исключает аки в GEC, так и в EC :3
let tt = x => x;
undefined
let tt = (x,y) => y;
VM180:1 Uncaught SyntaxError: Identifier 'tt' has already been declared
at <anonymous>:1:1
let ttt = {tt: function (x) {console.log(x)}, tt: function (x,y) {console.log(x, y)}}
console.log(
ttt.tt(5))
VM599:1 5 undefined // WTF is that shit "UNDEFINED"??? (C)
undefined
undefined
console.log(
ttt.tt(5, 6))
VM610:1 5 6
undefined
undefined