JavaScript closures are one of the language’s most powerful and sometimes confusing concepts. If you’re diving deeper into JavaScript, understanding closures is essential.
But what exactly is a closure, and how can you use it effectively in your code?
In this blog, we’re going to know the importance of closures and how they can be applied practically for the purpose of more efficient, readable, and maintainable code.
If you’re a newbie in JavaScript or if you already know your stuff, knowing what a closure is will make you realize much more of how things are going down under the hood.
So let’s start from scratch!
What Is a Closure?
A closure is a function that “remembers” the environment in which it was created. This means that it has access to the variables of the outer function even if the latter has completed execution.
Think of it as a snapshot of the environment, storing key information regarding the variables available to the function.
Why Should You Care About Closures?
Closures are powerful because they enable functions to “capture” values from their surrounding scope. This becomes super useful for scenarios, like encapsulation, callbacks, and even private data management.
But let’s make this a little less abstract by seeing an example!
Lexical Scoping – The Principle of Closures
JavaScript functions execute in a lexical environment. So, when you declare a function inside another one, the inner will be able to reference the outer’s variables.
Here is an example so you get the point
In the example above, the inner function can access the outerVar even after outer() finishes running. That’s because the inner function remembers the scope in which it was created—this is a closure in action!
The Power of Closures: A Real-Life Analogy
Think of a closure like a post- A note that’s attached to a function. When a function is created, it takes a “note” of its environment (variables, functions, etc.) so that it can use them later.
Even if the environment changes or the function has finished executing, the function still keeps the note.
Closures in Action: Practical Examples
Example 1: Function Factories
Let’s say you need to create multiple functions that add a fixed number to a variable. A closure can help here!
In this example, the closure lets us create functions like add5 that “remember” the value of x (in this case, 5), so we can use it later in the inner function.
Example 2: Emulating Private Variables
A cool trick with closures is emulating private variables. Before JavaScript classes, we could use closures to “hide” data, providing encapsulation.
In this example, the count variable is private and cannot be accessed directly from outside the counter object. We can only interact with it through the public functions provided.
Block Scoping with let and const
ES6 introduced the ability to block-scoped variables, which otherwise could lead to tricky bugs caused by closures in JavaScript. It is now possible to declare a variable that falls within a given block using the let and const keywords.
This avoids all the unexpected behaviors caused by the var keyword.
With let, the variable i is scoped to each iteration of the loop, so the closure remembers the correct value for each setTimeout.
Common Closure Pitfall: Loop and var
Before ES6, closures created inside loops using var didn’t behave as expected.
Let’s see how to fix that!
Buggy Example (with var)
Solution (with let)
Using let in the loop makes sure each closure captures the correct value of “i” during the iteration.
closures in Asynchronous Code
Closures are extremely powerful when working with asynchronous code. For example, they let you “remember” the values in the surrounding scope after asynchronous actions (like timeouts or AJAX requests) are finished.
Even though setTimeout happens after 2 seconds, the closure enables us to remember and access the data variable inside the asynchronous function.
Performance Considerations
Closures can be powerful but use them judiciously. Creating too many closures unnecessarily (especially in tight loops or recursive functions) can cause memory leaks or performance issues.
Always evaluate whether a closure is needed or if you can use simpler solutions, such as using prototypes or passing data directly.
Conclusion: Why Closures Matter
Closures are a core part of JavaScript, allowing you to write clean, modular, and scalable code. They help you capture state, hide data, and manage functions in a more sophisticated way.
Whether you’re working on small projects or building large-scale applications, understanding closures will significantly improve your JavaScript skills.
We teach closures at Codeneur and all the major concepts of JavaScript through our Ultimate Full Stack Developer’s Course. Learn how to build applications full-fledged and take your skills to a new height!
Key Takeaways
- A closure is a function that can access its surrounding lexical environment even after the outer function has stopped execution.
- Lexical scoping states how functions can refer to variables depending upon their declaration location.
- Modern JavaScript’s let, const, and arrow functions make the closure even more powerful and convenient.
- By mastering closures, you will have the most powerful tool to write very efficient, well-organized, and robust JavaScript code!