js definitive guide 6th -- ch8.3

gistfile1.js

function getIndexNames(o, a){ 
    var a = a || []; //if (a === undefined) a = [];
        
    for(var index in o){ 
        console.log("---"+ o[index]);
        a.push(index);
    };  

    return a;
}

var a = getIndexNames(["111"]);
console.log(a); //['0']

 
getIndexNames(["aaa", "bbb", "ccc"], a); 
console.log(a);//['0', '0', '1', '2']