Chapter 8. Function Prototype Property
Conceptual Overview of the Prototype Chain
The prototype property is an
object created by JavaScript for every Function() instance. Specifically, it links
object instances created with the new
keyword back to the constructor function that created them. This is done
so that instances can share, or inherit, common methods and properties.
Importantly, the sharing occurs during property lookup. Remember from
Chapter 1 that every time you look up or
access a property on an object, the property will be searched for on the
object as well as the prototype chain.
Note
A prototype object is created for every function, regardless of whether you intend to use that function as a constructor.
Below, I construct an array from the Array() constructor, and then I invoke the
join() method.
<!DOCTYPE html><html lang="en"><body><script>
var myArray = new Array('foo', 'bar');
console.log(myArray.join()); // logs 'foo, bar'
</script></body></html>The join() method is not defined
as a property of the myArray object
instance, but somehow we have access to join() as if it were. This method is defined
somewhere, but where? Well, it is defined as a property of the Array() constructor’s prototype property. Since join() is not found within the array object
instance, JavaScript looks up the prototype chain for a method called
join().
Okay, so why are things done this way? Really, it is about efficiency and reuse. Why should every array instance created from the array ...
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