Private setter typescript?

87
votes

Is there a way to have a private setter for a property in TypeScript?

class Test
{
    private _prop: string;
    public get prop() : string
    {
        return this._prop;
    }

    private set prop(val: string)
    {
        //can put breakpoints here
        this._prop = val;
    }
}

Compiler complains that visibility for getter and setter don't match. I know I can just set the backing field, but but then I can't set breakpoints when the value is set.

I though about using an interface to hide the setter, but interfaces can only define a property, not whether it has a getter on setter.

Am I missing something here? There doesn't seem to be any reason to not allow private setters, the resulting JS doesn't enforce visibility anyway, and seems better that the current alternatives.

Am I missing something? If not is there a good reason for no private setters?

2
typescriptsettergetter-setteraccessor

2 Answers

82
votes

The TypeScript specification (8.4.3) says...

Accessors for the same member name must specify the same accessibility

So you have to choose a suitable alternative. Here are two options for you:

You can just not have a setter, which means only the Test class is able to set the property. You can place a breakpoint on the line this._prop =....

class Test
{
    private _prop: string;
    public get prop() : string
    {
        return this._prop;
    }

    doSomething() {
        this._prop = 'I can set it!';
    }
}

var test = new Test();

test._prop = 'I cannot!';

Probably the ideal way to ensure private access results in something akin to a "notify property changed" pattern can be implemented is to have a pair of private get/set property accessors, and a separate public get property accessor.

You still need to be cautious about someone later adding a direct call to the backing field. You could get creative in that area to try and make it less likely.

class Test
{
    private _nameBackingField: string;

    private get _name() : string
    {
        return this._nameBackingField;
    }

    private set _name(val: string)
    {
        this._nameBackingField = val;
        // other actions... notify the property has changed etc
    }

    public get name(): string {
        return this._name;
    }

    doSomething() {
        this._name += 'Additional Stuff';
    }
}
10
votes

I also hope we could have public getter and private setter. Until we do, another way to handle this is to add additional private getter and setter:

class Test {
  _prop: string;
  public get prop(): string {
    return this._prop;
  }

  private get internalProp(): string {
    return this.prop;
  }

  private set internalProp(value: string) {
    this._prop = value;
  }

  private addToProp(valueToAdd: string): void {
    this.internalProp += valueToAdd;
  }
}