How can closures improve functional programming?

1 What are closures?

A closure is a function that can access and manipulate variables that are defined outside of its own scope, even after the outer function has returned. For example, in JavaScript, you can create a closure by defining a function inside another function and returning it. The inner function will have access to the variables of the outer function, even if they are not passed as arguments. Here is a simple example of a closure in JavaScript:

function makeCounter() {
  let count = 0; // this variable is in the scope of makeCounter
  return function() {
    count++; // this function can access and modify count
    return count;
  };
}
let counter = makeCounter(); // counter is a closure
console.log(counter()); // 1
console.log(counter()); // 2
console.log(counter()); // 3        

2 How can closures help functional programming?

One of the benefits of closures is that they can help you write more concise and expressive code by avoiding the need to pass extra arguments or create global variables. For example, if you want to create a function that adds a fixed amount to a given number, you can use a closure to store the amount in the outer scope and return a function that takes the number as an argument. This way, you can create different functions with different amounts without repeating the logic. Here is an example of using a closure to create a function that adds 10 to a number:

function makeAdder(amount) {
  return function(number) {
    return number + amount;
  };
}
let addTen = makeAdder(10); // addTen is a closure
console.log(addTen(5)); // 15
console.log(addTen(20)); // 30        

3 How can closures enable currying?

Another benefit of closures is that they can enable currying, a technique that allows you to transform a function that takes multiple arguments into a sequence of functions that each take one argument. Currying can help you create more modular and reusable code by allowing you to partially apply some arguments and defer the execution of the function until all the arguments are provided. For example, if you have a function that takes three arguments and returns their sum, you can use currying to create a function that takes one argument and returns another function that takes the second argument, and so on. Here is an example of using currying to create a function that adds three numbers:

function addThree(a, b, c) {
  return a + b + c;
}
function curry(fn) {
  return function(a) {
    return function(b) {
      return function(c) {
        return fn(a, b, c);
      };
    };
  };
}
let addThreeCurried = curry(addThree); // addThreeCurried is a closure
console.log(addThreeCurried(1)(2)(3)); // 6
console.log(addThreeCurried(4)(5)(6)); // 15        

5 What are some challenges of closures?

While closures can improve functional programming, they also come with some challenges that you need to be aware of. One of the challenges is that closures can create memory leaks if they hold references to variables that are no longer needed. For example, if you create a closure inside a loop and assign it to a variable, you might end up creating multiple closures that keep the loop variables in memory, even after the loop is done. To avoid this, you need to be careful about how you use closures and when you release them. Another challenge is that closures can make debugging harder if you are not familiar with how they work and how they affect the scope chain. For example, if you use a debugger to inspect a closure, you might see variables that are not in the current scope but are in the outer scope. To overcome this, you need to understand how closures capture and access variables and how they relate to the lexical environment.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?