Здравствуйте, Serginio1, Вы писали:
S>>Разве? Мне нравится TS, но конкретно в этом случае ошибки скорее всего не будет — ибо совместимость. Если только самому переопределить описание map, то да, будет нормальная проверка.
S> Подскажет, что параметры опциональны пометив как x? Сейчас проверю

Для

* Performs the specified action for each element in an array.
      * @param callbackfn  A function that accepts up to three arguments. forEach calls the callbackfn function one time for each element in the array.
      * @param thisArg  An object to which the this keyword can refer in the callbackfn function. If thisArg is omitted, undefined is used as the this value.
      */
    forEach(callbackfn: (value: T, index: number, array: T[]) => void, thisArg?: any): void;

Но вот для map куча перегрузок

  * Calls a defined callback function on each element of an array, and returns an array that contains the results.
      * @param callbackfn A function that accepts up to three arguments. The map method calls the callbackfn function one time for each element in the array.
      * @param thisArg An object to which the this keyword can refer in the callbackfn function. If thisArg is omitted, undefined is used as the this value.
      */
    map<U>(this: [T, T, T, T, T], callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): [U, U, U, U, U];
    /**
      * Calls a defined callback function on each element of an array, and returns an array that contains the results.
      * @param callbackfn A function that accepts up to three arguments. The map method calls the callbackfn function one time for each element in the array.
      * @param thisArg An object to which the this keyword can refer in the callbackfn function. If thisArg is omitted, undefined is used as the this value.
      */
    map<U>(this: [T, T, T, T], callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): [U, U, U, U];
    /**
      * Calls a defined callback function on each element of an array, and returns an array that contains the results.
      * @param callbackfn A function that accepts up to three arguments. The map method calls the callbackfn function one time for each element in the array.
      * @param thisArg An object to which the this keyword can refer in the callbackfn function. If thisArg is omitted, undefined is used as the this value.
      */
    map<U>(this: [T, T, T], callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): [U, U, U];
    /**
      * Calls a defined callback function on each element of an array, and returns an array that contains the results.
      * @param callbackfn A function that accepts up to three arguments. The map method calls the callbackfn function one time for each element in the array.
      * @param thisArg An object to which the this keyword can refer in the callbackfn function. If thisArg is omitted, undefined is used as the this value.
      */
    map<U>(this: [T, T], callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): [U, U];
    /**
      * Calls a defined callback function on each element of an array, and returns an array that contains the results.
      * @param callbackfn A function that accepts up to three arguments. The map method calls the callbackfn function one time for each element in the array.
      * @param thisArg An object to which the this keyword can refer in the callbackfn function. If thisArg is omitted, undefined is used as the this value.
      */
    map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): U[];
    /**

То есть для калбеков мы можем опускать параметры если их не используем