JavaScript was primarily created to enhance interactivity in web browsers. It allowed developers to manipulate web page elements and respond to user actions, making websites more dynamic. While JavaScript later found uses on the server-side as well, its initial purpose was to enhance the client-side user experience.

The for...in loop will also iterate over the enumerable properties of an object. Enumerable properties are those that can be iterated over using for...in, typically the object's own properties. Non-enumerable properties, private properties, and prototype properties are not included in this iteration.

If no case matches in a switch statement and there is no default case, the control is passed to the next statement after the switch. This behavior is important to understand because it determines what happens when none of the cases match the expression value.

In JavaScript, the "this" keyword typically refers to the parent object, which is the object that calls the function. It allows you to access and manipulate the properties and methods of that object within the function. This behavior is crucial for object-oriented programming in JavaScript.

In JavaScript, the Symbol data type can be used to create unique identifiers. Symbols are often used as object property keys when you need to ensure that the key is unique and not accidentally overwritten. They are not used to create objects, functions, or private variables.

In JavaScript, a callback function is a function that can be passed as an argument to another function. Callback functions can accept up to three arguments: the value of the element, the index of the element, and the array object being traversed. They are commonly used in asynchronous programming and array methods.

In terms of browser compatibility, the forEach() method should be avoided in Internet Explorer 8. This method was introduced in ECMAScript 5, and Internet Explorer 8 only supports ECMAScript 3. Other methods like map(), filter(), and indexOf() have more widespread support in older browsers.

The HTTP status code 200 OK indicates that the request has succeeded. It is the standard response for successful HTTP requests. The other options represent different HTTP status codes with different meanings, such as 'No Content,' 'Not Found,' and 'Internal Server Error.'

You should use Promise.catch() to handle errors in a Promise. This method allows you to specify a callback function that will be called when the Promise is rejected, allowing you to log an error message and gracefully continue execution without propagating the error to the outer scope.

You would use a for...of loop with Object.keys() to iterate over object properties while excluding prototype properties. Object.keys() returns an array of an object's own enumerable property names, ensuring that prototype properties are filtered out.