Typescript generic function. How to use with other types?

Question

typescript

/** 
Example array
*/
arr = [
  {
    name: 'John',
    author: 'Bender',
    year: 2003,
    text: [
        'Lorem ipsum',
        'Lorem ipsum',
        'Lorem ipsum'
      ]
  }
]

/** 
Example generic function

This function should sort array
*/

function sortArray<T extends [], K extends keyof T>  (originArr: T, propertyName: K, cb: (a: T) => T): T {
    
    let arr: T = JSON.parse(JSON.stringify(originArr));  //deep copy array
    let resultSort: T = arr;
    
    resultSort = arr.sort((a: K, b: K) => {

        let nameA = a[propertyName].toLowerCase(), nameB = b[propertyName].toLowerCase();

        if (nameA < nameB)
            return -1;
        if (nameA > nameB)
            return 1;
        return 0;
    });

    return cb(resultSort);
}

At the moment I only see the problem in the line:

let nameA = a[propertyName].toLowerCase(), nameB = b[propertyName].toLowerCase();

a[propertyName] : Type 'K' cannot be used to index type 'K'.
b[propertyName] : Type 'K' cannot be used to index type 'K'.

Screen of IDE

Answer 1

No, there are a lot of problems here.
T extends [], K extends keyof T- Kthere is something derived from number, because the keys (keyof) of the array ([]) are only numbers.
let resultSort: T = arr;- meaningless, since it arr.sortchanges the original array anyway.
a[propertyName].toLowerCase()- will fall if propertyNameit is year, because Number does not have a toLowerCase.
...
As a result, the function should look something like this:

function sortArray<T extends {[key: string]: unknown}, K extends keyof T>  (originArr: T[], propertyName: K, cb: (a: T[]) => T[]): T[] {
  let arr: T[] = JSON.parse(JSON.stringify(originArr));  //deep copy array

  arr.sort((a, b) => {
    let nameA = String(a[propertyName]).toLowerCase(),
        nameB = String(b[propertyName]).toLowerCase();

    if (nameA < nameB)
      return -1;
    if (nameA > nameB)
      return 1;
    return 0;
  });

  return cb(arr);
}

You can not cast strictly to String, but then instead unknownyou need to specify the specific types with which you are supposed to work, and take them into account in the sorting function itself.

Answer 2

Instead of T extends [] there should be T extends T[]
And throughout the code, the array is not T, but T[]