Dynamic Typing

  • One thing that separates JavaScript from a lot of programming languages, is that JavaScript is a dynamic language.

What do I mean by dynamic?

  • Well, we have two types of programming languages:

    • Static languages or

    • Dynamic languages

  • In static languages, when we declare a variable, the type of the variable is set and it cannot be changed in the future.

string name = "John";
  • In a dynamic language like JavaScript, the type of a variable can change at run time.
let name = 'John';

Let's see this in code:

let name = 'Pratham'; 
let age = 25; 
let isApproved = true; 
let firstName = undefined; 
let selectedColor = null;

  • So back in the example from the last lecture we have declared this name variable, and we have set that to a string, so the type of name is currently a string, but it can change in the future.

  • Let us take a look in console, we can execute some JavaScript code here, we have:

typeof
  • we have this type of operator, and with that we can check the type of a variable.

typeof operator is used to check the data-type of the variable in JavaScript

  • So after that we add the name of the variable after the typeof operator. In this case the name variable is string.

  • If you reassign name to a different value, like a number, and check it's type:

  • Observe that the type is now changed to a number, this is what we call a dynamic language (unlike static languages the type of these variables will be determined at runtime based on the values that we assigned to them).

  • Now, let's take a look at a few more examples of the typeof operator.

to clear the console, type command : ctrl + L

  • So now let's take a look at the type of age,

  • Now if we change age to a floating point number, and I know it doesn't make sense, but let's just stick to this for example, 30.1, and then look at type of age, it's still a number.

In JavaScript, unlike other programming languages we don't have two kinds of numbers, like integer or floating point number. All numbers are of type number.

  • Now, let's look at the type of is approved, it's a boolean as I told you before.

  • what about the first name, let's have a look:

It's undefined, and that's funny, because the value of this variable is undefined, but it's type is undefined as well. What does this mean??

  • Well, earlier I told you that we had two categories of types, we have:

    • primitives or value types, and

    • Reference types

  • In the primitive types category, we have strings, numbers, booleans, undefined and null. So undefined is actually a type, but it's also a value.

  • In this example, because we have set first name, to undefined as a value it's type is also undefined, But what about selected color??

  • Let's have a look:

  • The type of this variable is an object.

What is an object? That's a topic for the next lecture.