Edit in JSFiddle

var oldArr = [{first_name:"Colin",last_name:"Toh"},{first_name:"Addy",last_name:"Osmani"},{first_name:"Yehuda",last_name:"Katz"}];

function getNewArr(){
    
    var newArr = [];
    
    for(var i= 0, l = oldArr.length; i< l; i++){
        var item = oldArr[i];
        item.full_name = [item.first_name,item.last_name].join(" ");
        newArr[i] = item;
    }
    
    return newArr;
}

console.log(getNewArr());