Learn JavaScript Basics in Just 10 Min | 2021

JavaScript Comment

1. JavaScript comments
2. Advantage of javaScript comments
3. Single-line and Multi-line comments

The JavaScript comments are a meaningful way to deliver the message. It is used to add information about the code, warnings, or suggestions so that end-user can easily interpret the code.

The JavaScript comment is ignored by the JavaScript engine i.e. embedded in the browser.

Advantages of JavaScript comments

There are mainly two advantages of JavaScript comments.

1. To make code easy to understand It can be used to elaborate the code so that end-user can easily understand the code.
2. To avoid unnecessary code It can also be used to avoid the code being executed. Sometimes, we add the code to perform some action. But after some time, there may be a need to disable the code. In such a case, it is better to use comments.

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Types of JavaScript Comments

There are two types of comments in JavaScript.

1. Single-line Comment
2. Multi-line Comment

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JavaScript Single line Comment

It is represented by double forward slashes (//). It can be used before and after the statement.

Let’s see the example of single-line comment i.e. added before the statement.

1. <script>  
2. // It is single line comment  
3. document.write("hello javascript");  
4. </script>  

Let’s see the example of single-line comment i.e. added after the statement.

1. <script>  
2. var a=10;  
3. var b=20;  
4. var c=a+b;//It adds values of a and b variable  
5. document.write(c);//prints sum of 10 and 20  
6. </script>  

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JavaScript Multi line Comment

It can be used to add single as well as multi-line comments. So, it is more convenient.

It is represented by a forwarding slash with an asterisk then an asterisk with a forwarding slash. For example:

1. /* your code here  */  

It can be used before, after, and middle of the statement.

1. <script>  
2. /* It is a multi-line comment.  
3. It will not be displayed */  
4. document.write("example of javascript multiline comment");  
5. </script>  

JavaScript Variable

1. JavaScript variable
2. JavaScript Local variable
3. JavaScript Global variable

A JavaScript variable is simply a name of storage location. There are two types of variables in JavaScript: local variable and global variable.

There are some rules while declaring a JavaScript variable (also known as identifiers).

1. Name must start with a letter (a to z or A to Z), underscore( _ ), or dollar( $ ) sign.
2. After first letter we can use digits (0 to 9), for example value1.
3. JavaScript variables are case sensitive, for example x and X are different variables.

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Correct JavaScript variables

1. var x = 10;  
2. var _value="sonoo";  

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Incorrect JavaScript variables

1. var  123=30;  
2. var *aa=320;  

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Example of JavaScript variable

Let’s see a simple example of JavaScript variable.

1. <script>  
2. var x = 10;  
3. var y = 20;  
4. var z=x+y;  
5. document.write(z);  
6. </script>  

Output of the above example

30

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JavaScript local variable

A JavaScript local variable is declared inside block or function. It is accessible within the function or block only. For example:

1. <script>  
2. function abc(){  
3. var x=10;//local variable  
4. }  
5. </script>  

Or,

1. <script>  
2. If(10<13){  
3. var y=20;//JavaScript local variable  
4. }  
5. </script>  

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JavaScript global variable

A JavaScript global variable is accessible from any function. A variable i.e. declared outside the function or declared with window object is known as global variable. For example:

1. <script>  
2. var data=200;//gloabal variable  
3. function a(){  
4. document.writeln(data);  
5. }  
6. function b(){  
7. document.writeln(data);  
8. }  
9. a();//calling JavaScript function  
10. b();  
11. </script>  

JavaScript Global Variable

A JavaScript global variable is declared outside the function or declared with a window object. It can be accessed from any function.

Let’s see the simple example of global variable in JavaScript.

1. <script>  
2. var value=50;//global variable  
3. function a(){  
4. alert(value);  
5. }  
6. function b(){  
7. alert(value);  
8. }  
9. </script>  

Declaring JavaScript global variable within the function

To declare JavaScript global variables inside the function, you need to use a window object. For example:

1. window.value=90;  

Now it can be declared inside any function and can be accessed from any function. For example:

1. function m(){  
2. window.value=100;//declaring global variable by window object  
3. }  
4. function n(){  
5. alert(window.value);//accessing global variable from other function  
6. }  

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Internals of a global variable in JavaScript

When you declare a variable outside the function, it is added to the window object internally. You can access it through the window object also. For example:

1. var value=50;  
2. function a(){  
3. alert(window.value);//accessing global variable   
4. }  

Javascript Data Types

JavaScript provides different data types to hold different types of values. There are two types of data types in JavaScript.

1. Primitive data type
2. Non-primitive (reference) data type

JavaScript is a dynamic type language, which means you don't need to specify the type of the variable because it is dynamically used by the JavaScript engine. You need to use var here to specify the data type. It can hold any type of value such as numbers, strings, etc. For example:

1. var a=40;//holding number  
2. var b="Rahul";//holding string  

JavaScript primitive data types

There are five types of primitive data types in JavaScript. They are as follows:

JavaScript non-primitive data types

The non-primitive data types are as follows:

JavaScript Operators

We will have a great discussion on each data type later.

JavaScript operators are symbols that are used to perform operations on operands. For example:

1. var sum=10+20;  

Here, + is the arithmetic operator, and = is the assignment operator.

There are the following types of operators in JavaScript.

1. Arithmetic Operators
2. Comparison (Relational) Operators
3. Bitwise Operators
4. Logical Operators
5. Assignment Operators
6. Special Operators

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JavaScript Arithmetic Operators

Arithmetic operators are used to performing arithmetic operations on the operands. The following operators are known as JavaScript arithmetic operators.

JavaScript Comparison Operators

The JavaScript comparison operator compares the two operands. The comparison operators are as follows:

JavaScript Bitwise Operators

The bitwise operators perform bitwise operations on operands. The bitwise operators are as follows:

JavaScript Logical Operators

The following operators are known as JavaScript logical operators.

JavaScript Assignment Operators

The following operators are known as JavaScript assignment operators.

JavaScript Special Operators

The following operators are known as JavaScript special operators.

JavaScript If-else

The JavaScript if-else statement is used to execute the code whether the condition is true or false. There are three forms of if statement in JavaScript.

1. If Statement
2. If else statement
3. if-else if statement

JavaScript If statement

It evaluates the content only if the expression is true. The signature of the JavaScript if statement is given below.

1. if(expression){  
2. //content to be evaluated  
3. }  

Flowchart of JavaScript If statement

Let’s see the simple example of if statement in javascript.

1. <script>  
2. var a=20;  
3. if(a>10){  
4. document.write("value of a is greater than 10");  
5. }  
6. </script>  

The output of the above example

value of a is greater than 10

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JavaScript If...else Statement

It evaluates the content whether the condition is true or false. The syntax of the JavaScript if-else statement is given below.

1. if(expression){  
2. //content to be evaluated if the condition is true  
3. }  
4. else{  
5. //content to be evaluated if condition is false  
6. }  

Flowchart of JavaScript If...else statement

Let’s see the example of if-else statement in JavaScript to find out the even or odd number.

1. <script>  
2. var a=20;  
3. if(a%2==0){  
4. document.write("a is even number");  
5. }  
6. else{  
7. document.write("a is odd number");  
8. }  
9. </script>  

The output of the above example

a is even number

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JavaScript If...else if statement

It evaluates the content only if the expression is true from several expressions. The signature of JavaScript if else if statement is given below.

1. if(expression1){  
2. //content to be evaluated if expression1 is true  
3. }  
4. else if(expression2){  
5. //content to be evaluated if expression2 is true  
6. }  
7. else if(expression3){  
8. //content to be evaluated if expression3 is true  
9. }  
10. else{  
11. //content to be evaluated if no expression is true  
12. }  

Let’s see the simple example of if else if statement in javascript.

1. <script>  
2. var a=20;  
3. if(a==10){  
4. document.write("a is equal to 10");  
5. }  
6. else if(a==15){  
7. document.write("a is equal to 15");  
8. }  
9. else if(a==20){  
10. document.write("a is equal to 20");  
11. }  
12. else{  
13. document.write("a is not equal to 10, 15 or 20");  
14. }  
15. </script>  

The output of the above example

a is equal to 20

JavaScript Switch

The JavaScript switch statement is used to execute one code from multiple expressions. It is just like else if statement that we have learned on the previous page. But it is convenient than if..else.if because it can be used with numbers, characters, etc.

The signature of JavaScript switch statement is given below.

1. switch(expression){  
2. case value1:  
3.  code to be executed;  
4.  break;  
5. case value2:  
6.  code to be executed;  
7.  break;  
8. ......  
9.   
10. default:   
11.  code to be executed if the above values are not matched;  
12. }  

Let’s see the simple example of switch statement in javascript.

1. <script>  
2. var grade='B';  
3. var result;  
4. switch(grade){  
5. case 'A':  
6. result="A Grade";  
7. break;  
8. case 'B':  
9. result="B Grade";  
10. break;  
11. case 'C':  
12. result="C Grade";  
13. break;  
14. default:  
15. result="No Grade";  
16. }  
17. document.write(result);  
18. </script>  

Output of the above example

B Grade

The switch statement is fall-through i.e. all the cases will be evaluated if you don't use break statement.

Let’s understand the behaviour of switch statement in JavaScript.

1. <script>  
2. var grade='B';  
3. var result;  
4. switch(grade){  
5. case 'A':  
6. result+=" A Grade";  
7. case 'B':  
8. result+=" B Grade";  
9. case 'C':  
10. result+=" C Grade";  
11. default:  
12. result+=" No Grade";  
13. }  
14. document.write(result);  
15. </script>  

Output of the above example

B Grade B Grade C Grade No Grade

JavaScript Loops

The JavaScript loops are used to iterate the piece of code using for, while, do-while, or for-in loops. It makes the code compact. It is mostly used in the array.

There are four types of loops in JavaScript.

1. for loop
2. while loop
3. do-while loop
4. for-in loop

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1) JavaScript For loop

The JavaScript for loop iterates the elements for a fixed number of times. It should be used if several iterations are known. The syntax of for loop is given below.

1. for (initialization; condition; increment)  
2. {  
3.     code to be executed  
4. }  

Let’s see the simple example of for loop in javascript.

1. <script>  
2. for (i=1; i<=5; i++)  
3. {  
4. document.write(i + "<br/>")  
5. }  
6. </script> 

Output:

1
2
3
4
5

2) JavaScript while loop

The JavaScript while loop iterates the elements for an infinite number of times. It should be used if several iterations are not known. The syntax of the while loop is given below.

1. while (condition)  
2. {  
3.     code to be executed  
4. }  

Let’s see the simple example of while loop in javascript.

1. <script>  
2. var i=11;  
3. while (i<=15)  
4. {  
5. document.write(i + "<br/>");  
6. i++;  
7. }  
8. </script>  

Output:

11
12
13
14
15

3) JavaScript do-while loop

The JavaScript do-while loop iterates the elements for an infinite number of times like a while loop. But, code is executed at least once whether the condition is true or false. The syntax of the do-while loop is given below.

1. do{  
2.     code to be executed  
3. }while (condition);  

Let’s see the simple example of do while loop in javascript.

1. <script>  
2. var i=21;  
3. do{  
4. document.write(i + "<br/>");  
5. i++;  
6. }while (i<=25);  
7. </script>  

Output:

21
22
23
24
25

4) JavaScript for-in loop

The JavaScript for-in loop is used to iterate the properties of an object. We will discuss it later.

JavaScript Functions

JavaScript functions are used to perform operations. We can call the JavaScript function many times to reuse the code.

Advantage of JavaScript function

There are mainly two advantages of JavaScript functions.

1. Code reusability: We can call a function several times so it saves coding.
2. Less coding: It makes our program compact. We don’t need to write many lines of code each time to perform a common task.

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JavaScript Function Syntax

The syntax of declaring function is given below.

1. function functionName([arg1, arg2, ...argN]){  
2.  //code to be executed  
3. }  

JavaScript Functions can have 0 or more arguments.

JavaScript Function Example

Let’s see the simple example of function in JavaScript that does not has arguments.

1. <script>  
2. function msg(){  
3. alert("hello! this is message");  
4. }  
5. </script>  
6. <input type="button" onclick="msg()" value="call function"/>  

Output of the above example

Function Arguments

We can call function by passing arguments. Let’s see the example of function that has one argument.

1. <script>  
2. function getcube(number){  
3. alert(number*number*number);  
4. }  
5. </script>  
6. <form>  
7. <input type="button" value="click" onclick="getcube(4)"/>  
8. </form>  

Output of the above example

Function with Return Value

We can call function that returns a value and use it in our program. Let’s see the example of function that returns value.

1. <script>  
2. function getInfo(){  
3. return "hello javatpoint! How r u?";  
4. }  
5. </script>  
6. <script>  
7. document.write(getInfo());  
8. </script>  

Output of the above example

hello javatpoint! How r u?

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