gistfile1.js

Array.prototype.sayHi = function () {
    var say = 'Hi, ' + this.join(', ') + '!';
    window.alert(say);
}

var a = ['Bob', 'Alice', 'Emma'];
a.sayHi(); // this will alert: Hi, Bob, Alice, Emma!

for (var i in a) {
    console.log(i, a.hasOwnProperty(i), a[i]);
}

/* console will show the following:
 * "0" true "Bob"
 * "1" true "Alice"
 * "2" true "Emma"
 * "sayHi" false function
 */