JavaScript let and const

This tutorial will introduce you to the new JavaScript keywords let and const introduced in ES6. We will discuss the differences between them and why they are important.

Why should you use it?

It helps you to write more organized and readable code.
It provides you with better control over your variables.
It ensures that your variables are not accidentally modified.

Let

The let keyword is a way to declare a variable in JavaScript. It is a block-scoped variable, meaning it can only be accessed within the block of code in which it was declared.

The main difference between let and var is that let is only accessible within the block of code in which it was declared. This means that you can use let to declare variables that are only accessible within a specific block of code. This can be useful for preventing variables from being accessed outside of the block of code in which they were declared.

The let keyword is also used to declare variables that are not accessible outside of the block of code in which they are declared. This is useful for declaring variables that are only used within a specific block of code, such as a loop or a function.

Here is an example of how to use the let keyword to declare a variable:

script.js

let x = 10;

console.log(x);

In the example above, we declare a variable called x using the let keyword. The variable is only accessible within the block of code in which it was declared. If we try to access the variable outside of the block of code, we will get an error.

Here is another example of how to use the let keyword to declare a variable:

script.js

{
  let y = 20;
  
  console.log(y);
}

console.log(y); // ReferenceError: y is not defined

In this example, we declare a variable called y using the let keyword. The variable is only accessible within the block of code in which it was declared. If we try to access the variable outside of the block of code, we will get an error.

The let keyword is a powerful tool for declaring variables that are only accessible within a specific block of code. It can be used to prevent variables from being accessed outside of the block of code in which they were declared, and it can also be used to declare variables that are only used within a specific block of code.

To learn more about let, visit Let.

Const

The const keyword is used to create constants, which are variables that can not be reassigned. Constants are block-scoped, which means that they are only visible within the block of code in which they are declared.

A constant must be declared with an initial value, and once declared, it cannot be changed or reassigned. For example, the following code will throw an error because we are trying to reassign the constant MY_CONSTANT:

script.js

const MY_CONSTANT = 'foo';
MY_CONSTANT = 'bar';
console.log(MY_CONSTANT);

The value of a constant cannot be changed, but the properties of the object it references can be changed. For example, the following code will not throw an error:

script.js

const MY_OBJECT = {
  foo: 'bar'
};

MY_OBJECT.foo = 'baz';
console.log(MY_OBJECT.foo);

Constants can be declared with the same name in different scopes, but they cannot share the same scope. For example, the following code will throw an error because we are trying to declare two constants with the same name in the same scope:

script.js

const MY_CONSTANT = 'foo';
const MY_CONSTANT = 'bar';
console.log(MY_CONSTANT);

Constants can be declared with the same name in different scopes, but they cannot share the same scope. For example, the following code will not throw an error:

script.js

const MY_CONSTANT = 'foo';

if (true) {
  const MY_CONSTANT = 'bar';
  console.log(MY_CONSTANT);
}

console.log(MY_CONSTANT);

Constants are often used in place of variables when the value of a variable is not expected to change. This makes it easier to understand the code and helps to prevent accidental reassignment of variables.

To learn more about const, visit Const.