You are debugging a JavaScript application, and you find a variable that seems to be available even after its block has finished executing. What concept of JavaScript allows this to happen?
- Hoisting
- Closure
- Scope
- Shadowing
This behavior is due to the concept of "Closure" in JavaScript. Closures allow functions to maintain access to their lexical scope, even after the outer function has completed execution. This enables the variable to persist and be accessible outside its block.
Which of the following is a correct syntax for an arrow function?
- (param1, param2) => { return expression; }
- function(param1, param2) { return expression; }
- (param1, param2) { return expression; }
- (param1, param2) -> { return expression; }
The correct syntax for an arrow function in JavaScript is '(param1, param2) => { return expression; }'. Arrow functions are known for their concise syntax, especially when there's a single expression to return. The '=>' syntax is used to define arrow functions.
In the method myArray.map(callback), what is the second argument passed to the callback?
- Element's Index (optional)
- Current Array (optional)
- Previous Element (optional)
- Current Index (optional)
In the map() method, the second argument passed to the callback is the current array itself. While the first argument represents the current element, the second argument provides access to the entire array if needed for mapping logic.
During code review, you notice a function defined as an expression is being invoked before its definition in a set of sequential scripts. What might be a potential risk in this scenario?
- The function may not have access to outer variables.
- The function may throw a TypeError when invoked.
- The function may have undefined behavior.
- The function will work without any issues.
In JavaScript, when a function expression is invoked before its definition, it may result in undefined behavior. This is because the function's declaration is not hoisted to the top of its scope like function declarations, and it may not have access to variables declared after its invocation.
Which method creates a new array with all elements that pass a test implemented by the provided function?
- reduce()
- forEach()
- filter()
- find()
The filter() method creates a new array with all elements that pass the test implemented by the provided function. It's a non-mutating method, meaning it doesn't change the original array. In contrast, reduce(), forEach(), and find() serve different purposes.
You are refactoring a codebase and converting regular functions to arrow functions. In which of the following cases would you need to be most cautious about changing the function type due to the "this" keyword's behavior?
- When the function is used as an event handler
- When the function is a method in a class or object
- When the function uses "let" instead of "var"
- When the function is used for mathematical operations
When converting regular functions to arrow functions, you need to be cautious when the function is a method in a class or object. Arrow functions do not have their own "this" binding and instead inherit it from their enclosing scope. This can lead to unexpected behavior in object methods if "this" is used within the function. In other cases, such as event handlers or simple functions, arrow functions may be safely used.
What is the potential impact on performance when improperly handling asynchronous operations in JavaScript?
- Memory leaks and decreased performance.
- Improved performance and faster execution of tasks.
- Reduced complexity and optimized code.
- No impact on performance.
Improperly handling asynchronous operations in JavaScript can lead to memory leaks and decreased performance. If resources are not released correctly or if there are too many pending tasks, it can strain system resources and cause sluggishness. It's essential to manage asynchronous operations properly to avoid these performance bottlenecks.
The second statement of a "for" loop is the ________, which is checked before every iteration.
- initialization
- condition
- increment
- termination
In a "for" loop, the second part is the condition. This condition is checked before every iteration to determine if the loop should continue executing. If the condition evaluates to false, the loop terminates. The initialization is the first part, and the increment is the third part of the "for" loop.
To prevent an infinite loop, you should always modify the ________ variable inside a while loop.
- condition
- counter
- iterator
- sentinel
To prevent an infinite loop in a 'while' loop, you should always modify the 'counter' variable. This ensures that the loop will eventually terminate when the condition becomes false, preventing it from running indefinitely.
You encounter a bug in your code where the wrong block of code is being executed despite the condition being false. What could be a possible reason for this?
- Logical operator precedence
- Syntax error
- Variable scope
- Type coercion
One possible reason for the bug could be incorrect logical operator precedence. If the operators are not used in the right order, it can lead to unexpected results. Understanding operator precedence is crucial to avoid such issues and ensure that conditions are evaluated as intended.