SyntaxError: Unexpected token '='. Expected an opening '(' before a method's parameter list when using arrow functions as public field declarations (TC39 proposal) -- Safari

[Nashorn]

Safari 13 and below does not support public class field declarations on classes. Ex:

class T {
    foo = () => {
        //syntax error
    }
}

Copy and run that in Safaris console; will throw a syntax error:
"SyntaxError: Unexpected token '='. Expected an opening '(' before a method's parameter list."

See TC39 Proposal:
https://github.com/tc39/proposal-class-fields

Safari 14 beta does have support. Work around is to use ES6 working method syntax:

class T {
    foo() {
        //this works
    }
}

The benefit of arrow functions on classes is to preserve scope reference to [this] without having to .bind() your methods.
Example:

class T {
    constructor(){
        this.desc = "hello";
        document.addEventListener("click", this.foo)
    }
    foo() {
        alert(this.desc) // undefined
    }
}

Will alert undefined, because foo() will run in scope of document object which has no .desc property. But with arrow functions:

class T {
    constructor(){
        this.desc = "hello";
        document.addEventListener("click", this.foo)
    }
    foo =()=> {
        alert(this.desc) // "hello"
    }
}

WORK AROUND SOLUTION:

It works as you would expect. Now, to use the normal working method syntax and keep your reference to [this], you'll have to bind():

class T {
    constructor(){
        this.desc = "hello";
        document.addEventListener("click", this.foo.bind(this))
    }
    foo() {
        alert(this.desc) // "hello"
    }
}