Animation.prototype.finished polyfill

finished-polyfill.js

// only polyfill .finished in browsers that already support animate()
if (document.body.animate) {
  
  // Chrome does not seem to expose the Animation constructor globally
  if (typeof Animation === 'undefined') {
    window.Animation = document.body.animate({}).constructor;
  }

  if (Animation.prototype.finished === undefined) {
    Object.defineProperty(Animation.prototype, 'finished', {
      get() {
        if (!this._finished) {
          this._finished = this.playState === 'finished' ? 
            Promise.resolve() :
            new Promise((resolve, reject) => {
              this.addEventListener('finish', resolve, {once: true});
              this.addEventListener('cancel', reject, {once: true});
            });
        }
        return this._finished;
      }
    });
  }
}

I was getting an error in Safari 13 (tech preview):

Unhandled Promise Rejection: TypeError: The Animation.finished getter can only be used on instances of Animation

So I forked this to fix the error: https://gist.github.com/davej/23b0481016bcdb969df46423c6284771

👍 Good to see more (full?) animation API support in the next version of Safari.