Setting default values for class members in Typescript is important for ensuring consistent behavior and

Question

Setting default values for class members in Typescript is important for ensuring consistent behavior and

My model is pretty straightforward:

export class Profile extends ServerData {
  name: string;
  email: string;
  age: number;
}

Whenever I make a server call using Angular 4's $http, I consistently receive this response:

{
  name: string;
  email: string;
}

The property age is missing.

Is there a way to incorporate my model and set a default age if it's not included? I'd rather avoid creating separate models if possible.

I don't want to make the age an optional property - I require it, even if it means having an incorrect default.

UPDATE:

This is the request I'm sending to the server:

results-manager.component.ts:

this.resultsManagerService.getResults(this.config.request.action, this.pagingAndSorting, this.args).subscribe(
  results => {
    this.results = this.results.concat(results as Profile[]);

results-manager.service.ts:

getResults(action: string, pagingAndSorting: PagingAndSorting, args: string[]): Observable<Profile[]> {
return this.apiService.callServer(
  pagingAndSorting,
  action,
  ...args );

}

The request goes through successfully and I get the response, but any default values I define (as suggested by @msanford's answer) are removed when I receive the response in the component. Similarly, adding a constructor to the model (like Mike Tung's answer) doesn't work either.

It seems like the backend response completely overrides the model instead of just assigning the values.

How can I ensure that it only assigns the values to the model without removing any existing ones?

angular typescript

Answer 1

Answer №1

Absolutely, without a doubt, you can achieve this without the need to incorporate a class constructor.

export class Profile extends ServerData {
  name: string;
  email: string;
  age: number = 0;
}

The feature that sets a class apart from an interface is the capability to define default values.

To make this function properly, you must include new Profile() in your code somewhere; otherwise, a class instance won't be generated and defaults won’t be established. This is because the TypeScript above will translate into the following JavaScript:

var Profile = /** @class */ (function () {
    function Profile() {
        this.age = 0;
    }
    return Profile;
}());

Hence, just utilizing it for type assertion during compilation isn't enough to assign a default value at runtime.

Witness it in action on the TypeScript Playground.

Answer 2

Absolutely, without a doubt, you can achieve this without the need to incorporate a class constructor.

export class Profile extends ServerData {
  name: string;
  email: string;
  age: number = 0;
}

The feature that sets a class apart from an interface is the capability to define default values.

To make this function properly, you must include new Profile() in your code somewhere; otherwise, a class instance won't be generated and defaults won’t be established. This is because the TypeScript above will translate into the following JavaScript:

var Profile = /** @class */ (function () {
    function Profile() {
        this.age = 0;
    }
    return Profile;
}());

Hence, just utilizing it for type assertion during compilation isn't enough to assign a default value at runtime.

Witness it in action on the TypeScript Playground.

Answer 3

Answer №2

The issue of properties being overwritten is a result of TypeScript's type erasure. During runtime, TypeScript cannot determine the types of objects assigned to its variables, which may be confusing if you are familiar with languages like Java or C#.

This is because ultimately, TypeScript compiles down to JavaScript, and JavaScript does not enforce strict type checking.

To ensure that profile objects always have an age property, one approach is to create custom objects and then copy values from the response. This is a common strategy for mapping wire format data to domain objects.

Firstly, define your domain model by creating a Profile class with a default age property:

 export class Profile {
        constructor(
            public name: string,
            public email: string,
            public age: number = 0) { }
    }

Next, map the response data to instances of the domain model:

this.resultsManagerService.getResults(this.config.request.action, this.pagingAndSorting, this.args).subscribe(
  results => {
    let res = (results as Profile[]).map((profile) => new Profile(profile.name, profile.email, profile.age));
    this.results = this.results.concat(res);
 });

Answer 4

The issue of properties being overwritten is a result of TypeScript's type erasure. During runtime, TypeScript cannot determine the types of objects assigned to its variables, which may be confusing if you are familiar with languages like Java or C#.

This is because ultimately, TypeScript compiles down to JavaScript, and JavaScript does not enforce strict type checking.

To ensure that profile objects always have an age property, one approach is to create custom objects and then copy values from the response. This is a common strategy for mapping wire format data to domain objects.

Firstly, define your domain model by creating a Profile class with a default age property:

 export class Profile {
        constructor(
            public name: string,
            public email: string,
            public age: number = 0) { }
    }

Next, map the response data to instances of the domain model:

this.resultsManagerService.getResults(this.config.request.action, this.pagingAndSorting, this.args).subscribe(
  results => {
    let res = (results as Profile[]).map((profile) => new Profile(profile.name, profile.email, profile.age));
    this.results = this.results.concat(res);
 });

Answer 5

Answer №3

All you have to do is utilize a constructor in your code.

export class Details extends DataHandler {
  title: string;
  description: string;
  price: number;

  constructor(title: string = '', description: string = '', price: number = null) {
    this.title = title;
    this.description = description;
    this.price = price;
  }
}

By using this constructor, default parameters will be set for any missing values.

Answer 6

All you have to do is utilize a constructor in your code.

export class Details extends DataHandler {
  title: string;
  description: string;
  price: number;

  constructor(title: string = '', description: string = '', price: number = null) {
    this.title = title;
    this.description = description;
    this.price = price;
  }
}

By using this constructor, default parameters will be set for any missing values.

Answer 7

Answer №4

It's not possible to automate this process, but you can establish defaults for a function (such as the class constructor):

function createProfile({ name = '', email = '', age = 0 }) {
  const individual = new Profile;
  individual.name = name;
  individual.email = email;
  individual.age = age;
  return individual;
}

So when you use

createPerson({ name: 'name', email: 'email' })

, it will generate a Person object with age = 0.

Answer 8

It's not possible to automate this process, but you can establish defaults for a function (such as the class constructor):

function createProfile({ name = '', email = '', age = 0 }) {
  const individual = new Profile;
  individual.name = name;
  individual.email = email;
  individual.age = age;
  return individual;
}

So when you use

createPerson({ name: 'name', email: 'email' })

, it will generate a Person object with age = 0.

Answer 9

Answer №5

export class Cell {
    constructor(
        public title: string = "",
        public rows: number = 1,
        public cols: number = 1
    ){ }
}

export class Desk {
    public cell1 = new Cell()
    public cell2 = new Cell()
    public cell3 = new Cell()
    
}

Answer 10

export class Cell {
    constructor(
        public title: string = "",
        public rows: number = 1,
        public cols: number = 1
    ){ }
}

export class Desk {
    public cell1 = new Cell()
    public cell2 = new Cell()
    public cell3 = new Cell()
    
}

Setting default values for class members in Typescript is important for ensuring consistent behavior and

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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