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.

In JavaScript, numbers are represented in binary, and not all decimal fractions can be precisely represented in binary. Due to this limitation, when you perform operations like addition with decimal numbers, you might encounter tiny rounding errors that make 0.1 + 0.2 !== 0.3. This behavior is not unique to JavaScript and occurs in many programming languages with floating-point arithmetic.

In JavaScript, a prototype is a template object that is used for inheritance. Each object in JavaScript has a prototype, and it allows objects to inherit properties and methods from other objects, creating a hierarchy of objects. Prototypal inheritance is a fundamental concept in JavaScript's object model.

The primary purpose of using AJAX in web development is to make asynchronous requests to the server without requiring a full page refresh. This allows for dynamic content loading and improved user experience in web applications.

The primary difference between for...in and for...of loops is in their order of iteration and syntax. For...in is used for iterating over object properties, while for...of is used for iterating over the values of iterable objects, like arrays and strings, and maintains the order.