Differences between Let Var Const that you didn't know - ES6 [Video + Article]

In this post we will discuss the differences between the let, var and const along with code examples and their outputs

Video:

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What are Let Var and Const

In order to use a variable in JavaScript, you will have to declare that variable. Before ES6, we had only var using which we used to declare variables. From ES6 onwards let and const were introduced and there are some significant differences that you need to know among these.

The differences

We will look at the differences in three aspects:

1. Function or block scoped
2. Redeclaring
3. Redefining

1. Function or block scoped

Var: Function scoped: This means that once a variable is declared using var, it is accessible anywhere within that function. This sounds nice, but we will face problem when we use var in a for-loop, and the variable leaks out.

for (var i = 0; i < 5; i++) {
  console.log(i);
}

console.log(i); // i is still accessible here

Output

0
1
2
3
4
5

Let: Block Scoped: A block is nothing but a piece of code that is enclosed by the curly braces { }. So when a variable is declared using the let, it will stay within that block and doesn't leak out.

for (let i = 0; i < 5; i++) {
  console.log(i);
}

console.log(i); // the variable i is not accessible here

Output

0
1
2
3
4
console.log(i);
            ^
ReferenceError: i is not defined

Const: Block Scoped: The variable declared with const has a block scope just like let, and isn't accessible outside the scope.

{
  const i = 10;
  console.log(i);
}

console.log(i);

Output

10
console.log(i);
            ^
ReferenceError: i is not defined

Redeclaring

Var: Can be Redeclared: The variables declared using var can be declared again using var anywhere in the program.

var cat = "meow";
var cat = "psssss"; // no error

Let: Cannot be Redeclared: The variables declared using let cannot be redeclared within the same scope of it.

let dog;

let dog; // causes error

Output

let dog;
    ^
SyntaxError: Identifier 'dog' has already been declared

Const: Cannot be Redeclared: The variables declared using const cannot be redeclared within the same scope of it.

const lion;

const lion; // causes error

Output

const lion;
      ^
SyntaxError: Identifier 'lion' has already been declared

3. Redefining

Var: can be redefined: Defining is different from declaring in the sense that, defining assigns a value to the variable.

var dog = "boww";
dog = "voww"; // no error

Let: can be redefined: Defining is different from declaring in the sense that, defining assigns a value to the variable.

let cat = "meow";
cat = "prrr"; // no error

Const: cannot be redefined: This results in an error. This applied to the scope only.

const lion = "roar";
lion = "rooor"; // cannot redefine

Output

const lion = "roooor";
      ^
SyntaxError: Identifier 'lion' has already been declared

Summary

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