lib.d.ts:5704:5 
Duplicate identifier 'documentElement'.

My typescript class

export class ToggleFullScreen {
   viewFullScreenTriggerID: string;
   viewFullScreenClass: string;
   cancelFullScreenClass: string;
   viewFullscreenElem: any;
   activeIcon: string;
   notFullscreenIcon: string;
   isFullscreenIcon: string


   constructor() {
      this.viewFullScreenTriggerID = "#fullScreenTrigger";
      this.viewFullScreenClass = "not-fullscreen";
      this.cancelFullScreenClass = "is-fullscreen";
      this.notFullscreenIcon = "/assets/icon/fullscreen-enter.svg";
      this.isFullscreenIcon = "/assets/icon/fullscreen-exit.svg";
      this.activeIcon = this.notFullscreenIcon;
   }

   toggleFullScreen() {
      this.viewFullscreenElem = document.querySelector(this.viewFullScreenTriggerID);

      if (this.viewFullscreenElem.classList.contains(this.viewFullScreenClass)) {

         var docElm = document.documentElement;
         if (docElm.requestFullscreen) {
            docElm.requestFullscreen();
         } else if (docElm.msRequestFullscreen) {
            docElm.msRequestFullscreen();
         } else if (docElm.mozRequestFullScreen) {
            docElm.mozRequestFullScreen();
         } else if (docElm.webkitRequestFullScreen) {
            docElm.webkitRequestFullScreen();
         }

         this.viewFullscreenElem.classList.toggle(this.viewFullScreenClass);
         this.viewFullscreenElem.classList.toggle(this.cancelFullScreenClass);
         this.activeIcon = this.isFullscreenIcon;

      }

      else if (this.viewFullscreenElem.classList.contains(this.cancelFullScreenClass)) {
         if (document.exitFullscreen) {
            document.exitFullscreen();
         } else if (document.msExitFullscreen) {
            document.msExitFullscreen();
         } else if (document.mozCancelFullScreen) {
            document.mozCancelFullScreen();
         } else if (document.webkitCancelFullScreen) {
            document.webkitCancelFullScreen();
         }

         this.viewFullscreenElem.classList.toggle(this.viewFullScreenClass);
         this.viewFullscreenElem.classList.toggle(this.cancelFullScreenClass);
         this.activeIcon = this.notFullscreenIcon;
      }
   }
}

What is the proper way to get typescript compile errors to stop?

UPDATE: I found a workaround. Instead of trying to override documentElement, which is set to type HTMLElement, I extended HTMLElement and added the properties which were missing.

// Extend Document Typings
interface Document {
   msExitFullscreen: any;
   mozCancelFullScreen: any;
}


interface HTMLElement {
   msRequestFullscreen(): void;
   mozRequestFullScreen(): void;
}

You can't override existing properties of an existing interface, only add new ones.

Based on the MDN Using fullscreen mode and Element documentation you need to have:

Element.requestFullscreen()

Which exists in the lib.d.ts and lib.es6.d.ts.

If you're missing msRequestFullscreen and mozRequestFullScreen then you need to add them to Element:

interface Document {
    msExitFullscreen: any;
    mozCancelFullScreen: any;
}

interface Element {
    msRequestFullscreen(): void;
    mozRequestFullScreen(): void;
}

document.documentElement.mozRequestFullScreen(); // no error