Higher Order Function or First Class Function

Interview Bit

1. What is a first class function in Javascript?

When functions can be treated like any other variable then those functions are first-class functions. There are many other programming languages, for example, scala, Haskell, etc which follow this including JS. Now because of this function can be passed as a param to another function(callback) or a function can return another function(higher-order function). map() and filter() are higher-order functions that are popularly used.

Example

Here is an example of a first-class function in JavaScript:

// Define a function that takes another function as a parameter
function higherOrderFunction(callbackFunction) {
  console.log("The higher-order function was called!");
  callbackFunction();
}
 
// Define a function that can be passed as a parameter to another function
function callbackFunction() {
  console.log("The callback function was called!");
}
 
// Call the higher-order function and pass the callback function as a parameter
higherOrderFunction(callbackFunction);

In this example, higherOrderFunction is a first-class function because it takes another function (callbackFunction) as a parameter. callbackFunction is also a first-class function because it can be passed as a parameter to another function.