// Write a function called isUser that accepts an object, and returns true if it is

// a valid user object, false otherwise. A valid user object will contain a

// property called `name` which is a string, and a property called `screen_name`

// which is also a string. It should contain no other properties.

//

// isUser({ "name":"Semmy Purewal", "screen_name":"semmypurewal" });

// //=> true

//

// isUser({ "name":"Firstname Lastname", "screen_name":"user" });

// //=> true

//

// isUser("semmypurewal");

// //=> false

//

// isUser({ "age": 37, "name":"Semmy Purewal", "screen_name":"semmypurewal" });

// //=> false

//

var isUser = function () {

};

// Often when working with HTML, we'll want to take data stored in an object and

// display it as an HTML `div` element. For example, suppose we have a person

// object which contains a name and a screen_name. We'd like to produce a `div`

// that contains an `h1` element with the name, and an `h2` element with

// screen_name. For example:

//

// var user = { "name":"Semmy Purewal", "screen_name":"semmypurewal" };

//

// userToDiv(user);

// //=> "<div><h1>Semmy Purewal</h1><h2>semmypurewal</h2></div>"

//

// Write this function. It should throw an error if the user is not a valid

// user. It might be helpful to use some functions from previous sections.

//

var userToDiv = function () {

};

// Now suppose we have a user object that contains a list of tweets. In HTML, we

// may want to represent that as a list element with a set of list items. That

// looks something like this:

//

// <ul>

// <li>this is a tweet.</li>

// <li>this is another tweet.</li>

// </ul>

//

// Write a function that converts a user object containing tweets into an HTML

// `div` element representing that user.

//

// userWithTweetsToDiv({

// "name": "Semmy Purewal",

// "screen_name":"semmypurewal",

// "tweets": [

// "this is a tweet.",

// "this is another tweet!"

// ]

// });

// //=> "<div><h1>Semmy Purewal</h1><h2>semmypurewal</h2><ul><li>this is a tweet.</li><li>this is another tweet</li></ul></div>"

//

var userWithTweetsToDiv = function () {

};

// Write a function that accepts an array of strings, and returns an object that

// represents the number of times that each string appears in the array. This might

// sound confusing, but this is what we'd like to have happen.

//

// frequencies([ "hello", "world", "hello", "goodbye", "hello", "world", "thing" ]);

// //=> { "hello" : 3, "world" : 2, "goodbye": 1, "thing" : 1 }

//

// frequencies([]);

// //=> {}

//

// frequencies([ "hello", "world" ]);

// //=> { "hello" : 1, "world" : 1 }

//

// There are several ways you can do it, but it might be interesting to try it with

// the `reduce` method that starts with an empty object.

//

var frequencies = function () {

};