Basics of JavaScript

Ashwin Kumar
Jan 14 · 5 min read

Variable definition with (let, var, const).

Data Types


Type Conversions

1. Variable

Variable is named storage for data, we can use variables to store data in JavaScript, a variable can contain any data.

To create a variable in JavaScript, we use let, var and const keyword.

Declaring a variable using let, var and const keyword:

let message

var message;

const message;

So now, we can store some data in a variable, name called message by using the assignment operator =

let message;

message = “Welcome”;

We can access it using the variable name:

Also, we can declare multiple variables in one line

We can also change the value of the variable:

We can declare two variables and copy data from one into the other.

Naming a variable:

We can use camelCase for naming a variable, the name must contain only letters, digits, or the symbols $ and _.

And the main thing is that the 1st character must not be a number.

let firstName;

let abc123;

Use camelCase when there are multiple words, JavaScript is a case sensitive language where let name; and let Name; are different variables.

Some of the Reserved names in JavaScript:

let, var, const, class, function, and return, etc.

let — it is a modern type of variable declaration.

var — it is an old school type of variable declaration.

const — it is similar to let keyword, but the declared value can’t be changed.

2. Data types

Numbers, Strings, Boolean, Null, Undefined, Symbols and Objects

Primitive Datatypes

a. Numbers:

The number type represents both integer and floating-point numbers.


Many operations can be done using numbers i.e addition, subtraction, multiplication, division and so on.


b. Strings:

A string is one of JavaScript datatype which is enclosed by quotes.


let str = “hai”;

In JavaScript, there are three types of quotes.

Single quotes: let str1 = ‘hey’;

Double quotes: let str = “welcome”;

Note: Double quotes and Single quotes don't have any difference in JavaScript.

Backticks: let str2 = `hello`;

Backticks are like extended functionality quotes, we can embed variables and expression into a string by wrapping them in ${…}

Ex: embedded variable

let name = ‘Ashwin’;

alert(`Hai, ${name}`); // Hai, Ashwin

Ex2: embedded an expression

alert(`the result is, ${15+10}`); // the result is, 25

c. Boolean:

Boolean is a logical type, it has only two values true and false.


d. null:

It is a special kind of data type, which contains the null value.


let student = null;

The above code states that the value of “student” is empty.

e. undefined:

It is also a special data type, undefined means value is not assigned.

If a variable is declared, but not assigned, then its value is undefined


We can assign undefined to any variable

f. Symbols:

We never use symbols much in Java Script, symbols are a unique identifier.

Value can be created using Symbol()


let name = Symbol(); // name is a new symbol

Non — Primitive Datatypes

g. Objects:

An object is a collection of “key and values”, they are written within curly braces {}

Object properties are written as key: value, separated by commas.


3. typeof

typeof is an operator that can help to find the type of variable, It’s useful for a quick check.


typeof x // operator

typeof (x) // function


4. Type Conversions

Operators and functions automatically convert the values given to them called an implicit conversion.

Using alert() it automatically converts any value to a string and mathematical operations convert values to numbers.

Sometimes we need to explicitly convert a value to the expected type.

a. String Conversion:

When we need string from the value we can convert it explicitly to get the expected type i.e string.


To convert any value to String use keyword String()

String(7 + 10); // “17” which gives u string (converting number into string)

b. Numeric Conversion:

To convert for numeric we use Number() keyword, it is also an explicit conversion.


If the string is not a valid number, the result of such a conversion is NaN.


c. Boolean Conversion:

It is the simple conversion, using the Boolean() keyword we can explicitly convert.

The values that are 0, empty string, null undefined and NaN become false other values become true.



alert( Boolean(“0”) ) // true;

The string with zero “0” is true because a non-empty string is always true.

alert( Boolean(“ “) ); // true

Spaces, also true because it's not an empty string.


Here I have described about the Variable definition with (let, var, const), Data Types, typeof, and Type Conversions which helps to understand the javascript.


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