Higher Order Function in JavaScript

Higher Order Function (HOF) is a function that can take a function as an argument or a function that return another function.

Array.map is HOF, in below case, map receive an arrow function as an argument.

['cat', 'dog', 'bird'].map(animal => `Hi, ${animal}`)

Let's have a look another example

This function take num1 as an argument, then return a function function (num2) {return num1 * num2}. So it's HOF.

const multiplyBy = function (num1){
        return function (num2) {
            return num1 * num2
        }
}
const multiplyByTwo = multiplyBy(2)
const multiplyByFive = multiplyBy(5)
multiplyByTwo(5) // 10
multiplyByFive(5) // 25

Let's convert above method.

const multiplyBy = num1 => num2 =>  num1 * num2
multiplyBy(2)(5) // 10

Comments

[Jon Randy 🎖️]

A simpler way to write 'multiplyBy' would just be:

const multiplyBy = num1 => num2 => num1 * num2

[Mavis]

Thank you for correcting, I forgot to add return or remove {}.

[SeongKuk Han]

According to Wikipedia, there is a description about HOF.

In mathematics and computer science, a higher-order function is a function that does at least one of the following: takes one or more functions as arguments, returns a function as its result.

Your examples are about closure.

---

Below one is able to be one of examples of HOF.

const greeting = (funcA) => {
  return (name) => {
    funcA();
    console.log(`My name is ${name}.`);
  }
}

const welcome = () => console.log('Welcome!');

greeting(welcome)('Mavis');

const hello = greeting(welcome);
hello('Mavis');

---

In developer.mozilla.org/en-US/docs/W..., there is this example.

function makeAdder(x) {
  return function (y) {
    return x + y;
  };
}

const add5 = makeAdder(5);
const add10 = makeAdder(10);

console.log(add5(2)); // 7
console.log(add10(2)); // 12

[Jon Randy 🎖️]

The OP's example is a perfectly good example of a HOF, and is all but identical to the example from MDN - which is also a good example of a HOF. The fact that a closure is involved in the creation of the HOF is irrelevant.

Your own example also makes use of a closure, and actually does both of the things that the definition of a HOF requires at least one of: it takes a function as input, and also returns a function. Both things are not necessary for it to be a higher order function.

[SeongKuk Han]

Yes, I used a closure to implement a HOF. I just wanted to show that HOF takes a funtion as input.

[Jon Randy 🎖️]

That isn't correct, even by the definition you provided:

In mathematics and computer science, a higher-order function is a function that does at least one of the following: takes one or more functions as arguments, returns a function as its result.

It only needs take take a function as input, or return a function as output to be a HOF. It can do both too, but that's not necessary.

[SeongKuk Han]

Oh, return a function as output. I've written wrong information. Thank you for correcting me :)