After applying the map function in Typescript, the resulting values may vary in their

I work with three different interfaces

interface A {
  productId: string;
  productName?: string;
  quantity: number;
}
interface B {
  productId: string;
  productName?: string;
  company: string;
}

interface Product {
  id: string;
  name: string;

In my data structure, I have arrays defined for each interface. For example, here is an array for the Product interface:

[{productId: '1', productName: 'pen', productId: '2', productName: 'paper'}]

The goal is to iterate through the arrays of type A and B, filter to locate the corresponding product in the Product array, and append it to each item. Here's the function attempted:

const addProductNameToLineItem = (
  products: Product[],
  items: A[] | B[]
): A[] | B[] => {
  return items.map((item) => {
    const productInfo = products?.find(
      (product: Product) => product.id === item?.productId
    );
    return {
      ...item,
      productName: productInfo?.name
    };
  });
};

Currently encountering a type error where the result is not assignable to type A[] | B[]. An attempt was made using function overloading without success.

Upon splitting the function into duplicates expecting only type A or B items separately, the operation works as intended.

Answer №1

Ensuring your function is generic is crucial:

const updateLineItemWithProductName = <Item extends X | Y>(
  products: Product[],
  items: Item[]
): Item[] => {

By making the return type coherent with what you pass - for instance, providing X[] should result in receiving X[]. Previously, the function returned X[] | Y[] even when given X[], which was incorrect.

Interactive Example

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