How do I convert or cast `{}` to `T` and preserve default values In TypeScript?

Question

Say I have Product with three properties and their default values. How do I convert or cast {} to Product with respect to default value when there is no value in {}?

export class Product{
  Price:number=20;
  Label:string="No name";
  Details:string="no desc";
}
let laptop = {Label:'Laptop'} as Product;
//I would like to get this laptop ={Label:'Label',Price:20,Details:'no desc'}

Answer 1

This is not possible with type casting. All you're doing when you cast an object as Product is saying to the compiler, "This thing is a Product even though it doesn't have the same properties as a Product".

If you want default values, you need to put a constructor on your class, like this:

export class Product {
  price: number;
  label: string;
  details: string;

  constructor(obj: {price?: number, label?: string, details?: string}) {
    this.price = obj.price || 20;
    this.price = obj.label || "No name";
    this.details = obj.details || "No description";
  }
}

Then you can pass any partial configuration object and the other default values will get set.

let laptop = new Product({label: 'Laptop'}); 
// {label: 'Laptop', price: 20, details: 'No description'}

Now, laptop will automatically be of type Product and you don't even have to cast it.

Tip: you can use the Partial type to make typing your constructor parameter easier.

type Partial<T> = {
  [P in keyof T]?: T[P];
}

Then your constructor parameter would look like constructor(obj: Partial<Product>)

For more info on type assertions (aka type casting), read the 'Type Assertions' section of this article: https://www.typescriptlang.org/docs/handbook/basic-types.html.