I have looked up other posts but I cannot seem to get their solutions to work. I have this union type and this function that uses the object[] from that union type, but it throws errors saying length doesn't exists because both my types User and Query are not an array of objects.

type Results = object[] | User | Query;

class Pagination implements IPagination
{
    totalPages: any[];
    readonly itemsPerPage = 5;
    
    public async Paginate(items:Results, page: number):Promise<Page>
    {
        const pages = Math.ceil(items.length / this.itemsPerPage); //<- error here

        const paginatedItems = Array.from({ length: pages }, (_, index) => 
        {
            const start = index *  this.itemsPerPage;
            return items.slice(start, start    this.itemsPerPage); //<- And here because slice is also an array method.
        });

        this.totalPages = [...paginatedItems];
        return { totalPages: this.totalPages, currentPageRows: this.totalPages[page], itemsPerPage: this.itemsPerPage };
    }
}

CodePudding user response：

You can carry out checks to ensure that the type of an object is correct before you do any type-specific operations.

More generally, you shouldn't use a union of types in a place where you actually want a very specific type and have code that relies on a certain type being passed.

Something like the following can be used at the top of the method and will prevent invalid types from being used, but a better approach is to ensure that your method signature only accepts types which it can actually use.

if (!Array.isArray(items)) {
  return;
}