Javascript var let const

Comparison of `var`, `let`, and `const` in JavaScript

In JavaScript, var, let, and const are keywords used to declare variables, but they have important differences related to scope, redeclaration, and mutability of values. Below is a detailed comparison:

1. Scope

var:
- Has function scope: a var variable is only effective within the function it was declared.
- Does not have block scope: if declared inside a block ({}), the variable can still be accessed outside of that block.
- Example:
```
if (true) {
  var x = 10;
}
console.log(x); // 10
```
let and const:
- Have block scope: the variable is only effective within the block ({}) it was declared.
- Example:
```
if (true) {
  let y = 20;
}
console.log(y); // Error: y is not defined
```

2. Redeclaration (Khả năng tái khai báo)

var: Can be redeclared within the same scope without causing an error.
```
var z = 5;
var z = 10; // No error
console.log(z); // 10
```

let and const: Cannot be redeclared within the same scope.

let a = 5;
let a = 10; // Error: Identifier 'a' has already been declared

3. Reassignment

var and let: Can be reassigned after declaration.

var x = 5;
x = 10; // Valid

let y = 15;
y = 20; // Valid

const: Cannot be reassigned after declaration.

const z = 30;
z = 40; // Error: Assignment to constant variable

Note: for objects or arrays, the contents can change, but the variable itself cannot be reassigned.

const obj = { name: 'Alice' };
obj.name = 'Bob'; // Valid
obj = { age: 25 }; // Error

4. Hoisting (Cơ chế đưa lên đầu)

var: Is “hoisted” (moved to the top of the scope) but not initialized. The variable’s value will be undefined if accessed before being assigned.

console.log(x); // undefined
var x = 10;

let and const: Are also “hoisted”, but cannot be accessed before initialization (Temporal Dead Zone).

console.log(y); // Error: Cannot access 'y' before initialization
let y = 10;

5. When to Use?

var: Should be avoided as it can lead to errors due to loose scope and hoisting behavior.
let: Use when you need to declare a variable that may change its value.
const: Use when declaring a variable whose value should not change, especially for constants, objects, or arrays.

Summary

Attribute	`var`	`let`	`const`
Scope	Function	Block	Block
Redeclaration	Allowed	Not Allowed	Not Allowed
Reassignment	Allowed	Allowed	Not Allowed
Hoisting	Yes (undefined value)	Yes (Temporal Dead Zone)	Yes (Temporal Dead Zone)

Using let and const will make your code safer and easier to maintain.

Comparison of var, let, and const in JavaScript