Javascript Closure in Five Levels

Introduction

To understand the concept of closures in JavaScript, it’s beneficial to have an understanding of the following concepts, listed in an approximate order from basic to more complex:

  1. Variables: Understanding what variables are and how they are used to store and manipulate data is fundamental before going further into more advanced concepts.

  2. Functions: Understanding how to declare functions, call them, and pass data to them (through parameters and arguments) is crucial. This includes understanding the return statement.

  3. Scope: Scope refers to the visibility or accessibility of variables, functions, and objects in some particular part of your code during runtime. This encompasses both global and local scope.

  4. Nested functions: Closures make the most sense in the context of nested functions (a function within another function). Understanding how these work is key before diving into closures.

  5. The this keyword: While not always necessary for understanding closures, getting a grasp of the this keyword can help, especially as it can behave differently inside a closure.

After grasping these concepts, you’ll be well-prepared to dive into closures and understand their usage and benefits within JavaScript.

Socrates Teaches Closure

The Socratic method encourages a dialog where I pose questions and you come up with the answers, promoting critical thinking and illuminating ideas. Let’s start:

  1. Do you understand the concept of a function in programming? Could you explain it briefly?

  2. Good, now can you tell me what local variables are and their scope inside a function?

  3. Perfect. Now, imagine you’ve created a function inside another function. What do you think the scope of the inner function would be? Would it be able to access the variables of the outer function?

  4. That’s right. Now, suppose that the outer function returns the inner function, and you assign this returned function to a variable. Now if you call this variable like a function, do you think it will still have access to the variables in the outer function, even after the outer function has finished execution?

  5. That’s exactly what a closure is! It’s the ability of a function to ‘remember’ and have access to variables from its parent scope, even after the parent function has finished execution. Can you see how this might be useful in programming?

Closure in Javascript

The concept of closures in JavaScript at different levels of complexity:

1. To a Child:

Imagine you have a magic box that keeps your secret treasures. Even when you show the box to your friends, they can’t see or touch what’s inside - only you can. In JavaScript, a closure is like this magic box. It’s a special kind of function that can remember and access variables from another function, even after that function has finished running.

2. To a Teenager:

You know when you’re texting with a friend, and you both understand a private joke or a code word that no one else gets? A closure in JavaScript is like that. It’s a function that can remember and use variables that were defined in another function, even after that function has finished running. It’s like having access to a secret code that only your function knows about.

3. To an Undergraduate Computer Science Major:

In JavaScript, a closure is a function that has access to its own scope, the outer function’s scope, and the global scope. This allows the function to “remember” the environment in which it was created, even after the outer function has finished executing. Closures are commonly used to create private variables or functions, implement function factories, and in other patterns where you need to encapsulate data and behavior.

4. To a Graduate Computer Science Student:

Closures in JavaScript are a fundamental concept that underpins a number of important JavaScript features and patterns. They occur when a function is declared inside another function and has access to its outer function’s variables even after the outer function has returned. This is a result of JavaScript’s lexical scoping and first-class function support, which allows functions to be returned as values and maintain their lexical environment. Closures are used in module patterns, currying, function factories, and many other advanced JavaScript concepts.

5. To a Colleague (Professional Developer):

A closure in JavaScript is the combination of a function bundled together with references to its surrounding state or lexical environment. This includes any local variables that were in-scope at the time the closure was created. Closures are commonly used in JavaScript for data privacy, in event handlers and callback functions, and in any situation where you need to preserve state over the lifespan of an asynchronous operation. They provide an excellent way to create data that’s isolated and protected, while still being accessible to a specific function or set of functions.

Closure Explained using Ladder of Abstraction

Climbing the Ladder of Abstraction for the concept of closure in programming:

Ground Level: Concrete Example

Let’s start with a very concrete example. Suppose you have a function in your code that calculates sales tax. The rate of tax depends on the state, but once defined, it remains the same for all calculations. You can create a function for this calculation that ‘remembers’ the rate for a state even after the function is executed.

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def tax_calculator(tax_rate):
    def calculate_tax(amount):
        return amount * tax_rate
    return calculate_tax

tax_in_NY = tax_calculator(0.08)
print(tax_in_NY(100)) # This will output 8.0

Here, tax_calculator is creating a calculate_tax function that ‘remembers’ the tax_rate for NY. This ‘remembering’ is the closure.

Step Up: More Abstract

At a more abstract level, closure is a technique in programming where a function has access to a value from its containing lexical scope, even when it’s called outside that scope.

Step Up: Abstract Definition

Closures in programming refer to the ability of a function to preserve and access variables from its parent scope, even after the parent function has finished execution. They are a way to carry state around with a function.

Step Up: Philosophical Level

At a philosophical level, closures represent a blend of data and behavior. They embody the principle that sometimes, what you do (the function) is intrinsically tied to what you know (the data). By bundling them together, we ensure that the function always has the context it needs to execute correctly, thus promoting more modular and understandable code.

Top of the Ladder: In Relation to Other Concepts

At the top, we see closures in the context of larger programming concepts. They’re closely tied to the idea of lexical scoping and are a key feature of functional programming languages. They enable techniques like function factories and decorators, and play a crucial role in effective asynchronous programming, such as in JavaScript callbacks and promises.

Richard Feynman Explanation

Alright, let’s put this into a fun context to explain closures, Feynman style.

Think about a circus, where there’s this magician who performs a really unique magic trick. The magician has a special box - let’s call it the “MagicBox”. Now, this MagicBox can do something pretty fantastic. You give it a number, and it will remember that number and add to it.

Let’s say you give the MagicBox the number 3. The MagicBox takes that 3, and from that moment on, whenever you ask it to, it will add to that number. Ask it to add 5, and it’ll tell you the result is 8.

Now, here’s where the real magic comes in. You see, even after the magician finishes the trick, packs up the MagicBox, and leaves the stage, the MagicBox still remembers that number 3. Even if you open the MagicBox in another city, on another day, it still remembers that number. That’s pretty incredible, right?

In JavaScript, this MagicBox is what we call a “closure”. It’s a function that remembers the “surroundings” or “environment” in which it was created, even after it leaves that environment.

Just like the MagicBox remembers the number 3, a closure in JavaScript remembers the variables that were in scope when the function was created. It “closes over” the variables it needs, hence the name “closure”.

So, if we create a function within another function, the inner function forms a closure. It can access and remember the variables of the outer function, even after the outer function has finished running.

This magic trick - it’s not really magic, of course. It’s just the way JavaScript works. But it’s a handy trick to have up your sleeve, don’t you think?

Closure and Functional Programming

You said functional programming separates data from logic. However it carry around the state due to closure. So it does seem to encapsulate its own state with logic. Is this correct?

Yes, you’re correct that closures in functional programming do allow for the encapsulation of state with logic. However, this isn’t the primary or typical way that data and logic are handled in functional programming.

In functional programming, data and logic are often kept separate because functions are typically designed to be “pure,” meaning that they don’t have any side effects and their output is determined solely by their input. This is a different model than in object-oriented programming, where methods often operate on and modify the state of the objects they belong to.

Closures can be used to encapsulate state in a functional style of programming, and this can be a useful tool for creating things like private variables or factory functions. But even then, the general principle of keeping data and logic separate still mostly applies. The data in the closure isn’t usually being transformed or modified by the function, but rather, it’s being captured and used by the function in a predictable and repeatable way.

However, as with many aspects of programming, there’s a great deal of flexibility and variation, and different situations call for different techniques. So while functional programming often promotes the separation of data and logic, there are indeed scenarios, like closures, where the two can be more tightly coupled.

Closure and Coding

Developers don’t think about closure when programming. It’s not like I need some closure here, let’s use this coding construct. So what problem does closure in functional programming solve?

Closure is a powerful concept in functional programming and it offers a number of benefits.

One major use of closures is in data encapsulation and privacy. In languages that don’t have built-in support for private properties or methods, closures can be used to emulate them. Here’s an example:

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function makeCounter() {
    let count = 0; // this variable is "private" - it's inaccessible outside this function

    return function() {
        return count++; // but it can be accessed and modified by this function
    };
}

const counter = makeCounter();
console.log(counter()); // 0
console.log(counter()); // 1

In this case, makeCounter returns a function, and that function has “closed over” the count variable - it has access to count, even after makeCounter has finished executing.

Another common use of closures is in event handlers and callbacks. When you register an event handler, it often needs access to the context in which it was created, and closures allow that:

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let name = "Alice";

button.addEventListener('click', function() {
    console.log("Hello, " + name + "!");
});

In this case, the function we’re passing to addEventListener closes over the name variable. Even if we change name later, the function will still remember the value of name at the time it was created.

Closures are also very useful in functional programming constructs like function factories (i.e., functions that return other functions), and decorators (i.e., functions that take a function as input, modify it in some way, and return the modified function).

So, while developers might not think explicitly about creating a closure, they are often using them implicitly when they use functions as values in JavaScript. It’s a fundamental concept in functional programming, and understanding it can help write more expressive and powerful code.

Problems

A closure in JavaScript is a function that has access to its own scope, the outer function’s scope, and the global scope. Closures can solve various problems or enhance the functionality of JavaScript applications. Here are some key examples:

  1. Data Privacy and Encapsulation: Closures can help to emulate private methods or variables, a fundamental concept in object-oriented programming that JavaScript does not natively support.
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function counter() {
  let count = 0;
  return function() {
    count++;
    return count;
  };
}

let incrementCounter = counter();

console.log(incrementCounter()); // Outputs: 1
console.log(incrementCounter()); // Outputs: 2

In this example, count acts like a private variable. It’s not accessible from outside the counter function, but it’s accessible and can be modified by the inner function that counter returns.

  1. Preserving State: Closures allow you to preserve a function’s state, even after the function has returned. This can be very useful for data that needs to be used across multiple function calls.
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function greet(name) {
  return function(message) {
    console.log(`${message}, ${name}!`);
  };
}

let greetJohn = greet('John');

greetJohn('Hello'); // Outputs: Hello, John!
greetJohn('Goodbye'); // Outputs: Goodbye, John!

In this example, the name parameter’s value is preserved in the returned function, even after greet has finished executing.

  1. Creating Function Factories: Closures can also be used to create function factories, functions that return other functions with specific behaviors.
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function multiplyBy(factor) {
  return function(num) {
    return num * factor;
  };
}

let double = multiplyBy(2);
let triple = multiplyBy(3);

console.log(double(5)); // Outputs: 10
console.log(triple(5)); // Outputs: 15

Here, multiplyBy is a function factory that creates and returns new functions for multiplying numbers by a specific factor. Each of the returned functions preserves its specific multiplication factor via a closure.

Closures can be a complex topic for newcomers to JavaScript, especially those without a background in computer science. However, they’re a powerful feature that can enable more flexible, efficient, and maintainable code.

Lazy Evaluation

Is lazy execution one of the key characteristics of Closure in functional programming languages?

Lazy evaluation, also known as call-by-need, is a strategy where expressions are not evaluated until their results are needed. However, it’s important to note that not all functional languages implement lazy evaluation. For example, languages like Haskell employ lazy evaluation by default, but others like Scheme, Python, or JavaScript, are eager (also known as strict) by default, meaning they evaluate expressions as soon as they are able to.

Closures, on the other hand, are a different concept that pertains to the scope of a function. A closure occurs when a function has access to variables from an outer function that has already finished its execution. This is possible because the function “closes over” or captures the outer function’s variables it refers to, hence the term closure.

While closures are a key characteristic of functional programming languages and play an important role in strategies such as higher-order functions and currying, they are not inherently related to lazy evaluation. That being said, closures can be used to achieve lazy evaluation in languages that don’t support it natively. For example, in JavaScript, you might use a closure to create a function that doesn’t compute its result until it’s called:

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function lazyAdd(a, b) {
    var result;
    return function() {
        if (result === undefined) {
            console.log("Computing result...");
            result = a + b;  // The computation is delayed until needed.
        }
        return result;
    };
}

var adder = lazyAdd(2, 3);
console.log(adder());  // "Computing result..." then "5"
console.log(adder());  // "5", no computation done this time

In this JavaScript code, lazyAdd returns a closure that encapsulates the computation a + b. This computation isn’t done until the returned function is called. If you call the function again, it remembers the result from the previous call and doesn’t recompute it. This is a form of lazy evaluation, implemented using closures.

In the context of functional programming, the concept of “Closure” is intimately linked with several other concepts:

  1. First-Class Functions: In languages that support first-class functions (functions that can be passed around like any other variable), closures become possible. This is because closures essentially “capture” variables from their surrounding scope, and if functions can’t be treated as variables, you can’t have closures.

  2. Lexical Scoping: Closures are a consequence of lexical scoping (or static scoping). This means that functions have access to variables defined in their outer scope, even if the function is executed in a different context. It’s the principle that makes closures work.

  3. Higher-Order Functions: These are functions that accept other functions as parameters, or return them as results. Closures are often used in the context of higher-order functions, because they allow the “inner” function to maintain access to its original scope, even when it’s being passed around as a parameter or return value.

  4. Immutability: In functional programming, immutability (the idea that a variable, once created, cannot change) is a key principle. Closures fit well with this concept because they capture and maintain state from their surrounding scope in a way that can’t be tampered with from outside the closure.

  5. Currying and Partial Application: These are techniques often used in functional programming where a function with multiple arguments is transformed into a series of functions each with a single argument. Closures are used in both techniques as they allow each function in the sequence to remember the arguments of the previous functions.

  6. Lambda Calculus: This is the theoretical foundation of functional programming, and closures can be seen as a practical implementation of some of the concepts from lambda calculus.

Each of these concepts is related to the idea of closures in functional programming and contributes to understanding them more fully.