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How Closures Work and Why It Matters

Level of difficulty Medium
Reading time 8 min
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They might sound complicated, but they are actually a fundamental part of the language. In this article, we’ll explore closures in a straightforward and practical way. Let’s clear up common misunderstandings. Walk through real-world examples. Nail those tricky interview questions about closures. By the end, you’ll see closures not as a hurdle, but as a valuable part of your JavaScript toolkit.

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Common Misconceptions Debunked: Clarifying What Closures Are Not

Understanding closures in JavaScript is crucial, especially in a dynamic work environment like Luxoft. Thanks to our internal courses, which keep us updated on the latest in JavaScript, I’ve deepened my understanding of closures beyond the common myths. Let’s debunk some of these myths with clear examples:

Myth 1: Closures Are Functions Returning Functions

Example:

function sayAlex() {
  console.log("alex");
}
function nameGenerator() {
  return sayAlex;
}

var myName = nameGenerator();
myName();

Reality: Here, nameGenerator returns the function sayAlex, but there’s no closure because nothing is closed over the function. A closure involves a function retaining access to its lexical scope, not just returning another function.

Myth 2: Closures Are About Functions Remembering Their Own Variable Values

Example:

function nameGenerator() {
  var name = "alex";
  function sayAlex() {
    console.log(name);
  }

  name = "jane";
  return sayAlex;
}

var myName = nameGenerator();
myName(); // Logs 'jane'

Reality: This example demonstrates a closure. The function sayAlex closes over the variable name, not its value at the time of definition. The closure retains a reference to the variable, which is why it logs ‘jane’ instead of ‘alex’.

Myth 3: You Can Only Create a Closure with a Function Expression

Reality: Closures can be formed with both function declarations and expressions. The defining characteristic of a closure is its ability to access variables from an outer function’s scope, regardless of how the function was defined.

Myth 4: Closures Are Complex and Rarely Needed

Reality: Closures are a fundamental part of JavaScript, essential in many common programming scenarios. They are frequently used in:

  • Event Handlers: To remember the state of certain elements.

  • Callbacks: Especially in asynchronous operations.

  • Modules: To create private variables and functions.

  • Memoization: Storing the results of expensive function calls.

  • Libraries/Frameworks: Many JavaScript libraries use closures.

  • Bundlers like Webpack: To manage module references.

Understanding closures is key to excelling in JavaScript programming and technical interviews. They are not just an advanced topic but a daily tool in a developer’s toolkit. In the next chapter, we’ll explore practical examples of closures at various experience levels.

Closures in Action: Practical Examples

This chapter showcases how closures function at different levels of coding expertise. These examples will show the versatility and power of closures in JavaScript.

Junior Level: The Slot Machine Function

Consider a simple slot machine game function:

function createSlotMachine() {
  var symbols = ["🍒", "🍍", "🍌"];
  var combination = [];

  function spin() {
    combination = [];
    for (var i = 0; i < 3; i++) {
      var randomIndex = Math.floor(Math.random() * symbols.length);
      combination.push(symbols[randomIndex]);
    }
    console.log(`Combination: ${combination.join("")}`);
  }

  return spin;
}

var spin = createSlotMachine();

// Example usage:
spin(); // 'Combination: 🍒🍌🍍'
spin(); // 'Combination: 🍍🍍🍌'
spin(); // 'Combination: 🍌🍒🍍'

In this example, spin is a closure that remembers the variable of combination from its parent function createSlotMachine. Each time spin is called, it accesses and modifies the combination variable, illustrating how closures capture local state from their containing function.

Middle Level: The Wrapper Function

At a slightly more advanced level, consider a function that uses setTimeout:

function createDebouncedAlert() {
  var timeoutId: NodeJS.Timeout | null = null;
  var delay = 3 * 1000; // 3 seconds

  function debounceAlert(message: string) {
    if (timeoutId) {
      clearTimeout(timeoutId); // Clear the previous timer if it exists
    }

    timeoutId = setTimeout(function () {
      console.log(message);
      timeoutId = null; // Reset the timer ID
    }, delay);
  }

  return debounceAlert;
}

const debounceAlert = createDebouncedAlert();

// Example usage:
debounceAlert("Alert 1");
debounceAlert("Alert 2");
debounceAlert("Alert 3"); // only the last one is shown

Here, debounceAlert is a closure that captures the value of timeoutId from createDebouncedAlert. Despite the asynchronous setTimeout, each debounceAlert function remembers the correct value of its createDebouncedAlert. This illustrates closures in an asynchronous context, a common scenario in real-world JavaScript applications.

Senior Level: Unique Execution Contexts

For a more advanced example:

function createTraffic() {
  var vehicles = ["🚐", "🚕", "🚗", "🚎", "🛵"];
  var traffic = [];

  function increasePopulation() {
    var randomIndex = Math.floor(Math.random() * vehicles.length);
    traffic.push(vehicles[randomIndex]);
    console.log(traffic.join(" "));
  }

  return increasePopulation;
}

var increasePopulation = createTraffic();
increasePopulation(); // '🚗'
increasePopulation(); // '🚗 🚕'

var decreasePublicTransport = createTraffic();
decreasePublicTransport(); // '🚐'
decreasePublicTransport(); // '🚐 🚕'

This example highlights execution contexts created by closures. Each createTraffic call generates a new closure with its own traffic variable. The increasePopulation and decreasePublicTransport instances are separate, demonstrating how closures can encapsulate state uniquely across different instances.

These practical examples show how closures are not theoretical constructs but powerful tools for maintaining state, handling asynchronous operations, and creating modular and maintainable code. In the next chapter, we’ll explore real-world applications of closures to provide further context on their usefulness in everyday programming.

Applications of Closures: Real-World Scenarios

In this chapter, we’ll delve into the real-world applications of closures. By understanding how closures are used in everyday coding, you’ll be better prepared for technical interviews and practical JavaScript development.

Data Privacy and Encapsulation

One of the primary uses of closures is to create private data and methods. This is crucial in many programming paradigms, especially in modular JavaScript design. Consider a simple module:

function createCounter() {
  var count = 0;

  return {
    increment: function() { count++; },
    getCount: function() { return count; }
  };
}

var counter = createCounter();
counter.increment();
console.log(counter.getCount()); // 1

In this module, count is private. External code can’t access or change it, only through increment and getCount. This pattern is used in creating APIs and modules, where you need to protect the internal state from external interference.

Event Handling

Closures are used in event handling. They allow the event handler to access the context in which it was created. For instance:

function setupAlert(buttonId, alertMessage) {
  var button = document.getElementById(buttonId);
  button.addEventListener('click', function() {
    alert(alertMessage);
  });
}

Here, the anonymous function inside addEventListener is a closure. It has access to alertMessage from the outer setupAlert function.

Callbacks and Asynchronous Programming

Closures are integral to asynchronous programming, especially with callbacks and promises. They ensure that callback functions have access to the surrounding local variables at the time they are executed.

function fetchData(callback) {
  // Fetch some data asynchronously
  setTimeout(() => {
    callback('Data retrieved');
  }, 1000);
}

fetchData(data => {
  console.log(data); // 'Data retrieved'
});

In this example, the callback function is a closure that accesses the data variable when it’s executed.

Memoization

Closures allow the implementation of memoization, an optimization technique that caches the results of expensive function calls:

function memoize(fn) {
  var cache = {};
  return function(...args) {
    var key = JSON.stringify(args);
    if (!cache[key]) {
      cache[key] = fn.apply(this, args);
    }
    return cache[key];
  };
}

var complexCalculation = memoize(function(arg) {
  // Perform some complex calculation here
});

In this pattern, the inner function has access to cache, making it possible to store and retrieve values based on arguments.

Libraries and Frameworks

Many JavaScript libraries and frameworks make extensive use of closures. For example, jQuery uses closures for event handling and managing state. React leverages closures for hooks like useState and useEffect, allowing components to maintain state across renders.

Module Management in Bundlers

Bundlers like Webpack use closures to wrap modules. Each module is wrapped in a function closure, which helps in isolating module scope and managing dependencies.

Through these examples, closures emerge as practical tools for JavaScript developers. At Luxoft, applying our understanding of closures within the Scrum methodology streamlines our development process, allowing for more efficient sprint planning and reduced coding time. Closures aid in data encapsulation, event handling, and asynchronous programming. This knowledge is vital for both technical interviews and daily coding, enhancing efficiency and innovation in our projects.

Typical Closure Questions and How to Answer Them

In this final chapter, we’ll explore some typical closure-related interview questions and how to answer them. These questions will not only test your understanding of closures but also your ability to apply them in practical scenarios.

Question 1: What is a Closure in JavaScript?

How to Answer:

Begin by defining a closure: A closure in JavaScript is when a function retains access to its lexical scope even when that function is executed outside of its original scope. Then, provide a simple example, like the createCounter function from a previous chapter, to illustrate the concept.

Question 2: Can Closures Be Used for Data Privacy?

How to Answer:

Affirm that closures are a great tool for data privacy in JavaScript. Use the module pattern as an example, where you return an object with methods to interact with private variables, without exposing the variables themselves.

Question 3: How Do Closures Work with Asynchronous Code?

How to Answer:

Explain that closures are very useful in asynchronous code, especially in callbacks and promises, as they allow the callback function to access the scope in which it was declared. Provide an example of using closures in an fetch request or with setTimeout.

Question 4: Describe a Scenario Where Closures Might Be Preferable to Global Variables

How to Answer:

Discuss the benefits of closures in avoiding global variables, which can lead to code conflicts and security issues. Use an example like encapsulating functionality within a function to maintain state between function calls, without polluting the global namespace.

Question 5: How Do Closures Work in Loops, Especially with var and let?

How to Answer:

Explain that var shares the same variable across loop iterations, leading to closures capturing the last value of the variable. let creates a new binding for each iteration, allowing each closure within the loop to capture a unique copy of the loop variable. This difference is crucial for ensuring closures within loops behave as expected.

Question 6: How Are Closures Used in Functional Programming?

How to Answer:

Closures are a cornerstone of functional programming in JavaScript, particularly in higher-order functions and currying. Explain how closures enable functions to be returned from other functions with “remembered” environments.

Getting ready for these interview questions does more than just prepare you for interviews; it helps you really understand JavaScript closures. At Luxoft, I’m happy to share what I know about it. It’s important to not only know the answers in technical interviews but to explain them well, often using real examples. If you’re interested, I can help guide you through Luxoft’s technical interview process and share more about what I’ve learned.

References

  1. “You Don’t Know JS” by Kyle Simpson

  2. Mozilla Developer Network (MDN) Web Docs on Closures

  3. ECMAScript Specification

  4. “ES6 In Depth: let and const” by Mozilla Hacks

  5. Flavio Copes’ Blog on JavaScript ES6

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