Add concept exercise objects

concepts/objects/.meta/config.json

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+{
+  "blurb": "Besides primitive data types like \"number\" and \"string\", there is another data type in JavaScript called \"object\". Objects are collections of key-value pairs but also the fundamental building blocks of the language.",
+  "authors": ["junedev"],
+  "contributors": []
+}

concepts/objects/about.md

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+# About
+
+## Explanation
+
+Besides primitive data types like `number` and `string`, there is another important data type in JavaScript called `object`.
+Objects are collections of key-value pairs.
+As such, they can be used as what is often referred to as maps or dictionaries in other languages.
+
+In other languages, all values in a map often need to have the same data type.
+In JavaScript, only the type of the key is restricted: it has to be a string.
+The values inside one object can have different types.
+They can be primitive types like numbers but also arrays, other objects or even functions.
+This makes objects very versatile so that they are also key entities for [object-oriented programming][oop] (OOP) in JavaScript.
+
+In the following we will focus on objects as collections or maps. Other use cases of objects will be covered in other concepts, see e.g., [Classes][concept-classes].
+
+## Creating an Object
+
+You create an object [literal][literal] using curly brackets.
+You can also directly include some entries (key-value pairs).
+For that, state the key first, followed by a colon and the value.
+
+```javascript
+const emptyObject = {};
+
+const obj = {
+  favoriteNumber: 42,
+  greeting: 'Hello World',
+  useGreeting: true,
+  address: {
+    street: 'Trincomalee Highway',
+    city: 'Batticaloa',
+  },
+  fruits: ['melon', 'papaya'],
+  addNumbers: function (a, b) {
+    return a + b;
+  },
+};
+```
+
+The trailing comma after the last entry is optional in JavaScript.
+You can read about the advantages of using it [here][mdn-trailing-commas].
+
+You might wonder why the keys are not wrapped in quotation marks although they are supposed to be strings.
+This is a short-hand notation.
+If the key follows the naming rules for a JavaScript [identifier][mdn-identifier], you can omit the quotation marks.
+For keys with special characters in the name, you need to apply the usual string notation.
+
+```javascript
+const obj = {
+  '1keyStartsWithNumber': '...',
+  'key/with/slashes': '...',
+  'key-with-dashes': '...',
+  'key with spaces': '...',
+  '#&()[]{}èä樹keyWithSpecialChars': '...',
+};
+```
+
+Often you will want to add existing variables when declaring an object.
+This would often lead to key-value pairs like `name: name`.
+JavaScript has a short-hand notation that allows just writing `name` instead.
+There is also a short-hand notation for defining a function in an object.
+There you can omit the colon and the `function` keyword.
+
+```javascript
+const x = 1;
+const y = 2;
+
+const obj = {
+  x: x,
+  y: y,
+  calcSum: function (a, b) {
+    return a + b;
+  },
+};
+
+// Can be shortened to ...
+const obj = {
+  x,
+  y,
+  calcSum(a, b) {
+    return a + b;
+  },
+};
+```
+
+If you want to create an object literal with a key name that is stored in a variable or derived via an expression, you need to wrap the variable or expression in square brackets.
+
+```javascript
+const key = 'greeting';
+
+const obj = {
+  [key]: 'hello world',
+};
+console.log(obj);
+// => { greeting: 'hello world' }
+```
+
+## Retrieving a Value
+
+There are two ways to retrieve the value for a given key, dot notation and bracket notation.
+
+```javascript
+const obj = { greeting: 'hello world' };
+
+obj.greeting;
+// => hello world
+
+obj['greeting'];
+// => hello world
+
+// Bracket notation also works with variables.
+const key = 'greeting';
+obj[key];
+// => hello world
+```
+
+Using the dot notation as a short-hand has the same restriction as omitting the quotation marks.
+It only works if the key follows the identifier naming rules.
+
+If you try to retrieve a key that does not exist in the object, JavaScript returns `undefined`.
+See [Null and Undefined][concept-null-undefined] for more details on this.
+
+You can chain the keys if you want to retrieve a value from a nested object.
+
+```javascript
+const obj = {
+  address: {
+    street: 'Trincomalee Highway',
+    city: 'Batticaloa',
+  },
+};
+
+obj.address.city;
+// => 'Batticaloa'
+
+obj['address']['city'];
+// => 'Batticaloa'
+
+// Notations can also be mixed.
+obj.address['city'];
+// => 'Batticaloa'
+```
+
+## Adding or Changing a Value
+
+You can add or change a value using the assignment operator `=`.
+Again, there is dot and bracket notation available.
+
+```javascript
+const obj = { greeting: 'hello world' };
+
+obj.greeting = 'Hi there!';
+obj['greeting'] = 'Welcome.';
+
+obj.newKey1 = 'new value 1';
+obj['new key 2'] = 'new value 2';
+
+const key = 'new key 3';
+obj[key] = 'new value 3';
+```
+
+## Deleting an Entry
+
+You can delete a key-value pair from an object using the `delete` keyword.
+
+```javascript
+const obj = {
+  key1: 'value1',
+  key2: 'value2',
+};
+
+delete obj.key1;
+delete obj['key2'];
+```
+
+Note that although `undefined` is returned for non-existing keys, setting a key to `undefined` does not remove the entry.
+Always use `delete` instead.
+
+## Checking Whether a Key Exists
+
+You can check whether a certain key exists in an object with the `hasOwnProperty` method.
+
+```javascript
+const obj = { greeting: 'hello world' };
+
+obj.hasOwnProperty('greeting');
+// => true
+
+obj.hasOwnProperty('age');
+// => false
+```
+
+`hasOwnProperty` returns `false` for [inherited keys][concept-inheritance]. This is usually the desired behavior.
+If you want include inherited keys in the existence check you can use the [`in` operator][mdn-in-operator] instead.
+
+## Looping Through an Object
+
+There is a special `for...in` loop to iterate over all keys of an object.
+
+```javascript
+const obj = {
+  name: 'Ali',
+  age: 65,
+};
+
+for (let key in obj) {
+  console.log(key, obj[key]);
+}
+// name Ali
+// age 65
+```
+
+It might seem like `for...in` always visits the keys in the order in which they appear or were added to the object (insertion order).
+Nevertheless you should not rely on this because there are special cases where the order will differ from the insertion order.
+Always write your code as if the order would be arbitrary.
+If you need a reliable order, use a [`Map`][jsinfo-map] or a nested [array][concept-arrays] instead of an object.
+
+The `for…in` loop has a pitfall in case the object has [inherited keys][concept-inheritance].
+Those will also be visited by the `for…in` loop.
+You usually want to avoid this so you need to skip over them like shown below.
+
+```javascript
+for (let key in obj) {
+  if (!obj.hasOwnProperty(key)) {
+    continue;
+  }
+  // ...
+}
+```
+
+JavaScript also has a `for...of` loop, for example to iterate over arrays.
+It is important to note that `for...in` and `for...of` are not interchangeable, see [MDN documentation][mdn-for-in-for-of] for details on this.
+
+## Keys, Values and Entries
+
+The built-in object `Object` provides helper methods to retrieve all the keys, values or entries of a given object as arrays.
+
+```javascript
+const obj = {
+  name: 'Ali',
+  age: 65,
+};
+
+Object.keys(obj);
+// => [ 'name', 'age' ]
+
+Object.values(obj);
+// => [ 'Ali', 65 ]
+
+Object.entries(obj);
+// => [ [ 'name', 'Ali' ], [ 'age', 65 ] ]
+```
+
+## Truly Empty Object
+
+You might have noticed that an empty object in JavaScript is not completely empty.
+For example, it contains the `hasOwnProperty` method and other methods like `toString`.
+Usually that does not cause any problems but if you ever need to create a truly empty object use a [null object][mdn-null-object] that can be created via `Object.create(null)`.
+
+[oop]: https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Objects/Object-oriented_JS
+[concept-classes]: /tracks/javascript/concepts/classes
+[literal]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Grammar_and_types#literals
+[mdn-trailing-commas]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Trailing_commas
+[mdn-identifier]: https://developer.mozilla.org/en-US/docs/Glossary/Identifier
+[jsinfo-map]: https://javascript.info/map-set#map
+[concept-inheritance]: /tracks/javascript/concepts/inheritance
+[mdn-in-operator]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/in
+[mdn-null-object]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/create#custom_and_null_objects
+[mdn-for-in-for-of]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of#difference_between_for...of_and_for...in
+[concept-null-undefined]: /tracks/javascript/concepts/null-undefined