Unfortunately, most of the SoC devices are based on older WebKit (usually version <50). Because of that, you cannot use modern JS syntax (read more on ECMAscript).
We strongly recommend to transpile your code and avoid using polyfills. If you have to use polyfills we recommend using Babel.JS.
Automated transpilation in single file Applet via Web Editor
CLI-based Applet development
While developing on your device, you can easily add Babel.js as a transpilator of choice into your project and use it during building your code.
Check browser versions sorted by platforms
Here is a list of supported devices along with their browser versions.