introduction to Javascript
Tags: javascript, beginners
Published: 2022-11-08 20:10:56

> MOSE
## History of JavaScript
JavaScript was created at Netscape Communications ****by Brendan Eich in 1995. Netscape and Brendan Eich designed JavaScript as a scripting language for Netscape Navigator which was a leading web browser in the 1990s.
JavaScript's first name was Mocha. After that, it was known as Live Script and later known as JavaScript.


## JavaScript placement

- JavaScript can be placed in a html file  in the  body section using a script tag and placing you JavaScript code within the tags.

```
<body>
<script type="text/javascript">
<!--
document.write("Hello World")
//-->
</script>
<p>This is web page body </p>
</body>
```

- Can also be placed in an external file source with an extension ".js" e.g **filename.js** then include it in a html code using **script** tag and **src **attribute 


```
<html>
<head>
<script type="text/javascript" src="filename.js" ></script>
</head>
<body>
.......
</body>
</html>
```

## JavaScript Variables

- JavaScript has variables that you can declare with
 the optional var keyword.`var age="21years"`

- JavaScript variable shouldn't start with numbers but with letters or underscore. i.e can be declared as `_name="Moses"` and `name="Moses"` but **not **`1name="Moses"` which is an invalid variable

- They are case sensitive. i.e `Name` and `name` are two different variables.

- Don`t use reserved words as variables
NB: 

## Operators
let's declare variables
`A = 10
B = 20`

**1. +(Addition)**
Adds two operands. Ex:

`
result = a + b;
document.write("a + b = ");
document.write(result);
`
will give 
`a + b = 43`
**NOW TRY USING THE OTHER OPERATORS**

**2. -(Subtraction)**
Subtracts the second operand from the first
Ex: A - B will give -10

**3. *(Multiplication)**
Multiply both operands
Ex: A * B will give 200

**4. / (Division)**
Divide the numerator by the denominator
Ex: B / A will give 2

**5. % (Modulus)**
Outputs the remainder of an integer division
Ex: B % A will give 0

**6. ++ (Increment)**
Increases an integer value by one
Ex: A++ will give 11

**7. -- (Decrement)**
Decreases an integer value by one
Ex: A-- will give 9

## Comparison Operetors
A = 10
B = 20

**1. == (Equal**)
Checks if the value of two operands are equal or not, if yes, then 
the condition becomes true.
Ex:
`result = (a == b);
document.write("(a == b) => ");
document.write(result);`
THE OUTPUT FOR THE CODE WILL BE
`(a == b) => false`


**2. != (Not Equal)**
Checks if the value of two operands are equal or not, if the values 
are not equal, then the condition becomes true.
Ex: (A != B) is true.

**3. > (Greater than)**
Checks if the value of the left operand is greater than the value of the right operand, if yes, then the condition becomes true.
Ex: (A > B) is not true.
**4. < (Less than)**
Checks if the value of the left operand is less than the value of 
the right operand, if yes, then the condition becomes true.
Ex: (A < B) is true.

**5. >= (Greater than or Equal to)**
Checks if the value of the left operand is greater than or equal to 
the value of the right operand, if yes, then the condition becomes 
true.
Ex: (A >= B) is not true.

**6. <= (Less than or Equal to)**
Checks if the value of the left operand is less than or equal to the 
value of the right operand, if yes, then the condition becomes 
true.
Ex: (A <= B) is true.

## Conditional statements

## if else statement
The ‘if’ statement is the fundamental control statement that allows JavaScript to make decisions and execute statements conditionally.
**syntax**
`

```
if (expression){
 Statement(s) to be executed if expression is true
}else{
Statement(s) to be executed if expression is false
}
```
Example
```
<html>
<body>
<script type="text/javascript">
<!--
var age = 20;
if( age < 18 ){
 document.write("<b>You are not qualified to enter the site</b>")
}else{
welcome to the site
}
//-->
</script>
<p>Try using a different variable value</p>
</body>
</html>
```
**Output**
`You are not qualified to enter the site.
Set the variable to a different value and ten try...
`
**

## else if statement

else if statement allows JavaScript to make a correct decision out of several conditions.
**syntax**

```
if (expression 1){
 Statement(s) to be executed if expression 1 is true
}else if (expression 2){
 Statement(s) to be executed if expression 2 is true
}else if (expression 3){
 Statement(s) to be executed if expression 3 is true
}else{
 Statement(s) to be executed if no expression is true
}
```
Example

```
<html>
<body>
<script type="text/javascript">
<!--
var series = "outpost";
if( series == "blacklist" ){
 document.write("<b>TheBlacklist</b>");
}else if( series == "witcher" ){
 document.write("<b>The Witcher</b>");
}else if( series == "outpost" ){
 document.write("<b>The Outpost</b>");
}else{
 document.write("<b>Not Available</b>");
}
//-->
</script>
<p>Try using a different variable value</p>
</body>
</html>
```
output

`
The Outpost
Try using a different variable value
`