# Javascript quick revision

### **Point-1: var vs let vs const – What's the Difference?**

In JavaScript, `var`, `let`, and `const` are used to declare variables.  
There are 3 factors to analyze the differences between these keywords

1. Scope of variables:
    
2. Redeclaration and reassignment
    
3. Hoisting
    

| **Feature** | `var` | `let` | `const` |
| --- | --- | --- | --- |
| Scope | Function scope (accessible throughout the function they are declared in) | Block scope (accessible within the block they are declared in) | Block scope (accessible within the block they are declared in) |
| Redeclaration | Allowed (can be redeclared in the same scope) | Not allowed (results in SyntaxError) | Not allowed (results in SyntaxError) |
| Reassignment | Allowed (can be reassigned) | Allowed (can be reassigned) | Not allowed (cannot be reassigned) |
| Hoisting | Hoisted and initialized with `undefined` | Hoisted but not initialized | Hoisted but not initialized |

### **Point-2: Datatypes in javascript**

In JavaScript, data types specify the type of data that can be stored and manipulated within the language. JavaScript is a loosely typed or a dynamic language, meaning variables do not need to be declared with a data type explicitly, and their data types can change during the execution of the program. JavaScript supports two categories of data types:

1. **Primitive Data Types**: These are the basic data types that hold a single value.
    
    * **String**: Represents textual data, e.g., `"Hello, World!"`.
        
    * **Number**: Represents both integer and floating-point numbers, e.g., `42`, `3.14`.
        
    * **BigInt**: Represents integers with arbitrary precision, e.g., `9007199254740991n`.
        
    * **Boolean**: Represents a logical entity with two values: `true` and `false`.
        
    * **Undefined**: Represents a variable that has not been assigned a value.
        
    * **Null**: Represents the intentional absence of any object value.
        
    * **Symbol**: Represents a unique, immutable identifier, often used as the key of an Object property.
        
2. **Reference Data Types** (Objects): These data types are used to store collections of data or more complex entities.
    
    * **Object**: Represents instances of named values, e.g., `{ name: "Alice", age: 30 }`.
        
    * **Array**: A type of object used for storing sequences of values, e.g., `[1, 2, 3]`.
        
    * **Function**: Represents code that can be called by other code or by itself, e.g., `function hello() { console.log("Hello, World!"); }`.
        

JavaScript dynamically determines the data type based on the value assigned to the variable, allowing for a flexible approach to programming but also requiring careful management to avoid unexpected behavior or bugs.

### **Point-3: undefined vs null**

In JavaScript, `undefined` and `null` both represent absence of value, but in different ways:

**Keypoint to remember**: `undefined` represents the system-level, absent value, `null` is used by programmers to indicate the intentional absence of any object value.

Here's a table view summarizing the differences between `undefined` and `null` in JavaScript:

| **Aspect** | `undefined` | `null` |
| --- | --- | --- |
| **Description** | Indicates that a variable has not been assigned a value. | Explicitly represents the absence of any object value. |
| **Type** | `"undefined"` | `"object"` (considered a legacy bug) |
| **Equality** | `undefined == null` is `true` (non-strict equality) | `undefined == null` is `true` (non-strict equality) |
|  | `undefined === null` is `false` (strict equality) | `undefined === null` is `false` (strict equality) |
| **Default Value** | Default value for uninitialized variables. | Can be assigned to a variable as a representation of no value. |
| **Function Return** | Functions return `undefined` if no value is specified. | Can be returned by a function to explicitly indicate no meaningful value. |
| **Usage** | Used by the JavaScript engine to denote absence of value. | Used by programmers to denote absence of value intentionally. |