TypeScript遍历类对象属性赋值报错：类型'number'无法赋值给'()=>number'

问题代码 #

var roundedMachineData = new RoundedMachineData();

for (var prop in roundedMachineData) {
  roundedMachineData[prop as keyof RoundedMachineData] = 0;
}

export class RoundedMachineData {
  public constructor(init?: Partial<RoundedMachineData>) {
      Object.assign(this, init);
  }
  public running: number = NaN;
  public idle: number = NaN;
  public estop: number = NaN;
  public alarm: number = NaN;
  public operatorCall: number = NaN;
  public maintenance: number = NaN;
  public feedHold: number = NaN;
  public disconnected: number = NaN;
  public singleBlock: number = NaN;
}

运行上述代码时出现错误：Type 'number' is not assignable to type '() => number'，但RoundedMachineData类的所有属性均为number类型。

错误原因 #

for...in循环会遍历对象原型链上的所有可枚举属性，包括继承自Object原型的方法（比如toString、valueOf等）。这些方法的类型是函数（() => number或其他函数类型），当你尝试给它们赋值0（number类型）时，就会触发类型不匹配错误。

解决方法 #

方法1：用hasOwnProperty过滤原型链属性 #

在循环内添加判断，只处理实例自身的属性：

var roundedMachineData = new RoundedMachineData();

for (var prop in roundedMachineData) {
  // 仅处理实例自身的属性，排除原型链上的方法
  if (roundedMachineData.hasOwnProperty(prop)) {
    roundedMachineData[prop as keyof RoundedMachineData] = 0;
  }
}

export class RoundedMachineData {
  public constructor(init?: Partial<RoundedMachineData>) {
      Object.assign(this, init);
  }
  public running: number = NaN;
  public idle: number = NaN;
  public estop: number = NaN;
  public alarm: number = NaN;
  public operatorCall: number = NaN;
  public maintenance: number = NaN;
  public feedHold: number = NaN;
  public disconnected: number = NaN;
  public singleBlock: number = NaN;
}

方法2：用Object.keys遍历实例属性 #

Object.keys只会返回对象自身的可枚举属性名数组，直接遍历这个数组即可避开原型链问题：

var roundedMachineData = new RoundedMachineData();

// 仅遍历实例自身的属性键
Object.keys(roundedMachineData).forEach(prop => {
  roundedMachineData[prop as keyof RoundedMachineData] = 0;
});

export class RoundedMachineData {
  public constructor(init?: Partial<RoundedMachineData>) {
      Object.assign(this, init);
  }
  public running: number = NaN;
  public idle: number = NaN;
  public estop: number = NaN;
  public alarm: number = NaN;
  public operatorCall: number = NaN;
  public maintenance: number = NaN;
  public feedHold: number = NaN;
  public disconnected: number = NaN;
  public singleBlock: number = NaN;
}

内容的提问来源于stack exchange，提问作者Luke Cobbs