In JavaScript, functions are considered to be first-class citizens, meaning they are treated as values and can be assigned to variables, passed as arguments to other functions, and returned as values from functions.

A first-class function is a function that can be treated like any other value in the language. This means that you can pass a function as an argument to another function, return a function as a value from a function, and assign a function to a variable.

For example, consider the following code:

function square(x) {
  return x * x;
}

var f = square;

console.log(f(4)); // outputs 16

In this code, the square function is assigned to the variable f. f is now a reference to the square function, and it can be called just like any other function.

Another example of using first-class functions is the map function in JavaScript. The map function takes a function as an argument and applies that function to each element of an array, returning a new array with the results. Here's an example:

var numbers = [1, 2, 3, 4];

var squares = numbers.map(function(x) {
  return x * x;
});

console.log(squares); // outputs [1, 4, 9, 16]

In this code, the map function takes an anonymous function as an argument, which returns the square of each element in the numbers array. The map function then returns a new array (squares) with the results of applying the function to each element in the original array.