What is a first class function?

[montasim]

In short

[d4tdev]

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.

First-class functions are functions that are treated as variables. They can also be parsed into other functions as arguments.

In JavaScript, functions are treated as values or just another type of object

[Preetiraj3697]

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

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!