# Day 01
## Tasks Done
- `node` is installed
- `chrome` is installed
- Understood how to open console ```Command+Option+J```
- Console.log
```js
console.log('Hello, World!')
```
- Console.log with Multiple Arguments
The **`console.log()`** function can take multiple parameters separated by commas.
The syntax looks like as follows: **`console.log(param1, param2, param3)`**
```js
console.log('Hello', 'World', '!')
console.log('HAPPY', 'NEW', 'YEAR', 2020)
console.log('Welcome', 'to', 30, 'Days', 'Of', 'JavaScript')
```
- Comments
JavaScript does not execute the comment part of our code. In JavaScript, any text line starting with `//` is a comment, and anything enclosed like this `/*..*/` is also a comment.
- Single Line Comment
```js
// This is the first comment
// This is the second comment
// I am a single line comment
```
- Multiline Comment
```js
/*
This is a multiline comment
Multiline comments can take multiple lines
JavaScript is the language of the web
*/
```
- Arithmetics
```js
console.log(2 + 3) // Addition
console.log(3 - 2) // Subtraction
console.log(2 * 3) // Multiplication
console.log(3 / 2) // Division
console.log(3 % 2) // Modulus - finding remainder
console.log(3 ** 2) // Exponentiation 3 ** 2 == 3 * 3
```
- `Visual Studio Code` is installed
- Adding JavaScript to a Web Page
JavaScript can be added to a web page in three different ways:
- Inline JavaScript
```html
30DaysOfScript:Inline Script
```
- Internal Script
The internal script can be written in the _`head`_ or the _`body`_, but it is preferred to put it on the body of the HTML document.
```html
30DaysOfScript:Internal Script
```
- External Script
Similar to the internal script, the external script link can be on the header or body, but it is preferred to put it in the body.
First, we should create an external JavaScript file with .js extension. All files ending with .js extension are JavaScript files. Create a file named introduction.js inside your project directory and write the following code and link this .js file at the bottom of the body.
```js
// introduction.js
console.log('Welcome to 30DaysOfJavaScript')
```
```html
30DaysOfJavaScript:External script
```
- Multiple External Scripts
We can also link multiple external JavaScript files to a web page.
```js
// helloworld.js
console.log('Hello, World!')
```
```html
Multiple External Scripts
```
- Introduction to Data types
In JavaScript and also other programming languages, there are different types of data types. The following are JavaScript primitive data types:
- Numbers
- Integers: Integer (negative, zero and positive) numbers
Example:
`... -3, -2, -1, 0, 1, 2, 3 ...`
- Float-point numbers: Decimal number
Example
`... -3.5, -2.25, -1.0, 0.0, 1.1, 2.2, 3.5 ...`
- Strings
A collection of one or more characters between two single quotes, double quotes, or backticks.
**Example:**
```js
'a'
'Asabeneh'
"Asabeneh"
'Finland'
'JavaScript is a beautiful programming language'
'I love teaching'
'I hope you are enjoying the first day'
`We can also create a string using a backtick`
'A string could be just as small as one character or as big as many pages'
'Any data type under a single quote, double quote or backtick is a string'
```
- Booleans
A boolean value is either True or False. Any comparisons returns a boolean value, which is either true or false.
A boolean data type is either a true or false value.
**Example:**
```js
true // if the light is on, the value is true
false // if the light is off, the value is false
```
- Undefined
In JavaScript, if we don't assign a value to a variable, the value is undefined. In addition to that, if a function is not returning anything, it returns undefined.
```js
let firstName
console.log(firstName) // undefined, because it is not assigned to a value yet
```
- Null
Null in JavaScript means an empty value.
```js
let emptyValue = null
```
- Checking Data Types
To check the data type of a certain variable, we use the **`typeof`** operator. See the following example.
```js
console.log(typeof 'Asabeneh') // string
console.log(typeof 5) // number
console.log(typeof true) // boolean
console.log(typeof null) // object type
console.log(typeof undefined) // undefined
```
- Variables
Variables are _`containers`_ of data.
Variables are used to _store_ data in a memory location.
When a variable is declared, a memory location is reserved.
When a variable is assigned to a value (data), the memory space will be filled with that data.
To declare a variable, we use _`var`_, _`let`_, or _`const`_ keywords.
- _`let`_ is used for a variable that changes at a different time
- _`const`_ is used if the data does not change at all
For example, _PI_, _country name_, _gravity_
A valid JavaScript variable name must follow the following rules:
- A JavaScript variable name should not begin with a number
- A JavaScript variable name does not allow special characters except dollar sign and underscore.
- A JavaScript variable name follows a camelCase convention.
- A JavaScript variable name should not have space between words.
The following are examples of valid JavaScript variables.
```js
firstName
lastName
country
city
capitalCity
age
isMarried
first_name
last_name
is_married
capital_city
num1
num_1
_num_1
$num1
year2020
year_2020
```
Example of invalid variables:
```js
first-name
1_num
num_#_1
```
# 💻 Day 1: Exercises
1. Write a single line comment which says, _comments can make code readable_
```js
// comments can make code readable
```
2. Write another single comment which says, _Welcome to 30DaysOfJavaScript_
```js
// Welcome to 30DaysOfJavaScript
```
3. Write a multiline comment which says, _comments can make code readable, easy to reuse_
_and informative_
```js
/*
comments can make code readable, easy to resuse and informative
*/
```
4. Create a variable.js file and declare variables and assign string, boolean, undefined and null data types
```js
// variable.js
let stringVar = "I am a String Literal";
let booleanVar = true;
let undefinedVar;
let nullVar = null;
```
5. Create datatypes.js file and use the JavaScript **_typeof_** operator to check different data types. Check the data type of each variable
```js
// datatypes.js
console.log(typeof(stringVar));
console.log(typeof(booleanVar));
console.log(typeof(undefinedVar));
console.log(typeof(nullVar));
```
6. Declare four variables without assigning values
```js
let stringVar;
let booleanVar;
let undefinedVar;
let nullVar;
```
7. Declare four variables with assigned values
```js
let stringVar = "I am a String Literal";
let booleanVar = true;
let undefinedVar = "Finally Defined..!!";
let nullVar = null;
```
8. Declare variables to store your first name, last name, marital status, country and age in multiple lines
```js
let firstName = "Nishant";
let lastName = "Goyal";
let isMarried = true;
let country = "India";
let age = 31;
```
9. Declare variables to store your first name, last name, marital status, country and age in a single line
```js
let firstName = "Nishant", lastName = "Goyal", isMarried = true, country = "India", age = 31;
```
10. Declare two variables _myAge_ and _yourAge_ and assign them initial values and log to the browser console.
```js
let myAge = 31;
let yourAge = 30;
console.log("I am " + myAge + " years old.");
console.log("You are " + yourAge + " years old.");
```