如何为ES6类属性提供默认值？

Question

我有一个JavaScript类,我想使用对象提供默认值.如果没有为某些值提供用户输入,我只希望默认值成为类的一部分.但是,我不知道如何实现这一点.这是我的班级:

// Class definition, properties, and methods
class iTunesClient {
  constructor(options) {
    this.term = options.terms;
    this.country = options.country;
    this.media = options.media;
    this.entity = options.entity;
    this.attribute = options.attribute;
    this.callback = options.callback;
    this.limit = options.limit;
    this.lang = options.lang;
    this.version = options.version;
    this.explicit = options.explicit;
    this.url = options.url;
  }
}

这是我的默认值:

// Default values defined according to iTunes API
const defaults = {
  terms: 'default',
  country: 'US',
  media: 'all',
  entity: '',
  attribute: '',
  callback: '',
  limit: 50,
  lang: 'en-us',
  version: 2,
  explicit: 'yes',
  url: '',
};

我意识到这可以通过函数的默认参数来实现,但我宁愿提供一个包含默认值的对象.

Answer 1

执行此操作的典型方法是使用Object.assign()将传入值与默认值合并:

// Class definition, properties, and methods
class iTunesClient {
  constructor(options) {

    // Default values defined according to iTunes API
    const defaults = {
      terms: 'default',
      country: 'US',
      media: 'all',
      entity: '',
      attribute: '',
      callback: '',
      limit: 50,
      lang: 'en-us',
      version: 2,
      explicit: 'yes',
      url: '',
    };      

    let opts = Object.assign({}, defaults, options);
    this.term = opts.terms;
    this.country = opts.country;
    this.media = opts.media;
    this.entity = opts.entity;
    this.attribute = opts.attribute;
    this.callback = opts.callback;
    this.limit = opts.limit;
    this.lang = opts.lang;
    this.version = opts.version;
    this.explicit = opts.explicit;
    this.url = opts.url;
  }
}

解释如何Object.assign()在这里工作:

1. 它从{}作为目标(空对象)开始
2. 然后它将所有默认值复制到空对象中
3. 然后,它将所有传入的属性复制到同一目标中
4. 然后它返回用于初始化所有实例数据的目标对象

当然,如果您的实例属性名称与options对象中的属性名称相同,则可以采用更干燥的方式执行此操作,如下所示:

// Class definition, properties, and methods
class iTunesClient {
  constructor(options) {

    // Default values defined according to iTunes API
    const defaults = {
      terms: 'default',
      country: 'US',
      media: 'all',
      entity: '',
      attribute: '',
      callback: '',
      limit: 50,
      lang: 'en-us',
      version: 2,
      explicit: 'yes',
      url: '',
    };      

    let opts = Object.assign({}, defaults, options);

    // assign options to instance data (using only property names contained
    //  in defaults object to avoid copying properties we don't want)
    Object.keys(defaults).forEach(prop => {
        this[prop] = opts[prop];
    });
  }
}