Test: Scope and Hoisting - 2 - Software Development MCQ

The scope of a variable in JavaScript defines the region or part of the program where the variable is accessible or visible. It also determines the lifetime or duration of the variable, including when it is created and when it is destroyed.

Global scope in JavaScript allows a variable to be accessed from anywhere within the program. Variables declared outside of any function or block have global scope. They are accessible throughout the entire program, including within functions, blocks, and other scopes.

Hoisting in JavaScript is a behavioral characteristic of the language where variable and function declarations are moved to the top of their containing scope during the compilation phase, before the actual execution of the code.

Hoisting affects both variable and function declarations, but not their assignments or initializations. It means that while the declarations are hoisted, the assignments or initializations remain in their original position.

In JavaScript, if a variable is declared without using the 'var', 'let', or 'const' keyword, it is automatically assigned a global scope. This behavior is known as implicit global variable declaration.

When a variable is declared without any explicit declaration keyword ('var', 'let', or 'const'), it becomes a global variable. This means that the variable is accessible from anywhere within the program, including inside functions, blocks, and other scopes.

The behavior of variable hoisting differs between "var" and "let" declarations in JavaScript.

1. Hoisting with "var": When a variable is declared using the "var" keyword, its declaration is hoisted to the top of the nearest function scope. This means that the variable declaration is moved to the top of its containing function, regardless of where it appears in the code. However, the assignment or initialization of the variable remains in its original position.

   Example:
   function example() {
     console.log(x); // Output: undefined
     var x = 5;
   }

2. In the above code, even though the variable x is accessed before its declaration, the declaration is hoisted to the top of the function scope. This is why the output of console.log(x) is undefined instead of throwing a ReferenceError. Hoisting with "let": When a variable is declared using the "let" keyword, its declaration is hoisted to the top of the nearest block scope (inside curly braces {}). This means that the variable declaration is moved to the top of the block in which it appears, regardless of its position within the block. Similar to "var" variables, the assignment or initialization of the variable remains in its original position.
   Example:
   function example() {
     console.log(x); // Throws ReferenceError: Cannot access 'x' before initialization
     let x = 5;
   }
   In the above code, accessing the variable x before its declaration throws a ReferenceError. Unlike "var" variables, "let" variables are not accessible before their declaration within the block scope.

To summarize, "var" variables are hoisted to the top of the function scope, while "let" variables are hoisted to the top of the block scope. Understanding these differences is crucial for managing variable visibility and preventing unintended behavior in JavaScript code.

Variable declarations with "var" are hoisted to the top, but the assignment is not hoisted. Thus, at the time of the console.log statement, the variable x exists but has not been assigned a value, resulting in undefined.

The variable x is declared after the function call, so at the time of the console.log statement, it exists but has not been assigned a value yet. Hence, it outputs undefined.

The variable x is declared using "let" which has block scope. When the console.log statement is executed before the variable x is declared, a ReferenceError occurs.

The variable x is declared inside the function foo using "var," which creates a local scope. At the time of the first console.log statement, the variable x exists but has not been assigned a value yet, resulting in undefined.

The variable x is declared in the global scope, and the function foo has access to it. When the foo function is called, it logs the value of x, which is 5.

The variable x is declared in the function foo but is not assigned a value before the first console.log statement. Therefore, it outputs undefined. Inside the bar function, it logs the value of x, which is 10.

The variable x is declared in different scopes: 10 in the bar function and 30 in the innermost block. The console.log statement inside the bar function outputs the value of x within its scope, which is 30.

The function foo has access to the global variable x, which is 5. When the foo function is called from the bar function, it logs the value of x, which is 5.

The function foo is called before the variable x is assigned a value in the bar function. Therefore, at the time of the console.log statement in the foo function, x exists but has not been assigned a value yet, resulting in undefined.

The function bar is assigned to the variable bar, and when it is called, the "this" keyword refers to the global object (window in a browser environment). Since the global object does not have a property x, it outputs undefined.