Built-in functions in JavaScript are pre-defined functions that are included in the language and do not require any additional code to be written. They are available to use in any JavaScript program without the need to define or import them, and are useful for performing standard tasks and operations.

There are many built-in functions in JavaScript that serve a variety of purposes. For example, some functions are designed to work with arrays, like Array.map() or Array.filter(), while others operate on strings like String.charCodeAt() or String.toUpperCase().

1. console.log() - Used for logging messages to the browser console for debugging purposes.

2. alert() - Used for displaying a message in a pop-up box on the screen.

3. isNaN() - Used to determine if a value is not a number.

4. parseInt() - Used to convert a string to an integer.

5. parseFloat() - Used to convert a string to a floating-point number.

6. Math.random() - Used to generate a random number between 0 and 1.

7. Date.now() - Used to get the current date and time.

Overall, built-in functions in JavaScript are an essential part of the language and provide programmers with a powerful toolset to perform a wide variety of tasks with minimal effort.

In the context of JavaScript programming, a built-in function is a function that is already included in the JavaScript language and can be called without needing to define it again.

JavaScript has many built-in functions that can be used to perform a variety of tasks, such as manipulating strings, numbers, and arrays, and performing mathematical or logical operations.

1. alert() function: used to display a message to the user in a pop-up window.

2. parseInt() function: used to convert a string to an integer.

3. parseFloat() function: used to convert a string to a floating-point number.

4. Math.random() function: used to generate a random number between 0 and 1.

5. Date() function: used to get the current date and time.

It is important to note that while built-in functions can be useful, they may not always be suitable for a particular use case, and custom functions may need to be defined in certain programming scenarios.