Quick JavaScript Go through for Revision Purposes.
Tags: javascript, beginners, quickrevision, svelte
Published: 2023-09-11 07:48:57

> Talk is Cheap, Show me the Code - Linus Torvalds

# Introduction


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/45qmlczrqzncxs6yr25k.gif)



# What are Variables?

- Variables store information in a JavaScript program.
- They act like labeled boxes to hold data.

## Creating Variables

- Use let to create a variable.
- Example:
`let message;
`

## Assigning Values

- Put data into a variable using =.
- Example:
`let message = 'Hello';
`

## Accessing Data

- Retrieve data from a variable using its name.
- Example:

`let message = 'Hello';
alert(message); // Shows the variable content
`

## Declaring Multiple Variables

- Declare multiple variables on separate lines for better readability.
- Example:
`let user = 'John';
let age = 25;
let message = 'Hello';
`

## Variable Naming Rules

- Variable names contain letters, digits, $, and _.
- The first character can't be a digit.
- Use camelCase for multiple words.
- Examples:
`let userName;
let test123;
`

## Constants

- Declare constants using const.
- Constants cannot be changed after assignment.
- Example:

`const myBirthday = '18.04.1982';
`

## Uppercase Constants

- Use uppercase and underscores for constants with known values.

- Example:

`const COLOR_RED = "#F00";
`

## Variable Naming Best Practices

- Choose clear and meaningful variable names.
- Avoid short or cryptic names.
- Make your code self-explanatory.
- Example:

`let currentUser;
let shoppingCart;
`

## Avoid Reusing Variables

- Create new variables instead of reusing existing ones.
- Clear, separate variables lead to better code quality.


# JavaScript Data Types

![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/ofx1ligwf8tsk2nsvevw.gif)



## 1. String

- Used for text data.
- Enclosed in single (''), double ("") quotes, or backticks (``).
- Example:


`const name = 'John';
`

## 2. Number
- Represents whole numbers or decimals.
- Example:

`const age = 25;
const price = 19.99;
`
## 3. BigInt
- For very large numbers, append 'n' to the end.
- Example:

`const bigIntValue = 900719925124740999n;
`

## 4. Boolean
- Represents true or false values.
- Used for logical decisions.
- Example:

`const isRaining = true;
`

## 5. undefined
- Represents uninitialized variables.
- Example:

`let lastName;
`

## 6. null
- Represents an empty or unknown value.
- Example:

`const emptyValue = null;
`

## 7. Symbol
- Unique and immutable data type.
- Introduced in newer JavaScript versions (ES2015+).
- Example:

`const uniqueSymbol = Symbol('description');
`

## 8. Object
- Used to store collections of data.
- Consists of key-value pairs.
- Example:

`const person = { name: 'Alice', age: 30 };
`

# JavaScript Type
- JavaScript determines data types automatically.
- A variable can change its type during execution.

## JavaScript typeof
- Use typeof operator to check the type of a variable.
- Example:

`typeof(name); // returns "string"
typeof(age); // returns "number"
typeof(isRaining); // returns "boolean"
typeof(uniqueSymbol); // returns "symbol"
`

Use it as a quick Go through in case of Revision, etc.
Thanks!

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![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/7legfplrd8e9n5xmmu2e.gif)