When trying to declare a variable, an error of 'implicitly has any type' may occur if the data type is not specified and

My interface has the capability to accept either a number or an array as a value. The array format always starts with a number followed by a string, for example:

[136237, "Name of something"]
.

The type is defined as follows:

interface SaleOrder {
  ...,
  partner_id?: [number, string] | number;
}

In this particular scenario, the property arrives in the form of an array.

In my code implementation, I have:

const { name, partner_id, id, amount_total } = saleOrder as SaleOrder;

const order = {
      "Name": name as string,
      "Customer Name": partner_id[1] as string,
      "Total Sale": amount_total as number
    }

However, even though it is valid to use '1' as an index in an array, VS Code displays a warning. Why is that?

Answer №1

One common error occurs when you fail to check whether the partner_id is an array or a number. The message '

Property '1' does not exist on type 'number | [number, string]'
' appears because a '1' property does not exist in the number type; it can only exist in an array.

Avoid unnecessary casting whenever possible. Even if you are certain that the object received from the API contains an array, TypeScript during compilation may not have such certainty. It is better to implement checks like in the example below:

interface SaleOrder {
  partner_id?: [number, string] | number;
}

const saleOrder: SaleOrder = {
  partner_id: [1, 'text']
};
const { partner_id } = saleOrder;

const order = {
  "Customer Name": Array.isArray(partner_id) ? partner_id[1] : 'unknown name', //Ensure to decide what should be returned as a fallback.
}

Playground: TypeScript Playground Link

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