Argument of type '(priority1: number, priority2: number) => number' is not assignable to parameter of type '(a: unknown, b: unknown) => number'

I am receiving the below type error ( TypeScript - 3.7.5 ).

error TS2345: Argument of type '(priority1: number, priority2: number) => number' is not assignable to parameter of type '(a: unknown, b: unknown) => number'. Types of parameters 'priority1' and 'a' are incompatible. Type 'unknown' is not assignable to type 'number'.

Code:

public updatePriorities() {
  const priorities = this.fetchedData.map((id: IList) => id.priority);
  const uniquePriorities = [...new Set(priorities)];
  uniquePriorities.sort((priority1: number, priority2: number) => priority1 - priority2);
  const updatedPriorities = uniquePriorities.map((priority: number, index: number) => {
    return index + 1;
  });

  uniquePriorities.forEach((id: number, index: number) => {
    this.fetchedData.forEach((id1: IList) => {
      if (id1.priority === id) {
        id1.priority = updatedPriorities[index];
      }
    });
  });
}