NodeList prototype extension for WebAnimations (w/ stagger option)

nodelist-animate.ts

interface AnimationOptions {
    stagger?: number;
}

NodeList.prototype.animate = function(keyframes: AnimationKeyFrame | AnimationKeyFrame[], options: number | AnimationOptions) {
    const elements: HTMLElement[] = Array.prototype.slice.call(this);

    (function next(elements: Array<HTMLElement>) {
        if (elements.length < 1) return;

        setTimeout(() => {
            elements[0].animate(keyframes, options);

            // call next() recursively
            next(elements.slice(1));
        }, typeof options =="object" ? options.stagger : 0);
    })(elements);
};