如何在另一个类中实例化一个 JavaScript 类？

Question

I'm currently doing a school project, the project revolves around easy access to skills in an organisation. I have created a class called Employee, this class have a predefined set of employees stored in localStorage (because no access to databases). The list of employees is outputted via a function that creates a table of the list.

Now I want to create a class called Skills, I want the Skills class be instantiated in my Employee class. The purpose of this is that employees should be able to enter a site and write in their skills, the skills has to be stored to a employee and the saved in localStorage and the updated in the table. So my problem revolves around using the right syntax / method of instantiation a class into another class..

class Skills{
    constructor(sId){
        this.sId = sId;
    }
}
if (localStorage.getItem("Skills") == null) {
    var skillList =[];
    var skillListString = JSON.stringify(skillList);
    localStorage.setItem("skills",skillListString);
} else {
    var skillListString = JSON.parse(localStorage.getItem("Skills"));    

class Employee {
    // vi bruger en constructor funktion for at lave en opskrift på objekter af en bestemt type.
    //this metoden benyttes til at referere til det tilhørende objekt
    constructor(name, gender, department, yy, email, skills) {
        this.name = name;
        this.gender = gender;
        this.department = department;
        this.email = email;
        this.skills = [];

    }
}

//Employee Database "Localstorage"
if(localStorage.getItem("Employee") == null) {
    let employeeList = [];
    employeeList.push (new Employee("Simon", "Male", "HR", 1999, "2650@mail.dk"));
    employeeList.push (new Employee("Mads", "Male","IT", 1999,  "1234@email.com"));
    employeeList.push (new Employee("Jessica", "Female", "Sales",1998, "Mail2@mail.dk"));
    employeeList.push (new Employee("Benjamin", "Male","IT", 1997, "blabla@mail.dk"));

    if(localStorage.getItem("Employee") == null) {
        employeeListString = JSON.stringify(employeeList);
        localStorage.setItem("Employee", employeeListString);
        employeeList = JSON.parse(localStorage.getItem("Employee"));
    }
} else {
    employeeList = JSON.parse(localStorage.getItem("Employee"));
    document.querySelector("#employees").appendChild(buildTable(employeeList));
}

Answer 1

You instantiate a class from inside another class in the same way you always instantiate a class - with the keyword new .

Example:

class Foo {}

class Bar {
  constructor() {
    this.foo = new Foo()
  }
}

bar = new Bar()

In terms of design you may find it best to 'inject' your classes dependencies. This typically leads to more testable and maintainable code. clean-code-javascript has some nice examples.

Example:

class Foo {}

class Bar {
  constructor(foo) {
    this.foo = foo
  }
}

foo = new Foo()
bar = new Bar(foo)

Answer 2

You can definitely store instantiated classes as property:

// create instance in constructor
class Human {
  constructor(name, cat_name) {
    this.name = name
    this.cat = new Cat(cat_name)
  }
}

// or pass already constructed instance
class Human
  constructor(name, cat) {
    this.name = name
    this.cat = cat
  }
}

But there is less obvious problem: JSON doesn't have custom types, therefore it could only store plain JS objects, arrays, and a few primitives, like strings and numbers.

So custom class (and especially class stored in a property) would not survive default serialization/deserialization roundtrip. You cannot store references as easily as you can with plain JS objects. You'd have to do it yourself, for example, you could transform each class to {type:'human', fields: { name: 'John' }} object that could be safely serialized to JSON.

class Human {
  constructor(name, cat) {
    this.name = name
    this.cat = cat
  }
  serialize() {
    return { type: 'Human', fields: { cat: this.cat.serialize(), name: this.name }}
  }
}

And then deserialize according to type , invoking constructors as needed.