I have an array of Product.

The Product class:

class Product {
    id: string;
    type: string;
    price: number;
    constructor(id: string, type: string, price: number) {
        this.id = id;
        this.type = type;
        this.price = price;
    }
}

I try to group the array of product by its type property by using the array reduce function:

const groupedProducts = productList.reduce((group, prod) => {
    const prodType = prod.type;
    group[prodType] = group[prodType] ?? [];
    group[prodType].push(prod);
    return group;
}, {});

Above code gets compiler error:

No index signature with a parameter of type 'string' was found on type '{}'.

I see that it is because of the initial value {} has no explicit type for the object index, so I refactored the initial value part from {} to {string: Product[]}. But it doesn't help.

The full code is here.

How can I get rid of this error & get the grouped products as a result?

CodePudding user response：

The correct approach would probably be to specify the generic type of reduce explicitly.

const groupedProducts = productList.reduce<Record<string, Product[]>>((group, prod) => {
    const prodType = prod.type;
    group[prodType] = group[prodType] ?? [];
    group[prodType].push(prod);
    return group;
}, {});

This correctly types the group parameter and the return type as Record<string, Product[]>.

Playground