Arrays and for..in

Something Like that:
Built-in Function Constructors

JS

Array.prototype.myCustomFeature = "Cool!";
var arr = ['John', 'Jane', 'Jim'];

for(var prop in arr){
    document.write(prop + ': ' + arr[prop] + '<br>');
}

[codepen_embed height=”265″ theme_id=”0″ slug_hash=”prKRjj” default_tab=”result” user=”pradeepanvi”]See the Pen Arrays and for..in by Pradeep Kumar (@pradeepanvi) on CodePen.[/codepen_embed]

Now you can see

0: John
1: Jane
2: Jim
myCustomFeature: Cool!