React Typescript map loop interface error

Question

I am trying to interface a map loop in JavaScript, what I have done is:

interface RoutesType {
  path: string;
  name: string;
  icon: string;
  layout: string;
}

the code of the map loop is:

// creates the links that appear in the left menu / Sidebar
  const createLinks = (routes: object[]) => {
    return routes.map((prop: RoutesType, key: number) => {
      return (
        <NavItem key={key}>
          <NavLink to={prop.layout + prop.path} tag={NavLinkRRD} onClick={closeCollapse} activeClassName="active">
            <i className={prop.icon} />
            {prop.name}
          </NavLink>
        </NavItem>
      );
    });
  };

The error I cannot understand is the following

TypeScript error: Argument of type '(prop: RoutesType, key: number) => Element' is not assignable to parameter of type '(value: object, index: number, array: object[]) => Element'.
  Types of parameters 'prop' and 'value' are incompatible.
    Type '{}' is missing the following properties from type 'RoutesType': path, name, icon, layout

Could you help me understand this issue? Thanks, F.

Dacre Denny · Accepted Answer · 2019-10-02 22:48:55Z

The reason for this error is that the routes argument of type object[] cannot be cast to a type of RoutesType[].

The reason that TypeScript is attempting this type-cast is that type of routes is inferred and expected to be RoutesType[], due to the type of the prop argument on the map callback.

A fix would be to revise your createLinks function signature like this:

 /* Update routes to RoutesType[] */
 const createLinks = (routes: RoutesType[]) => {
    return routes.map((prop: RoutesType, key: number) => {
      return (
        <NavItem key={key}>
          <NavLink to={prop.layout + prop.path} tag={NavLinkRRD} 
                   onClick={closeCollapse} activeClassName="active">
            <i className={prop.icon} />
            {prop.name}
          </NavLink>
        </NavItem>
      );
    });
  };

This change will require that all calls to createLink throughout your project specficy a routes value that is strictly typed to RoutesType[]

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