To @chris-allnutt and refer to #56
An error ReferenceError: 'Proxy' is undefined will appear while using eosjs in IE11. We could reproduce this error in the simple boilerplate project:

Reproducing steps:
Clone repo https://github.com/EOSIO/eosio-project-boilerplate-simple and run ./quick_start.sh
Open http://localhost:3000/ in IE11
Add a note by using the first account in the listed account array.
Then you could see the error in the console and the note is not added to the blockchain.

FYI:
By looking into the source code of eosjs, the Proxy object is used in /src/ripemd.js line 382.
The ripemd is required from /src/eosjs-numeric.ts.

And Proxy is not supported in IE11 at all.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy

@ChristianDunst @rickykung