类中的函数表达式

Question

I understand what the difference is between a function expression and a function declaration but how are they different when it comes to the reference of a class? Like where can the below someFunction be used inside the class and where can it not? Which class can be instantiated where?

class xyz {
    someFunction(){
        // Function code
    }
}

vs

class xyz {
    var someFunction = function(){
        // Function code
    }
}

Answer 1

When you use a function declaration, the function goes to the class prototype.

class xyz {
    someFunction(){
        // Function code
    }
}

The above class can be represented as function constructor:

    function xyz() {
        //Code.
    }
    xyz.prototype.someFunction = function() {
      //Some code
    }

So someFunction() will be part of xyz's prototype.

Function expressions throws invalid syntax error when you use them in a class. But when you use a function expression in classic function constructors, the function is just a local variable and cannot be accessed by xyz instances.

function xyz()
 {
    var someFunction = function(){
        // Function code
    }
}

Answer 2

This case has nothing to do with the difference between a function expression and a function declaration

you will get Unexpected identifier because docs doesn't allow or define such syntax

class xyz {
    var someFunction = function(){
        // Function code
    }
}

In class syntax,you have three choices to write function:

class xyz {
  constructor() {
    // Function code
  }
  someFunction() {
    // Function code
  }
  static sayHi() {
    // Function code
  }
}

If you use other expressions or statements not allowed by docs,you will get error