In most of the scripting languages, the type is associated with the value, not with the variable itself. What it means? JavaScript and other languages such as Python, called weakly typed, does not need to specify which kind of data we will use to store in the variable. JavaScript has many ways to ensure the correct type of an object, including duck typing.

Let’s look at an example:

1.  var age = 26;
2.  age = "twenty-six";
3.  age = false;

In the preceding code, the defined variables accept any data type, because data types will be evaluated at runtime, so, for example, the age variable in line 1 will be an integer, will become a string in line 2 and, finally, Boolean. Sounds tricky? Don't worry, think of the variable as an empty vial without a label. You can put anything you want, cookies, milk, or salt. What you will store in the vial? Depending of your needs, if you want to make a breakfast, milk should be the better option. The only thing you must keep in mind, is remember what is containing this vial! We would hate to confuse salt with sweet.

If we need to ensure that the value belongs to some specific type, we can use the typeof operator to retrieve the data type of a given variable. Let's have a look at them:

JavaScript objects are based on associative arrays, improved with the prototyping inclusion. The properties and values can be changed at runtime. Another common way to create objects is using the JavaScript Object Notation (JSON) or using functions.

Let's see how an object created by JavaScript code looks, and its JSON representation:

// Let's create the person object
function Person(first, last, age) {
this.firstName = first;
this.lastName = last;
this.age = age;
}
var diego = new Person("Diego", "Arguelles", 27);

//JSON representation of the same object
{
    firstName: "Diego",
    lastName: "Arguelles",
    age: 27
}

A function is an object inside itself. They have properties, methods, and can include inner functions. It's a way to encapsulate a functionality you want to reuse in more than one place in your application; you just need to write the function name instead of all the code inside that, just like the following example:

function sum(numberA, numberB){
return numberA + numberB
}
sum(4,5) //9
sum(5,2) //7
sum(sum(5,1),9) //15

JavaScript uses prototypes instead of classes for inheritance. It is possible to emulate all OOP characteristics using just prototypes:

function Person(first, last, age) {
this.firstName = first;
this.lastName = last;
this.age = age;
}

var diego = new Person('Diego', 'Arguelles', 26)
diego.nationality = 'Peruvian'
console.log(diego) 
// Person {firstName: "Diego", lastName: "Arguelles", age: 26, nationality: "Peruvian"}

Person.prototype.career = 'Engineering'
console.log(diego.career) // Engineering

That being said, what is exactly a prototype? Different from objects, one prototype does not have a closed structure. In objects, we define standard properties and we just have these properties for work, since JavaScript is not completely an object-oriented language, we have the advantage to add, remove, or change properties and values of our prototypes depending on our needs.

We can modify prototype attributes at runtime. Note that even if you can modify any prototype, you should only modify yours. If you modify standard prototypes (for example, the  array prototype) you will encounter very weird bugs in your application.

Events allow you to add the real interaction on your web page. JavaScript allows you to attach event handlers on your HTML pages and execute custom code when they are triggered. For example, the given code will display an alert when the user clicks on your web page body:

document.querySelector('body').onclick = function() {
alert('You clicked the page body!!!');
}