TS8020

error TS8020: JSDoc types can only be used inside documentation comments.

Broken Code ❌

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function add(a: number, b: number, c: number?): number {
  return a + b;
}

Fixed Code ✔️

If you wanted to make c optional:

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function add(a: number, b: number, c?: number): number {
  return a + b;
}

If you wanted to document c with JSDoc:

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/**
 * @param a Initial quantity
 * @param b Amount to add
 * @param [c] Optional number to add
 */
function add(a: number, b: number, c: number): number {
  return a + b;
}