First-class Function

A programming language is said to have First-class functions when functions in that language are treated like any other variable. For example, in such a language, a function can be passed as an argument to other functions, can be returned by another function and can be assigned as a value to a variable.

Examples

Assigning a function to a variable

const foo = () => {
  console.log("foobar");
};
foo(); // Invoke it using the variable
// foobar

We assigned an Anonymous Function in a Variable, then we used that variable to invoke the function by adding parentheses () at the end.

Note: Even if your function was named, you can use the variable name to invoke it. Naming it will be helpful when debugging your code. But it won't affect the way we invoke it.

Passing a function as an argument

function sayHello() {
  return "Hello, ";
}
function greeting(helloMessage, name) {
  console.log(helloMessage() + name);
}
// Pass `sayHello` as an argument to `greeting` function
greeting(sayHello, "JavaScript!");
// Hello, JavaScript!

We are passing our sayHello() function as an argument to the greeting() function, this explains how we are treating the function as a value.

Note: The function that we pass as an argument to another function is called a callback function. sayHello() is a callback function.

Returning a function

function sayHello() {
  return () => {
    console.log("Hello!");
  };
}

In this example, we are returning a function from another function - We can return a function because functions in JavaScript are treated as values.

Note: A function that returns a function or takes other functions as arguments is called a higher-order function.