Why does this code use <function>.call()?

Question

Code is from mozilla website

function Product(name, price) {
  this.name = name;
  this.price = price;

  if (price < 0)
    throw RangeError('Cannot create product "' + name + '" with a negative price');
  return this;
}

function Food(name, price) {
   Product.call(this, name, price);/// why not this be Product(name,price)
   this.category = 'food';
}
Food.prototype = new Product();

may very silly thing, can't understand this line
Product.call(this, name, price);

Since both Product and Food are global functions why do we have to use Product.call

Answer 1

The Product function is a constructor, this would normally be called with new. Using new when calling a function creates a new empty object, and calls the function with the new object as the context, it also sets up the prototype chain.

In this example Food is sub-classing Product, it wants to run the Product constructor on the new instance of Food.

An example should make things clear:

var f = new Food ('apple', 10)

f.name = 'apple'
f.price = 10
f.category = 'food'

The name and price category is being added to the object by calling the Product constructor with the new instance of Food.