TypeScript Crash Course: Property Access Modifiers

There is no other great moment to head into the world of TypeScript instead of right now. Angular is in TypeScript, React is in TypeScript, and even Vue3 is in TypeScript. That means it's a skill we must equip with rather than wait and see.

This is the first post of my own TypeScript crash course, chances that it's your jam, stay tune;)

public, private, protected and readonly access modifier

• public the default access modifier for properties

• private lock the properties inside the cage, no one else except the class in where it's defined can access it. But we could approach it in JavaScript runtime, even by valueOf method.

• protected as the same as private, but open a backdoor for derived class.

• readonly the properties marked with readonly should be initialized either when declare in no time or inside constructor. But it only works in TypeScript.

  /* compile time */
  class User {
       readonly idCard: string
       constructor(idCard: string) {
           this.idCard = idCard
       }
  }
  
  let user = new User('123')
  user.idCard = '321' // error hint
  
  /* runtime */
  user.idCard = '321'
  console.log(user.idCard) // "321", the value of idCard has been changed.
  
  // solution for real readonly properties
  
  class User {
       readonly idCard: string
       constructor(idCard: string) {
           this.idCard = idCard
           Object.defineProperty(this, 'idCard', { writable: false })
       }
  }
  /* runtime */
  user.idCard = '321' // everything goes well
  console.log(user.idCard) // but it's still "123"
  

Define properties through constructor parameters

It's way too boring to put values into the properties when construct an instance like below

class User {
    private readonly idCard: string
    protected name: string
    age: number

    constructor(idCard: string, name: string, age: number) {
        this.idCard = idCard
        this.name = name
        this.age = age
    }
}

Fortunately, TypeScript has done that for us. When we specify public, private or other access modifiers on the constructor parameters, a corresponding property is created on the class and filled with the value of the parameter. So we could make the previous one much damn shorter like this.

class User {
    constructor(private readonly idCard: string, protected name: string, public age: number) {}
}

Pretty cool yet;)