Add a property to Array.prototype and then delete it.

gistfile1.js

if (!Array.prototype.hello) {
    
    Array.prototype.hello = function () {};
    
}

var arr = ['a', 'b', 'c'];

for (var k in arr) {
    console.log(k + ' : ' + arr[k]);
}

delete Array.prototype.hello;

console.log('=====After delete=========');

for (var j in arr) {
    console.log(j + ' : ' + arr[j]);
}