Why is the removal of this type assertion in TypeScript causing an issue?

Why is TypeScript refusing to compile this code snippet?

interface TaggedProduct {
    tag: string;
}
interface Product {
    tag?: string;
}
const tagProduct = (product: Product): TaggedProduct => {
    const tag: string = "anything";
    product.tag = tag;

    // Type 'Product' is not assignable to type 'TaggedProduct'.
    // Property 'tag' is optional in type 'Product' but required in type 'TaggedProduct'.ts(2322)
    return product; // Won't compile
}

I find it confusing that adding a tag field to a Product doesn't automatically create a TaggedProduct. Is there a way around this without using a type assertion?

typescripttypescasting

Answer №1

To avoid a type assertion, you can utilize the Object.assign method like so:

const tagProduct = (product: Product): TaggedProduct => {
  const tag: string = "anything";
  return Object.assign(product, {tag});
}

The reason this works is because the Object.assign function is defined with the following types:

interface ObjectConstructor {
  assign<T, U>(target: T, source: U): T & U;
}

This means that the resulting type of Product & {tag: string} is compatible with the TaggedProduct type.

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