Javascript 如何为ES6类属性提供默认值？

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。然而，我不知道如何实施这一点。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;
  }
}

以下是我的默认值：

// 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: '',
};

// 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;
  }
}

---

我意识到通过函数可以实现这一点，但我宁愿提供一个包含默认值的对象。

实现这一点的典型方法是使用

object.assign（）

将传入的值与默认值合并：

// 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: '',
};

// 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;
  }
}

要在此处解释

Object.assign（）

的工作原理，请执行以下操作：

它以

{}

作为目标（空对象）开始

然后将所有默认值复制到空对象中

然后它将所有传入的属性复制到同一个目标中

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对象中的名称相同，您可以这样做：

// 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];
    });
  }
}

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: '',
    };      

    this.term = opts.terms || defaults.terms;
    this.country = opts.country || defaults.country;
    this.media = opts.media || defaults.media;
    this.entity = opts.entity || defaults.entity;
    this.attribute = opts.attribute || defaults.attribute;
    this.callback = opts.callback || defaults.callback;
    this.limit = opts.limit || defaults.limit;
    this.lang = opts.lang || defaults.lang;
    this.version = opts.version || defaults.version;
    this.explicit = opts.explicit || defaults.explicit;
    this.url = opts.url || defaults.url;
  }
}

---

你可以这样做：

// 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];
    });
  }
}

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: '',
    };      

    this.term = opts.terms || defaults.terms;
    this.country = opts.country || defaults.country;
    this.media = opts.media || defaults.media;
    this.entity = opts.entity || defaults.entity;
    this.attribute = opts.attribute || defaults.attribute;
    this.callback = opts.callback || defaults.callback;
    this.limit = opts.limit || defaults.limit;
    this.lang = opts.lang || defaults.lang;
    this.version = opts.version || defaults.version;
    this.explicit = opts.explicit || defaults.explicit;
    this.url = opts.url || defaults.url;
  }
}

---

但您必须警惕“虚假”值，例如，如果

opts.limit

作为

0

传入，则

此.limit

将设置为

defaults.limit

值，即使opt已定义。

对于比上述示例更现代的内容，您可以使用解构和默认参数

class iTunesClient {
  constructor({
    term = '',
    country = 'US',
    media = 'all',
    entity = '',
    attribute = '',
    callback = '',
    limit = 50,
    lang = 'en-us,
    version = 2,
    explicit = 'yes',
    url = '',
  }) {
    this.term = terms;
    this.country = country;
    this.media = media;
    this.entity = entity;
    this.attribute = attribute;
    this.callback = callback;
    this.limit = limit;
    this.lang = lang;
    this.version = version;
    this.explicit = explicit;
    this.url = url;
  }
}

---

这样，任何不属于构造函数对象的参数都将设置为默认值。您甚至可以传递一个空对象，只需获取所有默认值。

我认为更好的解决方案

---

我认为，如果您只使用

|

阅读起来更清晰，问题代码有什么问题？通常，您会使用

Object.assign（）

将默认选项与传入的选项合并到一个新对象中，然后使用该对象分配所有值。这是否回答了您的问题？请标记为重复，而不是链接/解释现有答案。