Issue with Readonly modifier not functioning as expected in Angular/Typescript

Question

Issue with Readonly modifier not functioning as expected in Angular/Typescript

My goal is to create a component property that is read-only. However, I am facing an issue where the readonly modifier does not seem to have any effect.

View example on stackblitz

According to the documentation, once I initialize the cars property in the constructor() of AppComponent, it should not be modifiable.

What I have attempted:

Initially, I was able to modify the cars property in AppComponent's ngOnInit(). I assumed this might be allowed as the component is still under construction. To test further, I created a button that calls a function to modify the property and found that I could do so again. This led me to think that maybe the readonly modifier only applies when accessed by external classes. However, even in HelloComponent, I could still modify cars.

Am I misunderstanding how readonly works? How can I correctly implement it to make a property accessible publicly but non-modifiable (read-only) from outside sources?

angular typescript

Answer 1

Answer №1

Immutable The keyword "readonly" is used in interfaces or classes to designate a property as immutable.

class Employee {
    readonly empCode: number;
    empName: string;

    constructor(code: number, name: string)     {
        this.empCode = code;
        this.empName = name;
    }
}
let emp = new Employee(10, "John");
emp.empCode = 20; //Compiler Error
emp.empName = 'Bill'; //Compiler Error

It's important to note that an immutable property can only be assigned a value in the constructor or when initially declaring the property.

An immutable property cannot be reassigned once it has been set.

In the provided code example:

 modifyCars() {
    this.cars[0] = { id: -1, name: 'MODIFIED BY BUTTON' };
  }

Instead of reassigning values, you are mutating the array. To prevent array mutation, you can use the ReadOnly mapped type.

Replace

  readonly cars;

with

readonly cars: Readonly<{ id: number, name: string }[]>;

See full example here

Answer 2

Immutable The keyword "readonly" is used in interfaces or classes to designate a property as immutable.

class Employee {
    readonly empCode: number;
    empName: string;

    constructor(code: number, name: string)     {
        this.empCode = code;
        this.empName = name;
    }
}
let emp = new Employee(10, "John");
emp.empCode = 20; //Compiler Error
emp.empName = 'Bill'; //Compiler Error

It's important to note that an immutable property can only be assigned a value in the constructor or when initially declaring the property.

An immutable property cannot be reassigned once it has been set.

In the provided code example:

 modifyCars() {
    this.cars[0] = { id: -1, name: 'MODIFIED BY BUTTON' };
  }

Instead of reassigning values, you are mutating the array. To prevent array mutation, you can use the ReadOnly mapped type.

Replace

  readonly cars;

with

readonly cars: Readonly<{ id: number, name: string }[]>;

See full example here

Answer 3

Answer №2

One useful feature of TypeScript is the ReadonlyArray type, which functions just like an Array but prevents any changes from being made after creation.

 cars :ReadonlyArray<any>;

Forked Example: https://stackblitz.com/edit/angular-g8pmdr

Ref:https://www.typescriptlang.org/docs/handbook/interfaces.html#readonly-properties

Answer 4

One useful feature of TypeScript is the ReadonlyArray type, which functions just like an Array but prevents any changes from being made after creation.

 cars :ReadonlyArray<any>;

Forked Example: https://stackblitz.com/edit/angular-g8pmdr

Ref:https://www.typescriptlang.org/docs/handbook/interfaces.html#readonly-properties

Issue with Readonly modifier not functioning as expected in Angular/Typescript

Answer №1

Answer №2

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