What kind of change does this PR introduce?
bugfix
Did you add or update the examples/?
No
Summary
There were prior issues regarding the client files, after being upgraded to ES6, not working properly in IE11 (#1084). However, the fix (c9d32f8) left let and const in the client codebase, which works in IE11, but not in other ES5 compliant browsers (IE<11, UC Browser on Android).
Certain regions of the world, in particular China and developing countries, still has a large percentage of browsers that only support ES5 (UC Browser has over 500 million downloads on Google Play), so developers targeting customers in those countries still need to debug their applications in those browsers.
This PR ensures all client files are ES5 only (also enforces this via ESLint parser version).
Does this PR introduce a breaking change?
No.
Other information