Angular 2 typescript invoke javascript function

Is there a correct way to invoke a JavaScript function from a component in Angular 2 (TypeScript) ?

Here is my component :

import { ElementRef, AfterViewInit }       from '@angular/core';

export class AppComponent implements AfterViewInit {

    constructor(private _elementRef: ElementRef) {
    }

    ngAfterViewInit() {
        /**
         * Works but i have this error :
         * src/app.component.ts(68,9): error TS2304: Cannot find name 'MYTHEME'.
         * src/app.component.ts(69,9): error TS2304: Cannot find name 'MYTHEME'.
         */
        MYTHEME.documentOnLoad.init(); 
        MYTHEME.documentOnReady.init();

        /**
         * Works without error, but doesn't seem like a right way to do it
         */
        var s = document.createElement("script");
        s.text = "MYTHEME.documentOnLoad.init(); MYTHEME.documentOnReady.init();";
        this._elementRef.nativeElement.appendChild(s);
    }
}

Calling the JavaScript function directly result in an compilation error, but the syntax in the "compiled" JavaScript file (app.component.js) is correct :

AppComponent.prototype.ngAfterViewInit = function () {
    MYTHEME.documentOnLoad.init();
    MYTHEME.documentOnReady.init();
};

The 2nd way (appendChild) works without error, but i don't think (altering the DOM from typescript/angular) is the correct way to do it.

I found this : Using a Javascript Function from Typescript I tried declaring the interface :

interface MYTHEME {
    documentOnLoad: Function;
    documentOnReady: Function;
}

But the TypeScript doesn't seem to recognize it (no error in the interface declaration).

Thanks

Edit :

Following the answer by Juan Mendes this is what i ended with :

import { AfterViewInit }       from '@angular/core';

interface MYTHEME {
    documentOnLoad: INIT;
    documentOnReady: INIT;
}
interface INIT {
    init: Function;
}
declare var MYTHEME: MYTHEME;

export class AppComponent implements AfterViewInit {

    constructor() {
    }

    ngAfterViewInit() {
        MYTHEME.documentOnLoad.init(); 
        MYTHEME.documentOnReady.init();
    }
}