JavaScript Const: The Definitive Guide

When it comes to JavaScript, one of the most common keywords you will come across is const. const is a keyword that has been around in JavaScript for quite some time now, and it’s used to declare a constant variable. However, many developers are still confused about what exactly const does and how it differs from let. In this article, we will dive deep into the world of JavaScript const and explore everything there is to know about this keyword.

What is const in JavaScript?

const is a keyword in JavaScript that is used to declare a constant variable. A constant variable is a variable that cannot be reassigned once it has been assigned a value. In other words, the value of a const variable remains the same throughout the program’s execution.

Here is an example of how const works in JavaScript:

const PI = 3.14159;
PI = 3; // throws an error because PI is a constant and cannot be reassigned

In the above example, we have declared a const variable named PI and assigned it the value 3.14159. When we try to reassign PI to the value 3, JavaScript throws an error because we are trying to change the value of a constant variable.

How is const different from let?

The other keyword for declaring variables in JavaScript is let. While both const and let are used to declare variables, there are some key differences between the two.

Firstly, as we mentioned earlier, const variables cannot be reassigned once they are assigned a value. On the other hand, let variables can be reassigned multiple times throughout the program’s execution.

Secondly, const variables must be initialized when they are declared. This means that you have to assign a value to a const variable when you declare it, whereas you can declare a let variable without assigning a value to it.

Here is an example to illustrate the difference between const and let:

const PI = 3.14159;
let radius = 5;

// calculate the area of a circle
const area = PI * (radius ** 2);

// reassign radius
radius = 7;

// recalculate the area of the circle with the new radius
const newArea = PI * (radius ** 2);

console.log(area);    // outputs 78.53975
console.log(newArea); // outputs 153.93804002589985

In the above example, we have declared a const variable named PI and assigned it the value 3.14159, and a let variable named radius and assigned it the value 5. We then calculate the area of a circle using the const variable PI and the let variable radius. After that, we reassign the value of radius to 7 and recalculate the area of the circle using the new value of radius. As you can see, we can reassign the let variable radius, but we cannot reassign the const variable PI.

When should you use const?

Now that we know what const does and how it differs from let, let’s talk about when you should use const is useful when you want to declare a variable that will not change throughout the program’s execution. For example, if you have a variable that represents the value of pi, you can declare it as a const variable because its value will never

change. Similarly, if you have a variable that represents a configuration value or a constant value that is used throughout your program, you can declare it as a const variable.

Another use case for const is to declare variables that should not be accidentally reassigned. When you declare a variable as a const, you are signaling to other developers who read your code that this variable should not be changed. This can help prevent bugs and make your code more maintainable.

Best Practices for Using const

Now that we know when to use const, let’s talk about some best practices for using this keyword in our code.

Use Descriptive Variable Names

When you declare a const variable, make sure to use a descriptive name that indicates the purpose of the variable. This will make your code more readable and easier to understand for other developers who come across it.

For example, instead of declaring a const variable as x, you can declare it as numOfElements if it represents the number of elements in an array.

// bad
const x = 10;

// good
const numOfElements = 10;

Avoid Declaring Too Many const Variables in One Line

While you can declare multiple const variables in one line, it’s best to avoid doing so if the variables are not related to each other. This can make your code harder to read and understand.

For example, consider the following code:

// avoid
const firstName = 'John', lastName = 'Doe', age = 30;

It’s not immediately clear what these variables represent and how they are related to each other. It’s better to declare each variable on a separate line:

// better
const firstName = 'John';
const lastName = 'Doe';
const age = 30;

Use const for Objects and Arrays

const is especially useful when dealing with objects and arrays. When you declare an object or an array as a const, you can still modify the properties of the object or the elements of the array, but you cannot reassign the entire object or array.

For example, consider the following code:

const person = {
  firstName: 'John',
  lastName: 'Doe',
  age: 30
};

person.age = 31; // OK
person = {};     // throws an error

In the above example, we have declared an object named person as a const. We can still modify the properties of the object (in this case, we update the value of the age property), but we cannot reassign the entire object.

Similarly, you can declare an array as a const and still modify its elements:

const arr = [1, 2, 3];

arr[0] = 4; // OK
arr = [];   // throws an error

Use let for Variables That Will Change

When you have a variable that will change throughout your program’s execution, you should declare it as a let variable. This will signal to other developers who read your code that this variable can be reassigned.

For example, if you have a variable that represents a counter, you can declare it as a let variable:

let count = 0;

function increment() {
  count++;
}

increment();
console.log(count); // outputs 1

In the above example, we declare a let variable named count and initialize it to 0. We then define a function named increment that increments the value of count by 1. When we call the increment function and log the value of count, we see that the value has been updated to 1. If we had declared count as a const variable, we would not be able to update its value.

Use const in Block Scope

When you declare a const variable in a block (such as a for loop or an if statement), the variable is only accessible within that block. This can help prevent naming conflicts and make your code more organized.

For example:

for (const i = 0; i < 10; i++) {
  console.log(i);
}

In the above example, we declare a const variable named i within the for loop. This variable is only accessible within the loop and cannot be accessed outside of it.

Use const for Function Parameters

When you define a function that takes in parameters, you can declare the parameters as const variables. This will prevent you from accidentally modifying the values of the parameters within the function.

For example:

function square(const num) {
  return num ** 2;
}

console.log(square(5)); // outputs 25

In the above example, we define a function named square that takes in a const parameter named num. We then return the square of num. Since num is declared as a const variable, we cannot modify its value within the function.

Conclusion

In this article, we have explored the world of JavaScript const and learned everything there is to know about this keyword. We learned that const is used to declare a constant variable that cannot be reassigned once it has been assigned a value. We also learned that const differs from let in that const variables cannot be reassigned and must be initialized when they are declared.

We discussed when to use const and some best practices for using this keyword in our code. We learned that const is useful for declaring variables that will not change throughout the program’s execution and for preventing accidental reassignment of variables. We also learned that const is especially useful for objects and arrays, and that let should be used for variables that will change throughout the program’s execution.

By following these best practices and using const appropriately in our code, we can write more maintainable, bug-free, and organized JavaScript programs.

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