如何在 TypeScript 中为了干净的代码目的在构造函数外初始化只读成员？

Question

I have code like this:

class A {
  private readonly something: B

  constructor() {
    this.setup();
    // another setup codes ...  
  }

  private setup():void {
    this.something = new B();
    // another config codes...
  }
}

But this will be result an error:
Cannot assign to 'something' because it is a read-only property.

Is there any alternative solution to setup readonly private members outside constructor ?

Answer 1

You can assign the B property with a class field instead:

class B { }
class A {
  private readonly something = new B()

  constructor() {
    this.setup();
    // another setup codes ...  
  }

  private setup() {
    // another config codes...
  }
}

Answer 2

No you can't, that's the purpose of readonly. Here is the definition of readonly members and the source with more examples

Read-only members can be accessed outside the class, but their value cannot be changed. Since read-only members cannot be changed outside the class, they either need to be initialized at declaration or initialized inside the class constructor.

Source

Answer 3

The readonly properties must be initialized only once, and it must be always on the constructor.

You can check the official documentation regarding that matter here .

You might want to remove the readonly and add a setter , in that way, you only use the set function to change the property value:

class A {
  private _something: B;

  constructor() {
    this.setup();
    // another setup codes ...  
  }

  set something(value: B) {
        this._something = value;
  }

  private setup():void {
    // This setup a new value and makes sure that not other piece of code 
    // changes it. Only through the setter will work
    this.something(new B());
  } 
}