In ES6 there are two common ways to define class methods, but they behave completely different.

The first way is to define them as a standard class function.

class A {
    foo() {
        console.log('foo from A')
    }
}

But you can also define them as an instance specific function:

class A {
    foo = () => {
        console.log('foo from A')
    }
}

The first difference is pretty obvious if you know ES6’s fat arrow functions: It binds the this variable inside the function call to the this variable of the scope of where the function was defined. So in our case, this will be bound to the current class instance.

We can achieve the same by avoiding arrow functions and use a standard class method in addition to binding it in the constructor:

class A { 
    constructor() {
        this.foo = this.foo.bind(this)
    }

    foo() {
        console.log('foo from A')
    }
}

You might now think that the following class definitions have the same behavior:

However, they behave in a completely different way when we use JavaScript’s class inheritance.

Class Inheritance yields different results

Let’s run the following code:

class A { 
    constructor() {
        this.foo = this.foo.bind(this)
    }

    foo() {
        console.log('foo from A')
    }
}

class B extends A {
    foo() {
        super.foo();
        console.log('foo from B')
    }
}
new B().foo()

The expected result is logged:

foo from A
foo from B

Let’s try the same with fat-arrow functions now:

class A { 
    foo = () => {
        console.log('foo from A')
    }
}

class B extends A {
    foo = () => {
        super.foo();
        console.log('foo from B')
    }
}
new B().foo()

We get an error:

Cannot read property 'call' of undefined

What’s happening here?

Whenever you call super.someFunction in JavaScript, a lookup for a key someFunction is done in __proto__ - that’s the way JavaScript’s class inheritance is implemented. The reason why we get this error is because foo is not in __proto__ of the object instance of new B().

So, what this tells us is that fat arrow class functions cannot be used from subclasses. Fat arrow functions can always only be defined through class instance properties. So what our class with transform-class-properties actually does, is attaching the function first inside the constructor when a concrete class instance is created:

class A {
    constructor() {
        this.foo = () => {
            console.log('foo from A')
        }
    }
}

Of course, doing it this way, foo does never exist in __proto__ and makes it unusable in extended classes. If you need a method to be callable from a derived class (and you want to use the ES6 class syntax), you need to define that method as a normal class method using shorthand notation:

class A { 
    constructor() {
        this.foo = this.foo.bind(this)
    }

    foo() {
        console.log('foo from A')
    }
}

class B extends A {
    foo() {
        super.foo();
        console.log('foo from B')
    }
}
new B().foo()