In order to write a begin lookalike in ES6, we can take advantage of the fact that in JavaScript arguments are evaluated from left to right. This leads to:

const begin = (...args) => args[args.length - 1];  

We can therefore replicate SICP's make-withdraw procedure as:

const makeWithdraw = bal =>  
  amount => bal >= amount ? begin(bal = bal - amount, bal) 
                          : "Insufficient founds"