Higher-order functions
- Node.js course
- Beginner
- Lesson 78/116
- 5 min
Learn functions that take functions as parameters.
Higher-order functions
A higher-order function either takes a function as a parameter or returns a function. Functions that accept callbacks fall in this category; but those that return functions are just as important.
Example: function makeAdder(n) { return (m) => n + m; }. This function takes a number and returns another function. The returned function remembers n and adds the new parameter m: const add5 = makeAdder(5); add5(3) gives 8.
This pattern is also called a closure; the returned function remembers variables from where it was defined. You will see the details at the advanced level.
Closure in brief
A function 'remembers' the variables from the scope where it was defined. This is called a closure. Higher-order functions use it to carry outer values into the function they return.
Example: function makeMultiplier(factor) { return (x) => x * factor; }. Here makeMultiplier(3) returns a function that remembers factor as 3. That function later computes x * 3.
A closure creates a separate environment per call. makeMultiplier(3) and makeMultiplier(5) produce two independent functions with different factor values. This is what makes higher-order functions flexible.
Practice Return a multiplier function
Define a function makeMultiplier(factor). Its body returns the arrow function (x) => x * factor. Build a multiplier with const triple = makeMultiplier(3), then print the result of triple(7) with console.log.
Starter code
// Define makeMultiplier, build triple and print the call resultSample solution
function makeMultiplier(factor) {
return (x) => x * factor;
}
const triple = makeMultiplier(3);
console.log(triple(7));Expected output
21
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Add with a closure
function makeAdder(n) {
return (m) => n ____ m;
}function makeAdder(n) {
return (m) => n + m;
}Explanation: n + m adds the outer n and the parameter m.
2 Which is a higher-order function?
- function add(a, b) { return a + b; }
- function make(fn) { return fn; } Correct answer
- const x = 5;
- const arr = [1, 2, 3];
Explanation: make takes a function parameter and returns a function.
3 Match the HOF parts
| Term | Meaning |
|---|---|
makeAdder |
HOF name |
n |
Outer parameter |
(m) => n + m |
Returned function |
add5(3) |
Call giving 8 |