无法在 javascript 中的 Class 的 object 中声明对象

Question

class A{
    obj = {};
    obj.x = 0;
}

let a = new A()

Why we cannot do this(create an object inside a class and give it a property) inside a class?

Answer 1

You can define instance fields with that syntax (outside of the constructor), but not mutate them. Don't confuse the = syntax with the usual assignment syntax, as it really has different rules. For instance you can also use this syntax:

class A {
    ["obj"] = {};
}

If you want to mutate fields/properties, you'll have to do this inside the constructor or other method. On the other hand, nothing stops you to initialise the field with a more elaborate object literal.

NB: to create an instance you should use the new keyword.

So either do:

 class A { obj = { x: 0 }; } let a = new A(); console.log(a.obj.x);

Or do:

 class A { obj = {}; constructor() { this.obj.x = 0; } } let a = new A(); console.log(a.obj.x);