noImplicitOverride

noImplicitOverride là compiler option bắt buộc dùng override keyword khi override method.

• Mặc định: false
• Phiên bản thêm vào: 4.3

Giải thích

Bắt buộc phải viết keyword override trước method khi subclass override method của superclass. Điều này giúp phát hiện khi method đang được override bị xóa hoặc đổi tên trong superclass.

Ví dụ, giả sử có class của toggle button (button click để bật tắt lặp lại) như sau:

ts
class ToggleButton {
  protected _active: boolean;
 
  public constructor() {
    this._active = false;
  }
 
  public get active(): boolean {
    return this._active;
  }
 
  public enable(): void {
    this._active = true;
  }
 
  public disable(): void {
    this._active = false;
  }
 
  public push(): void {
    if (this._active) {
      this.disable();
      // ...
      return;
    }
    this.enable();
    // ...
  }
}

ts
class ToggleButton {
  protected _active: boolean;
 
  public constructor() {
    this._active = false;
  }
 
  public get active(): boolean {
    return this._active;
  }
 
  public enable(): void {
    this._active = true;
  }
 
  public disable(): void {
    this._active = false;
  }
 
  public push(): void {
    if (this._active) {
      this.disable();
      // ...
      return;
    }
    this.enable();
    // ...
  }
}

Giờ xét subclass ToggleCountButton có thể đếm số lần đã toggle on/off. ToggleCountButton sẽ như sau:

ts
class ToggleCountButton extends ToggleButton {
  private _counter: number;
 
  public constructor() {
    super();
    this._counter = 0;
  }
 
  public enable(): void {
    this._counter++;
    this._active = true;
  }
 
  public disable(): void {
    this._counter++;
    this._active = false;
  }
 
  public get counter(): number {
    return this._counter;
  }
}

ts
class ToggleCountButton extends ToggleButton {
  private _counter: number;
 
  public constructor() {
    super();
    this._counter = 0;
  }
 
  public enable(): void {
    this._counter++;
    this._active = true;
  }
 
  public disable(): void {
    this._counter++;
    this._active = false;
  }
 
  public get counter(): number {
    return this._counter;
  }
}

Giờ giả sử superclass ToggleButton được thay đổi thành "Không cần hai method để toggle on/off! Setter là đủ":

ts
class ToggleButton {
  protected _active: boolean;
 
  public constructor() {
    this._active = false;
  }
 
  public get active(): boolean {
    return this._active;
  }
 
  public set active(active: boolean) {
    this._active = active;
  }
 
  public push(): void {
    this._active = !this._active;
    // ...
  }
}

ts
class ToggleButton {
  protected _active: boolean;
 
  public constructor() {
    this._active = false;
  }
 
  public get active(): boolean {
    return this._active;
  }
 
  public set active(active: boolean) {
    this._active = active;
  }
 
  public push(): void {
    this._active = !this._active;
    // ...
  }
}

Khi đó các method enable(), disable() vốn được override trong subclass sẽ trở thành method vô nghĩa.

noImplicitOverride yêu cầu thêm keyword override vào method đang override để kiểm tra xem có method cùng tên trong superclass hay không. Khi có keyword override nhưng không tồn tại method để override sẽ báo lỗi như sau:

ts
class ToggleCountButton extends ToggleButton {
  private _counter: number;
 
  public constructor() {
    super();
    this._counter = 0;
  }
 
  public override enable(): void {
This member cannot have an 'override' modifier because it is not declared in the base class 'ToggleButton'.4113This member cannot have an 'override' modifier because it is not declared in the base class 'ToggleButton'.
    this._counter++;
    this._active = true;
  }
 
  public override disable(): void {
This member cannot have an 'override' modifier because it is not declared in the base class 'ToggleButton'.4113This member cannot have an 'override' modifier because it is not declared in the base class 'ToggleButton'.
    this._counter++;
    this._active = false;
  }
 
  public get counter(): number {
    return this._counter;
  }
}

ts
class ToggleCountButton extends ToggleButton {
  private _counter: number;
 
  public constructor() {
    super();
    this._counter = 0;
  }
 
  public override enable(): void {
This member cannot have an 'override' modifier because it is not declared in the base class 'ToggleButton'.4113This member cannot have an 'override' modifier because it is not declared in the base class 'ToggleButton'.
    this._counter++;
    this._active = true;
  }
 
  public override disable(): void {
This member cannot have an 'override' modifier because it is not declared in the base class 'ToggleButton'.4113This member cannot have an 'override' modifier because it is not declared in the base class 'ToggleButton'.
    this._counter++;
    this._active = false;
  }
 
  public get counter(): number {
    return this._counter;
  }
}