Javascript Scopes

8 Key excerpts on "Javascript Scopes"

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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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  JavaScript for Web Warriors

  • Patrick Carey, Sasha Vodnik, Patrick Carey(Authors)
  • 2021(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  The other general type of scope is global scope, in which the variable or function is defined outside of any command block or function and, thus, is accessible throughout the entire program. Variables with local scope are called local variables, while variables with global scope are called global variables. In the following code the user variable is a global variable and thus is accessible within any command block or func- tion in the program: let user = "Dawson"; function showUser() { document.write(user); //writes Dawson } If a global variable and a local variable share the same name, the local variable takes precedence. However, the value assigned to a local variable within a function or command block is not transferred outside of that context. In the fol- lowing example the local value assigned to the user variable is limited to the scope of the function while the global value is unchanged outside of the function: let user = "Dawson"; function showUser() { let user = "Reynolds"; document.write(user); // writes Reynolds } document.write(user); // writes Dawson Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. UNDERSTANDING VARIABLE SCOPE 47 Global variables are fine for smaller applications developed and maintained by a single programmer. They make it easier to share data between functions, freeing the programmer from having to declare variables within every function and command block. However, their use is strongly discouraged for larger applications, especially for applications that are managed by a team of programmers.

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  Start Programming Using HTML, CSS, and JavaScript

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

    (Publisher)

  The scheme defining the fashion in which elements are selected for a comparison is described by a sorting algorithm, which you don’t need to know in order to be able to use sort() . Mike : It’s beginning to worry me that with several function definitions in my program I might accidentally choose a parameter name that already exists as a variable else-where in my code. That could easily happen as the size of my code grows, couldn’t it? Professor : Luckily, you are not the only one who has thought of that, so there ex-ist so-called scoping rules to help you manage the name conflicts like the one you hypothesized. 10.4 Variable Scope Professor : You know, not every variable is accessible anywhere in your program source code. The scope of a variable specifies precisely in which parts of the code the variable is defined. If you declare a variable outside of any function, then that is a global variable, which has global scope. Global variables are accessible anywhere 196 Meeting 10. Understanding Functions in your program. On the other hand, if you declare a variable within a function body, then you declare a local variable, which has local scope. This is also called function scope in JavaScript because such a variable is defined only within the function body in which it is declared. Note that function parameters act as local variables and are thus only accessible within the function body. Now, a question arises: what if there are two variables, one local and the other global, using the same name? According to the just-mentioned rules, both should be acces-sible within the function body in which the local variable is declared. To solve the conflict, there’s an additional rule which says that a local variable takes precedence over a global variable with the same name.

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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)

  NODE_VERSION = 10.08 ) anywhere in our program:

  $ console.log(process.env.NODE_VERSION); // "10.08"

  Local Scope

  Variables defined within a function or a block are in a local scope. Only those functions that are defined inside that function or block can have access to those variables.

  Function-Level Scope

  In JavaScript, each function has its own scope. All the variables and functions defined inside that function will have access to only each other: // Function A function parent(arg1, arg2) { let name = "gaurav"; let age = 24; function print(){ console.log(name, age); } } // Function B function print(){ console.log(name, age); // Error : name, age variable not defined. }

  Here, both functions have different levels of scope. Function B can't access the variables defined in Function A . The scope of both functions is highlighted in the following figure:

  Figure 13.10: Example of function-level scope

  Block-Level Scope

  Block-level scope is similar to function-level scope, but in this type of scope we do not initialize any functions. We can separate the scope of variables by creating blocks in JavaScript: // 1st block { let name = "gaurav"; let age = 24; console.log(name, age); } // 2nd block { console.log(name, age); } Note We use curly braces to create blocks in JavaScript. The second block can't access any variables created in the first block.

  Hoisting

  We now know that the scope of variables and functions depends on where they are declared, but there is an interesting concept in JavaScript, which is called hoisting

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  JavaScript for Modern Web Development

  Building a Web Application Using HTML, CSS, and JavaScript

  • Alok Ranjan, Alok Ranjan, Abhilasha Sinha, Ranjit Battewad(Authors)
  • 2020(Publication Date)
  • BPB Publications

    (Publisher)

  Parameters get passed as values. JavaScript passes arguments using the pass-by-value approach:

  • Whenever you pass arguments, firstly, the values get copied, and thereafter they get assigned to the corresponding parameters. Any change made to the parameter value will impact only the parameter within the function but not have any impact on the external variable (from the outer scope), which you used to pass to the function.
  • Scopes in functions: Functions cannot access variables in each other's scopes even if one function may be invoked in another.
  • Nested scopes: When a function is defined inside another function, the inner function can access the outer function's variables. This behavior is called lexical scoping, and the inner function is called a closure. The outer function cannot access the inner function: function outer () { const outerVar = ‘The outer variable!’; function inner() { console.log(outerVar); } return inner(); } outer();

  Play with functions and variables to see in action, what is in scope, and what is not, as the following example:

  Figure 4.3: Global and local variable scopes in functions and blocks

  Arrays

  Arrays are a list of values that are represented with an index starting from 0. These are commonly used data type to handle the list of values. Most often, you will have a list of data to handle, and that's where arrays are very useful as you can handle the entire list in a single variable. They also provide order to your data list with the use of an index. The list can be any data type like numbers, strings, Booleans, and also objects.

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  Learn C Programming

  A beginner's guide to learning C programming the easy and disciplined way

  • Jeff Szuhay(Author)
  • 2020(Publication Date)
  • Packt Publishing

    (Publisher)

  There are many instances where it is appropriate to limit the availability of a function or the accessibility of a variable. For instance, some functions may operate on a given structure and should only ever be called by other functions that also operate on that structure; these functions would never be called by any other functions. Similarly, we might want a value to be accessible to all functions within a program or we might want to limit its access to just a group of functions, or even a single function.

  The visibility of functions and variables is known as scope . The scope of a variable or function depends upon several factors with a program: its visibility , its extent , and its linkage . In this chapter, we will explore the different kinds of scope as they apply to variables and functions.

  The following topics will be covered in this chapter:

  • Being able to define the three aspects of scope: visibility, extent, and linkage
  • Understanding the scope of variables declared within statement blocks
  • Understanding the scope of variables declared outside of statement blocks
  • Understanding special cases of a variable's scope
  • Demonstrating statement-block variables, function-block variables, and file/global variables
  • Understanding compilation units
  • Understanding file scope
  • Understanding program scope

  Technical requirements

  As detailed in the Technical requirements section of Chapter 1 , Running Hello, World! , continue to use the tools you have chosen.

  The source code for this chapter can be found athttps://github.com/PacktPublishing/Learn-C-Programming .

  Defining scope – visibility, extent, and linkage

  Often, when thescope of a variable or function is mentioned, it is referring only to the visibility of the variable or function. Visibility essentially determines which functions or statements can see the variable to either access it or modify it. If the variable is visible, it can be accessed and modified, except—as you may recall from Chapter 4 , Using Variables and Assignment —when it is declared as aconst variable, it can only be accessed but cannot be changed. As we will see, visibility is but one component of a variable's scope. The other components of scope are extent (or the lifetime of the variable) and linkage

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  eBook - ePub

  Learn C Programming

  • Jeff Szuhay(Author)
  • 2022(Publication Date)
  • Packt Publishing

    (Publisher)

  There are many instances where it is appropriate to limit the availability of a function or the accessibility of a variable. For instance, some functions may operate on a given structure and should only ever be called by other functions that also operate on that structure; these functions would never be called by any other functions. Similarly, we might want a value to be accessible to all functions within a program or we might want to limit its access to just a group of functions, or even a single function.

  The visibility of functions and variables is known as scope . The scope of a variable or function depends upon several factors within a program: its visibility , its extent , and its linkage . In this chapter, we will explore the different kinds of scope as they apply to variables and functions.

  The following topics will be covered in this chapter:

  • Being able to define the three aspects of scope: visibility, extent, and linkage
  • Understanding the scope of variables declared within statement blocks
  • Understanding the scope of variables declared outside of statement blocks
  • Understanding special cases of a variable's scope
  • Demonstrating statement-block variables, function-block variables, and file/global variables
  • Understanding compilation units
  • Understanding file scope
  • Understanding program scope

  Technical requirements

  As detailed in the Technical requirements  section of Chapter 1 , Running Hello, World! , continue to use the tools you have chosen.

  The source code for this chapter can be found at https://github.com/PacktPublishing/Learn-C-Programming-Second-Edition/tree/main/Chapter25 .

  Defining scope – visibility, extent, and linkage

  Often, when the scope  of a variable or function is mentioned, it is referring only to the visibility of the variable or function. Visibility essentially determines which functions or statements can see  the variable to either access it or modify it. If the variable is visible, it can be accessed and modified, except—as you may recall from Chapter 4 , Using Variables and Assignments —when it is declared as a const  variable, it can only be accessed but cannot be changed. As we will see, visibility is but one component of a variable's scope. The other components of scope are extent  (or the lifetime of the variable) and linkage

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  Professional JavaScript for Web Developers

  • Matt Frisbie(Author)
  • 2019(Publication Date)
  • Wrox

    (Publisher)

  const at the top level are not defined in the global context, but they are resolved identically on the scope chain. When an execution context has executed all of its code, it is destroyed, taking with it all of the variables and functions defined within it (the global context isn't destroyed until the application exits, such as when a web page is closed or a web browser is shut down).

  Each function call has its own execution context. Whenever code execution flows into a function, the function's context is pushed onto a context stack. After the function has finished executing, the stack is popped, returning control to the previously executing context. This facility controls execution flow throughout an ECMAScript program.

  When code is executed in a context, a scope chain of variable objects is created. The purpose of the scope chain is to provide ordered access to all variables and functions that an execution context has access to. The front of the scope chain is always the variable object of the context whose code is executing. If the context is a function, then the activation object is used as the variable object. An activation object starts with a single defined variable called arguments. (This doesn't exist for the global context.) The next variable object in the chain is from the containing context, and the next after that is from the next containing context. This pattern continues until the global context is reached; the global context's variable object is always the last of the scope chain.

  Identifiers are resolved by navigating the scope chain in search of the identifier name. The search always begins at the front of the chain and proceeds to the back until the identifier is found. (If the identifier isn't found, typically an error occurs.)

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