So say we had code like this:

  const safeSplice = (items, start, deleteCount, ...itemsToAdd) => {
    const copy = [...items];
    const deleted = copy.splice(start, deleteCount, ...itemsToAdd);
    return {
      deleted,
      list: copy,
    };
  };

We could put this as a jsdoc-style typescript comment just before the `const safeSplice` line:

  /** @type {<Item>(items: Array<Item>, start: number, deleteCount?: number, itemsToAdd?: Array<Item>) => { deleted: Array<Item>, list: Array<Item> }} */

or as a multi-line type declaration:

  /** @type {<Item>(
    *   items: Array<Item>,
    *   start: number,
    *   deleteCount?: number,
    *   itemsToAdd?: Array<Item>
    * ) => {
    *   deleted: Array<Item>,
    *   list: Array<Item>
    * }}
    */

Alternatively, you could do something like this in place of the above:

  /*
   * @template Item
   * @param {Array<Item>} items
   * @param {number} start
   * @param {number} [deleteCount]
   * @param {Array<Item>} itemsToAdd
   */

I started out doing the former because I liked using the typescript notation, but I ended up doing the latter more often as I could leave the return type off the signature, which I prefer to do