Why doesn't the following prototype declaration work?

Question

I have declared Array prototype myArrayMax and the code seems right after repeated verifications from other resources on w3schools.com :

The code is for finding the maximum number in a given array.

Yet, it doesn't work, and the console shows the following error message:

Uncaught TypeError: points.myArrayMax is not a function

I can't find the problem in this prototype declaration

var points = [40, 100, 1, 5, 25, 10];
document.getElementById("demo").innerHTML = points.myArrayMax();

var len, max;

Array.prototype.myArrayMax = function() {
  len = this.length;
  max = -Infinity;
  while (len--) {
    if (this[len] > max) {
      max = this[len];
    }
  }
  return max;
}

<h2>JavaScript Array Sort</h2>

<p>The highest number is <span id="demo"></span>.</p>

Answer 1

Because you're attempting to use Array.myArrayMax before you actually define it.

var points = [40, 100, 1, 5, 25, 10];

var len, max;

Array.prototype.myArrayMax = function() {
  len = this.length;
  max = -Infinity;
  while (len--) {
    if (this[len] > max) {
      max = this[len];
    }
  }
  return max;
}

console.log(points.myArrayMax());