some this reference for js

reference.js

function GroceryList(items){
  this.items = items;
  this.mapItems = (fn) => {
    let newItems = [];
    // could use map but it would not fit for this example
    for (let i = 0; i < this.items.length ; i++){
      newItems = [...newItems, fn(this.items[i])];
    }
    this.items = newItems;
    return this;
  }
  return this;
}