In JavaScript, the ________ object represents the eventual
- EventEmitter
- Promise
- Callback
- Observable
In JavaScript, the Promise object represents the eventual completion or failure of an asynchronous operation. Promises are widely used for handling asynchronous tasks, providing a cleaner and more structured way to work with asynchronous code.
Which method is used to select an element by its ID in JavaScript?
- getElementByTag
- getElementById
- getElementByClass
- querySelector
The correct method to select an element by its ID in JavaScript is getElementById. This method retrieves an element by its unique ID attribute. The other options do not select elements by their IDs.
You're attending a tech conference, and a speaker mentions that JavaScript was initially met with skepticism because of its relation to a more established language. Which language are they referring to?
- Java
- C#
- Python
- Ruby
The skepticism surrounding JavaScript's name is due to its early association with Java. JavaScript was initially named "LiveScript" and was renamed to capitalize on Java's popularity. However, the two languages are quite different in terms of their usage and capabilities.
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.