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Variable Scope / Closures

Advanced Functions: Variable Scope / Closures

What is variable scope in JavaScript?​

View Answer:
Interview Response: Variable scope in JavaScript refers to the context in which a variable is accessible or visible within a program, determined by where it's declared (global, function, or block scope).

What are the three types of variable scope in JavaScript?​

View Answer:
Interview Response: The three types of variable scope in JavaScript are global, function, and block scope, each with its own rules for variable accessibility and visibility.

What are the three types of variable declarations in JavaScript?​

View Answer:
Interview Response: The three types of variable declarations in JavaScript are var, let, and const, each with its own rules for variable assignment, reassignment, and scope.

Code Example: Declaration

let arr = [1, 2, 3];

const str = 'Hello';

var num = 1;

How do the keywords "var", "let", and "const" affect variable scope?​

View Answer:
Interview Response: "var" creates function or global scoped variables, while "let" and "const" create block scoped variables, offering better control over variable visibility and usage.

What is hoisting in JavaScript and how does it affect variable scope?​

View Answer:
Interview Response: Hoisting is JavaScript's mechanism of moving variable and function declarations to the top of their scope before code execution, affecting variable initialization and function availability.

What is shadowing in the context of variable scope in JavaScript?​

View Answer:
Interview Response: Shadowing occurs when an inner-scoped variable has the same name as an outer-scoped variable, causing the inner variable to temporarily "hide" or "shadow" the outer variable within the inner scope.

What are the best practices for managing variable scope in JavaScript?​

View Answer:
Interview Response: Best practices include using "let" and "const" over "var", minimizing global variables, keeping functions small, and avoiding unintended closures to prevent memory leaks and improve code readability.

What is a code block or block statement (compound statement) in JavaScript?​

View Answer:
Interview Response: A code block, also known as a block statement or compound statement, is a group of statements enclosed by curly braces in JavaScript. We can use a block statement or compound statement to group zero or more statements encapsulated in curly brackets.

Code Example: Code Block

// do some job with local variables that should not be seen outside
{
  let message = 'Hello'; // only visible in this block

  console.log(message); // Hello
}

console.log(message); // Error: message is not defined

// We can use this to isolate a piece of code
{
  // show message
  let message = 'Hello';
  console.log(message); // returns Hello
}

{
  // show another message
  let message = 'Goodbye';
  console.log(message); // returns Goodbye
}

What are the benefits of a JavaScript code block or block statement?​

View Answer:
Interview Response: JavaScript code blocks, enclosed by curly braces, provide organization, scope control, and readability. They enable conditional execution, loops, and function declarations, promoting maintainability, modularity, and structured programming.

Code Example: No code block control results in an error

// show message
let message = 'Hello';
console.log(message);

// show another message
let message = 'Goodbye'; // Error: variable already declared
console.log(message);

Does JavaScript support nested functions? How do they work?​

View Answer:
Interview Response: Yes, JavaScript supports nested functions. They are functions defined inside another function, having access to the outer function's variables and parameters, enabling closure, encapsulation, and modular code organization.

Code Example:

function sayHiBye(firstName, lastName) {
  // helper nested function to use below
  function getFullName() {
    return firstName + ' ' + lastName;
  }

  console.log('Hello, ' + getFullName());
  console.log('Bye, ' + getFullName());
}
note

Nested functions are pretty standard in JavaScript because of their dynamic abilities, which allow an extension of the originating function.

Can you explain what is the Lexical Environment is in JavaScript?​

View Answer:
Interview Response: The Lexical Environment is a theoretical specification object, comprising the environment record (holding identifier-variable mappings) and a reference to the outer lexical environment, explaining scope and closure behavior.

Technical Response: "Lexical Environment" is a specification object: it only exists "theoretically" in the language specification to describe how things work. We cannot access this object in our code and manipulate it directly. JavaScript engines may also optimize it, discard unused variables to save memory, and perform other internal tricks if the visible behavior remains as described. The Lexical Environment object consists of two parts, including the environment record and a reference to the outer lexical environment. The Environment Record is an object that stores local variables as its properties (and some other information like the value of this).
note

The Environment Record is an object that stores all local variables as its properties and some other information like the value of this.

Can you explain how a variable works in the context of a JavaScript engine?​

View Answer:
Interview Response: A variable is a symbolic container for storing data. It's created using keywords (var, let, const) and given a name. The JavaScript engine allocates memory, associates the name with the memory address, and stores/retrieves values.

Code Example:

let word = 'Hello';
// Lexical Environment Record --> property = word: "Hello"; outer --> null

// ENGINE EXECUTION

// execution start --> word: <uninitialized> - outer --> null
let word; // word: undefined
word = 'Hello'; // word: "Hello"

How does a function declaration behave differently from a variable declaration during the initialization process?​

View Answer:
Interview Response: Function declarations are hoisted and fully initialized upon script load, while variable declarations are hoisted but remain uninitialized until assigned a value during code execution.
note

You should be aware that this only applies to function declarations, not function expressions.

In JavaScript, what is a closure?​

View Answer:
Interview Response: A closure in JavaScript is a function that has access to its own scope, the outer function's scope, and the global scope, even after the outer function has finished execution.

Technical Response: A closure is a function that remembers its outer variables and can access them. In some languages, that is impossible, or a function is written in a special way to make it happen. In JavaScript, all functions are naturally Closures (there is only one exception, The "new Function" syntax). Functions automatically remember where their point of creation is using a hidden [[Environment]] property, and then their code can access outer variables.
note

This should not be confused with a code block.

How does garbage collection work with the Lexical Environment and nested functions in JavaScript?​

View Answer:
Interview Response: In JavaScript, garbage collection occurs when an object is unreachable. For nested functions, if an outer function's variables are referenced by a nested function (closure), they're not garbage collected.

Technical Response: After the function call completes, a Lexical Environment typically disappears from memory and all variables. This behavior occurs because there are no references to it, and it is only retained in memory while it is accessible, just like any other JavaScript object. If a nested function is reached after the end of a function, it contains the [[Environment]] attribute, which refers to the lexical environment. In such a situation, the Lexical Environment is still available even after the function completes, therefore, it remains alive. When a Lexical Environment object becomes inaccessible, it dies (like any other object). In other words, it persists as long as there is at least one nested function that refers to it.

Code Example:

function f() {
  let value = 123;

  return function () {
    console.log(value);
  };
}

let g = f(); // g.[[Environment]] stores a reference to the Lexical Environment
// of the corresponding f() call

function f() {
  let value = 123;

  return function () {
    console.log(value);
  };
}

let g = f(); // while g function exists, the value stays in memory

g = null; // ...and now the memory is cleaned up

What are the common use cases of closures in JavaScript?​

View Answer:
Interview Response: Common use cases for closures include data encapsulation, function factories, memoization, implementing decorators, and maintaining state in asynchronous programming.

What are the advantages of using closures in JavaScript?​

View Answer:
Interview Response: Closures in JavaScript enable data privacy, function factories, and use of function state after execution. They help create powerful, maintainable and modular code, benefiting from JavaScript's functional nature.

What are the disadvantages or pitfalls when working with closures in JavaScript?​

View Answer:
Interview Response: Disadvantages of closures include potential memory leaks due to retained references to outer environments and increased complexity in code readability and debugging.

How do closures affect JavaScript's garbage collection mechanism?​

View Answer:
Interview Response: Closures retaining references to outer environments prevent garbage collection until the closure itself becomes unreachable, potentially causing memory leaks if not managed properly.

How can you mitigate memory leaks when working with closures in JavaScript?​

View Answer:
Interview Response: To mitigate memory leaks with closures, ensure proper release of references to outer environments by nullifying them when no longer needed and avoid creating unnecessary closures.

Is the lexical environment a theoretical object?​

View Answer:
Interview Response: The lexical environment is a conceptual, theoretical object used to describe the way scoping and variable lookup work in JavaScript. It represents the mapping between variable names and their values within a given scope.