fix: wrong usage about `this` in ES6+ (#395)
fix: wrong usage about `this` in ES6+ (#395)

Co-authored-by: Chen Fengyuan <2b0fafad6853deb5f868b051e6c1d2f19553bbc9@outlook.com>
gdh1995/viewerjs,fengyuanchen/viewerjs,gdh1995/viewerjs,fengyuanchen/viewerjs