We know that both Let and Const are block scoped, block scoped means a peice of code that is inside the same block, in between the { }. Anything between curly braces is a block.  

## Understanding Let

Let allow us to update the value in the same block but does not allow us to re declare the variable in the same block. 

Let us understand this by code examples - 

This code is not correct - 


```
function f1() {
    let abc = "1";
    let abc = "2";
  }

f1()
```
as we are trying to re declare the same variable. 

But the below code is correct -


```
function f1() {
    let abc = "1";
    abc = "2";
}

f1()
```
as here we are only updating the value of Let and not re declaring it. 

Let but allow us to re declare a valiable in an inner scope. 

So, we can do this - 


```
function f1() {
    let abc = "1";
    if (true) {
      let abc = "2";
    }
}

f1()
```

here, let abc = 2 is declared in an inner block and so it is allowed. 

Now let us find the output values from our example -


```
function f1() {
    let abc = "1";
    console.log("abc1", abc);
    if (true) {
      let abc = "2";
      console.log("abc2", abc);
    }
    abc = "3";
    console.log("abc3", abc);
}

f1()
```

Here the output value of abc1 will be 1 and abc2 will be 2 and abc3 will be 3, abc2 is re declared in a different block so it has its own value, whereas abc3 has an updated value from parent block. 

let us take another example - 


```
function f1() {
    let abc = "1";
    console.log("abc1", abc);
    if (true) {
      abc = "2";
      console.log("abc2", abc);
    }
    console.log("abc3", abc);
}
  
f1()
```

Here the output value of abc1 will be 1 but both the output values of abc2 and abc3 will be 2, the point to note here is that we are not re declaring abc2 in the inner block but simply updating the value. and so the value of abc3 is also 2.  



## Understanding Const

Const is also similar to Let in terms of scope but the major difference is that Const doest not either allow us to update the value of re declare the variable. 

In case of Const both of the following peice of code is wrong 

```
function f1() {
    const abc = "1";
    const abc = "2";
}

function f2() {
    const abc = "1";
    abc = "2";
}

f1()

f2()
```

Again as in case of Let, we can also re declare Const in an inner block and so the following code is correct - 


```
function f1() {
    const abc = "1";
    console.log("abc1", abc);
    if (true) {
      const abc = "2";
      console.log("abc2", abc);
    }
    console.log("abc3", abc);
}

f1()
```

the output value here will be abc1 = 1, abc2 = 2, and abc3 = 1.


There is one more important concept of Const and that is -


```
function f1() {
  const abc = {
      first: "first",
      second: "second",
   };
  console.log("abc1", abc);
  abc.first = "first-first";
  console.log(abc);
}

f1()
```

we can update the object values as shown above. 

Here the output of abc1 will be {first:"first", second:"second"} and of abc2 will be {first:"first-first", second:"second"}

We can do things showed in above code, update the object value but we can not assign a new property to a const object. 

The following code in not correct - 


```
  function f1() {
    const abc = {
      first: "first",
      second: "second",
    };
    abc.third = "first-first";
    console.log("abc2", abc);
  }

f1()
```

Here we are trying to add a new property called "third" which we can not do. 


I hope this is clear, and you all have understood the difference between Let and Const.

Thank You!

We will see what is Let and Const in JavaSCript Es6 and try to understand the difference between both.

Difference between Let and Const in JavaScript

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