Const vs Var vs Let: When and where to use them?
Tags: javascript, webdev, tutorial, beginners
Published: 2022-12-31 12:33:28

In JavaScript, `const`, `var`, and `let` are three keywords that are used to declare variables. These keywords are used to specify the scope and lifetime of the variables that they declare, and they also have different rules for reassignment and redeclaration. Understanding the differences between these keywords and how they are used can be important for writing effective and efficient JavaScript code.

So, today, we're just gonna do that :)

### **Const**

`const` is a keyword that is used to declare a constant variable. Such a variable is one that cannot be reassigned or redeclared once it has been declared. This means that once a variable has been declared using the `const` keyword, its value cannot be changed, and it cannot be redeclared within the same scope. For example:

```javascript
const x = 5;
x = 20; // this will throw an error, because x is a constant and cannot be reassigned
const x = 20; // this will also throw an error, because x has already been declared as a constant
```

`const` variables must be initialized with a value when they are declared, and this value cannot be changed later. However, it is important to note that the value of a `const` variable can be mutable, which means that it can be modified in place. For example:

```javascript
const x = [1, 2, 3];
x.push(4); // this is allowed, because x is an array and we are modifying it in place
x = [1, 2, 3, 4]; // this will throw an error, because x is a constant and cannot be reassigned
```

### **Var**

`var` is a keyword that is used to declare a variable in JavaScript. Unlike `const` variables, `var` variables can be reassigned and redeclared within the same scope. For example:

```javascript
var x = 10;
x = 20; // this is allowed, because x is a var and can be reassigned
var x = 20; // this is also allowed, because x has already been declared as a var and can be redeclared
```

`var` variables do not have to be initialized with a value when they are declared, and they can be declared without being initialized at all. However, it is generally a good practice to initialize `var` variables with a value when they are declared, in order to avoid issues with undeclared variables.

### **Let**

`let` is a keyword that was introduced in JavaScript in ECMAScript 6 (ES6) and is similar to `var` in many ways. Like `var` variables, `let` variables can be reassigned and redeclared within the same scope. However, `let` variables have a different lifetime than `var` variables, which means that they are only accessible within the block in which they are declared. This is known as block-scoping, and it is similar to the way that `const` variables are scoped.

For example:

```javascript
if (true) {
  let x = 10;
  console.log(x); // this will output 10
}
console.log(x); // this will throw an error, because x is not defined outside of the block in which it was declared
```

`let` variables can also be used in for loops in a similar way to `var` variables, and they will be re-initialized on each iteration of the loop:

```javascript
for (let i = 0; i < 10; i++) {
  console.log(i); // this will output 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
}
console.log(i); // this will throw an error, because i is not defined outside of the block in which it was declared
```

One **major difference between** `let` **and** `var` is that `let` variables are not hoisted, which means that they are not accessible before they are declared. This can be a useful feature in some cases, because it prevents the use of variables before they are defined, which can help to prevent certain types of bugs.

### **Conclusion**

All in all, `const`, `var`, and `let` are all useful keywords for declaring variables in JavaScript, and they each have their own unique properties and features. `const` is useful for declaring variables that should not be reassigned or redeclared, `var` is useful for declaring variables that can be reassigned and redeclared and `let` is useful for declaring variables that have a block-scoped lifetime and are not hoisted.

Whatever you choose to use, choose wisely!

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