Javascript Closures

7 Key excerpts on "Javascript Closures"

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  Mastering JavaScript

  • Ved Antani(Author)
  • 2016(Publication Date)
  • Packt Publishing

    (Publisher)

  Traditionally, closures have been a feature of purely functional programming languages. JavaScript shows its affinity with such functional programming languages by considering closures integral to the core language constructs. Closures are gaining popularity in mainstream JavaScript libraries and advanced production code because they let you simplify complex operations. You will hear experienced JavaScript programmers talking almost reverently about closures—as if they are some magical construct far beyond the reach of the intellect that common men possess. However, this is not so. When you study this concept, you will find closures to be very obvious, almost matter-of-fact. Till you reach closure enlightenment, I suggest you read and reread this chapter, research on the Internet, write code, and read JavaScript libraries to understand how closures behave—but do not give up.

  The first realization that you must have is that closure is everywhere in JavaScript. It is not a hidden special part of the language.

  Before we jump into the nitty-gritty, let's quickly refresh the lexical scope in JavaScript. We discussed in great detail how lexical scope is determined at the function level in JavaScript. Lexical scope essentially determines where and how all identifiers are declared and predicts how they will be looked up during execution.

  In a nutshell, closure is the scope created when a function is declared that allows the function to access and manipulate variables that are external to this function. In other words, closures allow a function to access all the variables, as well as other functions, that are in scope when the function itself is declared.

  Let's look at some example code to understand this definition: var outer = 'I am outer'; //Define a value in global scope function outerFn() { //Declare a a function in global scope console.log(outer); } outerFn(); //prints - I am outer

  Were you expecting something shiny? No, this is really the most ordinary case of a closure. We are declaring a variable in the global scope and declaring a function in the global scope. In the function, we are able to access the variable declared in the global scope—outer . So essentially, the outer scope for the outerFn() function is a closure and always available to outerFn()

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  Abstraction and Closure in Computer Science

  • (Author)
  • 2014(Publication Date)
  • Learning Press

    (Publisher)

  ________________________ WORLD TECHNOLOGIES ________________________ Chapter 2 Closure (Computer Science) In computer science, a closure (also lexical closure , function closure or function value ) is a function together with a referencing environment for the nonlocal names (free variables) of that function. Such a function is said to be closed over its free variables. The referencing environment binds the nonlocal names to the corresponding variables in scope at the time the closure is created, additionally extending their lifetime to at least as long as the lifetime of the closure itself. The concept of closures was developed in the 1960s and was first fully implemented as a language feature in the programming language Scheme to implement lexically-scoped first-class functions. Since then, many languages have been designed to support closures. The explicit use of closures is associated with functional programming and with languages such as ML and Lisp. Imperative languages have traditionally had no support for closures as these languages neither support nonlocal names (which can be introduced only in nested or anonymous functions) nor higher-order functions. Closures are used to implement continuation passing style, and in this manner, hide state. Constructs such as objects and control structures can thus be implemented with closures. In some languages, a closure may occur when a function is defined within another func-tion, and the inner function refers to local variables of the outer function. At run-time, when the outer function executes, a closure is formed, consisting of the inner function’s code and references to any variables of the outer function required by the closure; such variables are called the upvalues of the closure. The term closure is often mistakenly used to mean anonymous function.

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  JavaScript for Sound Artists

  Learn to Code with the Web Audio API

  • William Turner, Steve Leonard(Authors)
  • 2017(Publication Date)
  • Routledge

    (Publisher)

  Closures 49 The first thing to notice is that the function is wrapped in parentheses This is optional, but is considered best practice because it helps differentiate the con- struct syntactically from non-IIFE functions (function run() { return "data"; }()); The next thing to notice is the parentheses toward the end of the function before the closing, encapsulating parenthesis This syntax is what invokes the function (function run() { return "data"; }()); To add parameters and arguments, you put parameters in the first set of parentheses and arugments in the second set of parentheses (function add(a, b) { //_____ parameters return a + b; }(2, 3)); //___________________arguments Closures One of the most difficult aspects of the JavaScript language for new program- mers to grasp is closures Understanding closures will ultimately allow you to write cleaner code while giving you a powerful tool to solve a host of prob- lems you will inevitably run into Understanding the concept of closure can be a bit difficult at first But in the long term, the benefits are worth the time investment What Is a Closure? A closure is an inner function that has access to the scope of its outer environ- ment even after that outer environment has returned To understand what this means, you must first solidify your understanding of scope The following exam- ple demonstrates how a function has access to its local scope, the global scope, and its local arguments var globalVariable = "global variable"; function doSomething(argInput) { var localVariable = "local variable"; console.log(argInput); console.log(globalVariable); console.log(localVariable); } doSomething("argument input"); /*_________This outputs: "argument input" "global variable" "local variable" because the function has access to its own scope and the outer scope.*/

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  The JavaScript Workshop

  A New, Interactive Approach to Learning JavaScript

  • Joseph Labrecque, Jahred Love, Daniel Rosenbaum, Nick Turner, Gaurav Mehla, Alonzo L. Hosford, Florian Sloot, Philip Kirkbride(Authors)
  • 2019(Publication Date)
  • Packt Publishing

    (Publisher)

  var keyword:

  Figure 13.12: var keyword usage

  We see that using the var keyword gives the same undefined output for both blocks.

  Closures

  A closure is a feature in JavaScript where a function defined inside another function has access to the parent function's variables. A closure has three scope chains:

  • Own scope : Variables defined between its curly brackets
  • Parent function: Properties defined in the parent function
  • Global variables: Properties defined in the global scope

  Let's have a look at an example: function outer(arg) { let count = 1; function inner() { console.log(arg, '=', count++);     } return inner; } var funA = outer('A');  // outer() invoked the first time var funB = outer('B');  // outer() invoked the second time funA(); // function funA called for first time funA(); // function funA called for second time funA(); // function funA called for third time funB(); // function funB called for first time

  We have one main function, which is outer . Then, we have declared a count variable with a value of 1 . We have one more function, inner , which is using and incrementing the value of count . Then, we return the inner function. The output of the preceding code is as follows:

  Figure 13.13: Example of closures

  When we first call the outer('A') function , we are creating a scope for the count variable and the inner function. Then, we return the inner function and save its address in funA() . We did the same with funB() .

  When we called funA() first, it had access to the count variable because it was in its parent function. So, it printed the value of count and updated it by adding 1 . When we called it again, it again accessed the parent scope and got the updated value of count . JavaScript is able to achieve this because of closures.

  In this section, we have learned about a lot of the basic features of JavaScript. We started with prototypes and used them to implement inheritance. Then, we learned about anonymous and named functions and how to use them. We also learned about different types of data scopes in JavaScript. Finally, we learned about hoisting and closures, which are among the most complicated and important features provided by JavaScript.

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  Basic Concepts in Computer Science (Data & Data Structures)

  • (Author)
  • 2014(Publication Date)
  • Learning Press

    (Publisher)

  (This last scenario, referring to exactly two subtrees, a left subtree and a right subtree, assumes specifically a binary tree.) Closure (computer science) In computer science, a closure is a first-class function with free variables that are bound in the lexical environment. Such a function is said to be closed over its free variables. A closure is defined within the scope of its free variables, and the extent of those variables is at least as long as the lifetime of the closure itself. The explicit use of closures is associated with functional programming and with languages such as ML and Lisp. Closures are used to implement continuation passing style, and in this manner, hide state. Constructs such as objects and control structures can thus be implemented with closures. The concept of closures was developed in the 1960s and was first fully implemented as a language feature in the programming language Scheme. Since then, many languages have been designed to support closures. In some languages, a closure may occur when a function is defined within another func-tion, and the inner function refers to local variables of the outer function. At runtime, when the outer function executes, a closure is formed, consisting of the inner function’s code and references to any variables of the outer function required by the closure; such variables are called the upvalues of the closure. ________________________ WORLD TECHNOLOGIES ________________________ The term closure is often mistakenly used to mean anonymous function. This is probably because most languages implementing anonymous functions allow them to form closures and programmers are usually introduced to both concepts at the same time. These are, however, distinct concepts. Closures are closely related to function objects; the transformation from the former to the latter is known as defunctionalization or lambda lifting.

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  Learning TypeScript 2.x

  • Remo H. Jansen(Author)
  • 2018(Publication Date)
  • Packt Publishing

    (Publisher)

  Closures are one of the most powerful features available at runtime, but they are also one of the most misunderstood. The Mozilla developer network defines closures as follows: "Closures are functions that refer to independent (free) variables. In other words, the function defined in the closure 'remembers' the environment in which it was created". We understand independent (free) variables as variables that persist beyond the lexical scope from which they were created. Let's look at an example:

  function makeArmy() { const shooters = []; for (let i = 0; i < 10; i++) { const shooter = () => { // a shooter is a function console.log(i); // which should display it's number }; shooters.push(shooter); } return shooters; }

  The preceding example is a JavaScript example, not a TypeScript example.

  We have declared a function named makeArmy. Inside the function, we have created an array of functions named shooters. Each function in the shooters array will display a number, the value of which was set from the variable i inside a for statement. We will now invoke the makeArmy function:

  const army = makeArmy();

  The variable army should now contain the array of the function's shooters. However, we will notice a problem if we execute the following piece of code:

  army[0](); // 10 (expected 0) army[5](); // 10 (expected 5)

  The preceding code snippet does not work as expected because we made one of the most common mistakes related to closures. When we declared the shooter function inside the makeArmy function, we created a closure without being aware of it.

  The reason for this is that the functions assigned to shooter are closures; they consist of the function definition and the captured environment from the makeArmy function's scope. We have created ten closures, but all of them the same environment. By the time the shooter functions are executed, the loop has run its course, and the i variable (shared by all the closures) has been left pointing to the last entry (10).

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  eBook - PDF

  Start Programming Using HTML, CSS, and JavaScript

  • Iztok Fajfar(Author)
  • 2015(Publication Date)
  • Chapman and Hall/CRC

    (Publisher)

  If that important variable is kept local and you provide a special function to control it, then unwanted changes to it are much less likely. The rule of thumb is that you try to avoid global variables unless there’s a really good reason for them. Maria : You mentioned closures. What exactly is a closure? 10.6. The Scope Chain and Closures 203 Professor : Oh, sorry, I forgot to give the formal definition. A closure is a function together with scope from which it can use variables. If you want, you can also check which variables are accessible inside the current closure in the DevTools: You should place a breakpoint inside the anonymous function and reload the page to see the situation like on this screenshot. Notice how the variable i is not a local variable of the anonymous function but is accessible inside its closure as you can see at the right side of the DevTools window. Incidentally, if you want i to get incremented to two, then you must step over several function calls by pressing F10 several times. However, if there was no breakpoint inside the function, then you wouldn’t be able to enter the function using the Step Over command. To enter a function, you can use the Step Into debugger command or press F11 . 10.7 Homework Professor : For homework I want you to write and use a function that will help you to generate a math worksheet for kids to practice addition, subtraction, multiplication, and division. The function should accept two arguments: the desired operation and the maximum allowed value for the two operands. Let the function generate and return a random equation in the form of a string together with the corresponding solution. You can pack both in a two-element array. Additionally, take care that the result of a subtraction is never negative and that the result of a division is always an integer with no remainder.

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