How can i loop through a nested object, using the for in loop and return each property string concatenated?

You don't need a loop, just concatenate the properties.

var fullname = person.name.first + ' ' + person.name.middle + ' ' + person.name.last;

Using a for-in loop would be a bad idea, because objects aren't guaranteed to maintain their order. So you might end up with Doe John P. instead.

these type of questions have been posted millions of times, do some research before asking.

anyway:

alert(person.name.first + ' ' + person.name.middle + ' ' + person.name.last);

you can use object.reduce for this

check this snippet

var person = {
  name: {
    first: 'John',
    middle: 'P',
    last: 'Doe'
  },
  age: 35,
  homeTown: 'Nashville, TN'
};


var nameObject = person.name;

var fullname = Object.keys(nameObject).reduce(function(previous, key) {
  return previous +" "+ nameObject[key];
}, "");

console.log(fullname);

Hope it helps

You could use an array for the wanted property names (this keeps the order) and map the values and join it to a space separated string.

var  person = { name: { first: 'John', middle: 'P', last: 'Doe' }, age: 35, homeTown: 'Nashville, TN' };

console.log(['first', 'middle', 'last'].map(function (k) {
    return person.name[k];
}).join(' '));

You can use destructuring assignment

var  person = {
    name: {
        first: 'John',
        middle: 'P',
        last: 'Doe'
    },
    age: 35,
    homeTown: 'Nashville, TN'
};

var {first, middle, last} = person.name;

var fullname = `${first} ${middle} ${last}`;

console.log(fullname);

As Barmar's answer suggests, your example only needs a simple concatenation to give you your results.

However, in the more general case, you would want to iterate through each property of an object, and if the property is an object, iterate through that object as well.

For instance:

function iterateThroughAllProperties(obj) {
    Object.keys(obj).forEach(function(key, index) {
        if(typeof obj[key] !== null && typeof obj[key] === 'object') {
            iterateThroughAllProperties(obj[key]);
        }
        else {
            // Do something with the property.
            console.log(obj[key]);
        }
    });
}