accessing nested objects with a for in loop

Question

Within javascript, I am trying to print the values from a few objects listed in an array in an object. The question has to do with drilling into objects/arrays with a for in loop. More specifically I'd like to avoid using a for loop if possible (I just learned for/in and am trying to understand it well)

The object looks like this:

var users = {
  'Students': [ 
    { first_name:  'Michael', last_name : 'Jordan'  },
    { first_name : 'John',    last_name : 'Rosales' },
    { first_name : 'Mark',    last_name : 'Guillen' },
    { first_name : 'KB',      last_name : 'Tonel'   }
  ],
  'Instructors': [
    { first_name : 'Michael', last_name : 'Jackson' },
    { first_name : 'Martin',  last_name : 'Puryear' }
  ]
};

How could i write a function that returns all of the names burried in this object? Id like to console log:

Students
1 - MICHAEL JORDAN
2 - JOHN ROSALES
3 - MARK GUILLEN
4 - KB TONEL
Instructors
1 - MICHAEL JACKSON
2 - MARTIN PURYEAR

Ive tried writing a couple for/in loops but I keep getting unexpected results. Thanks in advance for taking a look at this.

What sort of "couple for/in loops"? What manner of "unexpected results"? — slim
– slim, Commented Dec 20, 2016 at 17:05

slim · Accepted Answer · 2016-12-20 17:41:37Z

5

You can break a problem like this into smaller parts. First, how to we get at the individual elements of your map?

for (var key in users) {
   console.log(key);
}

logs:

students
teachers

So now we can get at the members of users one at a time, inside that loop:

var userArray = users[key];

That extracts an array each time. Now you can loop through the array, by adding this inside the loop we already have:

for(var i in userArray) {
    console.log(userArray[i]);
}

That prints out individual items from your list, e.g. {first_name: 'Michael', last_name : 'Jordan'}

Now you can print it in a different way.

 var item = userArray[i];
 console.log(i + ' ' + item[first_name] + ' ' + item[last_name]);

Put all these pieces together and you are very close to your goal (You just need to uppercase -- Google it!)

So we have everything we need to achieve in Javascript, what this pseudocode sums up:

for each type of user in "users"
   for each user record in the list
       print the index, firstname and lastname
   end
end

You can make this tidier by extracting the inner loop into a function of its own. You can do it in more advanced ways by using functional programming constructs like array.map() to translate one array into another. But the pieces above are good "beginners' way" to tackle the problem.

answered Dec 20, 2016 at 17:41

slim

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1 Comment

maftoun95 Over a year ago

This helped big time, thank you so much! I was tetering on the edge an answer and this response was the appropriate amount of hand holding to get me there.

Ori Drori · Accepted Answer · 2016-12-20 17:11:16Z

0

Use array#map to extract the names from the arrays:

function getNames(arr) {
  return arr.map(function(o, i) { // is the object, i is the index in the array
    return (i + 1) + ' - ' + o.first_name + ' ' + o.last_name;
  });
}

var users = {
 'Students': [ 
     {first_name:  'Michael', last_name : 'Jordan'},
     {first_name : 'John', last_name : 'Rosales'},
     {first_name : 'Mark', last_name : 'Guillen'},
     {first_name : 'KB', last_name : 'Tonel'}
  ],
 'Instructors': [
     {first_name : 'Michael', last_name : 'Jackson'},
     {first_name : 'Martin', last_name : 'Puryear'}
  ]
 };

var students = getNames(users.Students);

var instructors = getNames(users.Instructors);

console.log('Students\n', students);
console.log('Instructors', instructors);

answered Dec 20, 2016 at 17:11

Ori Drori

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4 Comments

slim Over a year ago

It doesn't help anyone to spoon-feed beginners like this.

Ori Drori Over a year ago

@slim - you are welcome to write your own non spoon feeding answer :)

slim Over a year ago

@OriDori too late now. OP isn't going to do any thinking now they have something to copy/paste. Nor do beginners tend to bother upvoting, sorry.

Ori Drori Over a year ago

@slim - a copy/paste user will always stay a copy/paste user no matter how hard learning you use on him. A learning user will try to understand my code, and will learn Array#map, and accessing object properties on the way.

Mr. Polywhirl · Accepted Answer · 2016-12-20 17:55:07Z

You can simply loop over your keys via:

for (var userGroup in users) { ... }

And you can get the list of a user's group via:

users[userGroup].forEach((item, index) => item);

var users = {
  'Students': [ 
    { first_name:  'Michael', last_name : 'Jordan'  },
    { first_name : 'John',    last_name : 'Rosales' },
    { first_name : 'Mark',    last_name : 'Guillen' },
    { first_name : 'KB',      last_name : 'Tonel'   }
  ],
  'Instructors': [
    { first_name : 'Michael', last_name : 'Jackson' },
    { first_name : 'Martin',  last_name : 'Puryear' }
  ]
};

createLists(users, document.body, function(user) {
  return (user.first_name + ' ' + user.last_name).toUpperCase();
});

function createLists(dataMap, target, formatFn) {
  for (var groupName in dataMap) {
    target.appendChild(createTextEl('H3', groupName));
    var listEl = document.createElement('OL');
    addListItems(dataMap[groupName], listEl, formatFn);
    target.appendChild(listEl);
  }
}
function addListItems(dataList, target, formatFn) {
  dataList.forEach((item, index) => target.appendChild(createTextEl('LI', formatFn(item))));
}
function createTextEl(tagName, text) {
  var el = document.createElement(tagName);
  el.appendChild(document.createTextNode(text));
  return el;
}

* { font-size: 0.95em; }

<link href="https://cdnjs.cloudflare.com/ajax/libs/normalize/5.0.0/normalize.min.css" rel="stylesheet"/>

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