Array.length vs Array.prototype.length

Question

I found that both the Array Object and Array.prototype have the length property. I am confused on using the Array.length

Answer 1

Array.length provides the number of declared parameters in Array function definition. Since Array function definition has only one size parameter, hence it will return 1 irrespective of your array content.

Array.prototype.length provides the number of elements in array data. It is dependent on the array content.

var arr=new Array();
console.log(Array.length);//returns 1
console.log(arr.length);//returns 0 as array has 0 elements

Answer 2

Array.length is how many arguments the function Array() takes and Array.prototype.length is an instance method that gives you the length of your array. When you check ['foo'].length you're actually checking Array.prototype.length with the this argument being your array ['foo']

var myArray = ['a','b','c']
console.log(myArray.length); //logs 3

Answer 3

Array is a constructor function.

All functions have a length property that returns the number of declared parameters in the function definition.

Answer 4

If you have an instance of an Array, it inherits all properties from the Array.prototype thanks to Javascript's use of prototypical inheritance.

Take the following example:

function MyClass() {
  this.foo = Math.random();
}

MyClass.prototype.getFoo = function() {
  return this.foo;
}

// Get and log
var bar = new MyClass();
console.log(bar.getFoo());

This declares a function (serving as the constructor) for a class. That function provides the prototype for each instance of the class. When we assign a method (getFoo) to that prototype, every instance of the class will have the method.

You can then call the method on an instance and it will be applied to the data that class contains. In the case of arrays, the length property will get the length of the array you call it on:

[1, 2, 3, 4].length == 4; // Every array has a length

However, because functions behave largely like objects and may have their own properties, Array itself may have properties. That is what you see when using Array.length, which gets the number of parameters the Array (constructor) function expects to receive. Every function has a length property.