Var Vs Let Vs Const

Var

var a = 11;
{
var a = 8;
};
console.log(a);// 8
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var a = 11;
{
a = 28;
};
console.log(a);// 28

Variables declared with var are in the global scope. We can access a var variable even outside a block because it is not block-scoped. Additionally, we can redeclare and reassign a var variable both inside and outside a block.

Let

{
    let a = 24;
}
console.log(a);// ReferenceError: a is not defined
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{
    let a = 24;
    console.log(a);// 24
}
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{
    let a = 24;
    a = 20;
    console.log(a);// 20
}// 
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{
    let a = 24;
    let a = 20;
    console.log(a);//SyntaxError: Identifier 'a' has already been declared
}
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let a = 20;
{
let a = 24;
console.log(a);// 24
}

let has a separate memory space and block scope. Variables declared with let cannot be accessed outside the block because they are not in the global scope. We can reassign a let variable. However, we cannot redeclare the same variable within the same block, but we can redeclare it in a different block.

Const

{
  const x = 4;  
}
console.log(x);//ReferenceError: x is not defined
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{
  const x = 4;  
  console.log(x) ;// 4
}
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{
  const x = 4;  
  const x = 2;  
}
console.log(x);//SyntaxError: Identifier 'x' has already been declared
-------------------------------------------------------------------------
{
  const x = 4;   
}
const x = 2;
console.log(x);// 2
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const x = 2;// we can access the global(x)
{
  const x = 4;   // we cannot access the outside block
}
console.log(x);// 2

const has a separate memory space and is block-scoped. Once a value is declared and initialized with const, it cannot be redeclared or reassigned. We cannot access a const variable outside its block because it is not in the global scope. We cannot redeclare the variable within the same block, but we can redeclare it outside the block.