Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

<p>Click the button to create an array, call the new ucase() method, and display the result.</p>

<button onclick="myFunction()">Try it</button>

<p id="demo"></p>

<script>
Array.prototype.myUcase = function() {
    var i;
    for (i = 0; i < this.length; i++) {
        this[i] = this[i].toUpperCase();
    }
};

function myFunction() {
    var fruits = ["Banana", "Orange", "Apple", "Mango"];
    fruits.myUcase();
    document.getElementById("demo").innerHTML = fruits;
}
</script>

</body>
</html>