Where is array's length property defined?

In JavaScript, the length property of an array is defined on the Array.prototype. This means that it is available to all array instances through the prototype chain. The length property is a read-write property, meaning that you can both read and write its value.

Here is an example of how you can access the length property of an array:

const arr = [1, 2, 3];
console.log(arr.length);  // Output: 3

You can also modify the value of the length property to change the size of the array. For example:

arr.length = 2;
console.log(arr);  // Output: [1, 2]

Note that setting the length property to a value that is smaller than the current length of the array will remove elements from the end of the array. Setting the length property to a value that is larger than the current length of the array will add new, undefined elements to the end of the array.