They're probably using a preprocessor like Babel[1] or something like es6-shim[2] 1: https://babeljs.io/ https://babeljs.io/ 2: https://github.com/paulmillr/

They're probably using a preprocessor like Babel[1] or something like es6-shim[2] 1: https://babeljs.io/ https://babeljs.io/ 2: https://github.com/paulmillr/es6-shim/ https://github.com/paulmillr/es6-shim/