You are troubleshooting an issue where the Fetch API call to a third-party API is not returning any data, and you suspect it might be due to a CORS policy. How might you validate and debug this issue?

  • Use a CORS proxy
  • Check the browser console
  • Modify the server's CORS policy
  • Reboot the server
To validate and debug a CORS (Cross-Origin Resource Sharing) issue, you can check the browser console for error messages, which often provide information about the CORS policy violation. Modifying the server's CORS policy or using a CORS proxy can help resolve such issues. Rebooting the server is unlikely to fix CORS problems.

What will be the output of console.log(typeof null); in JavaScript?

  • "object"
  • "null"
  • "undefined"
  • "number"
The expression console.log(typeof null); in JavaScript will output "object." This is a quirk in JavaScript because typeof null returns "object," even though null is not an object but a special value representing the absence of a value.

In which scenario might you prefer to use Object.create(null) over {} to create an empty object?

  • When you need an empty object with no prototype chain (no inherited properties or methods)
  • When you need an empty object with default prototype properties
  • When you need an object with prototype properties
  • When you need an object with getter and setter methods
Object.create(null) is used when you want an empty object with no prototype chain. This is useful when you want to create a clean slate object without inheriting any properties or methods from the Object prototype.

The insertBefore method is used to insert an element before the _________ child of a specified parent.

  • first
  • last
  • next
  • previous
The insertBefore method is used to insert an element before the specified next child of a parent element. It allows you to precisely control the position of the new element within the parent's children.

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 main difference between stopPropagation and stopImmediatePropagation?

  • stopPropagation stops the event from further propagation but allows other event listeners on the same element to execute.
  • stopPropagation prevents default behavior in addition to stopping propagation.
  • stopImmediatePropagation stops further propagation and prevents other event listeners on the same element from executing.
  • stopPropagation is used for touch events, while stopImmediatePropagation is used for mouse events.
stopPropagation and stopImmediatePropagation are methods used to control event propagation. The main difference is that stopPropagation stops further propagation but allows other event listeners on the same element to execute. In contrast, stopImmediatePropagation not only stops further propagation but also prevents other event listeners on the same element from executing, even if they are in the same event phase. This can be useful when you want to ensure that certain listeners are not triggered.

You are debugging a JavaScript application and notice that the call stack refers to an anonymous function, making it difficult to trace the error. What could you do to make future debugging easier without altering the function's behavior or structure significantly?

  • Convert the anonymous function into a named one
  • Use a breakpoint in the browser's debugger
  • Add more comments to the anonymous function
  • Change the code structure to eliminate the function
To make future debugging easier without altering the function's behavior significantly, you can convert the anonymous function into a named one. Giving a function a meaningful name can help you identify it more easily in the call stack and error messages, making debugging less challenging. Renaming the function doesn't alter its behavior or structure but enhances code readability and maintainability.