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Master JavaScript higher-order functions – functions that take or return functions. Learn map, filter, reduce, compose, curry, and build custom HOFs with real-world examples.
Introduction
Higher-order functions (HOFs) are one of the most powerful features in JavaScript — and they are what make JavaScript a functional programming language.
They allow you to:
- Write more readable and reusable code
- Build abstractions that separate "what to do" from "how to do it"
- Power built-in methods like
map,filter, andreduce
If you've ever used .map() or .filter() on an array, you've already used higher-order functions!
Why Higher-Order Functions?
Example
// WITHOUT higher-order functions – repetitive, brittle
function doubleAll(arr) {
const result = [];
for (let i = 0; i < arr.length; i++) {
result.push(arr[i] * 2);
}
return result;
}
function squareAll(arr) {
const result = [];
for (let i = 0; i < arr.length; i++) {
result.push(arr[i] * arr[i]);
}
return result;
}
// WITH higher-order function – reusable, clean
function transformAll(arr, fn) { // fn = function parameter!
const result = [];
for (let i = 0; i < arr.length; i++) {
result.push(fn(arr[i]));
}
return result;
}
console.log(transformAll([1, 2, 3], n => n * 2));
console.log(transformAll([1, 2, 3], n => n * n));
Output
[2, 4, 6] [1, 4, 9]
One function does the work of many — just pass different logic as an argument.
Real World Analogy
Higher-order functions are like a factory machine with interchangeable toolheads. The machine (HOF) does the mechanical work (loop, iterate, manage state). You plug in a different tool (function) to produce different output. 🏭 Machine + 🔪 Blade tool = Sliced products 🏭 Machine + 🔥 Heat tool = Heated products 🏭 Machine + 🎨 Painting tool = Painted products Same machine. Different behavior. Controlled by the tool you pass in.
Built-in Higher-Order Functions
1. Array.prototype.map() – Transform Every Element
map creates a new array by applying a function to each element.
Example
const prices = [100, 250, 80, 430];
// Apply 10% discount to each price
const discounted = prices.map(price => price * 0.9);
console.log(discounted);
Output
[90, 225, 72, 387]
Example
HOW MAP WORKS INTERNALLY:
prices = [100, 250, 80, 430]
callback = price => price * 0.9
index 0: 100 × 0.9 = 90
index 1: 250 × 0.9 = 225
index 2: 80 × 0.9 = 72
index 3: 430 × 0.9 = 387
Returns new array: [90, 225, 72, 387]
Original array UNCHANGED: [100, 250, 80, 430]
2. Array.prototype.filter() – Keep Matching Elements
filter creates a new array with only elements that pass a test.
Example
const ages = [15, 22, 13, 30, 18, 17, 25];
const adults = ages.filter(age => age >= 18);
console.log(adults);
Output
[22, 30, 18, 25]
3. Array.prototype.reduce() – Reduce to a Single Value
reduce accumulates array elements into one result.
Example
const cart = [
{ name: "Book", price: 250 },
{ name: "Pen", price: 30 },
{ name: "Bag", price: 850 }
];
const total = cart.reduce((sum, item) => sum + item.price, 0);
console.log("Total:", total);
Output
Total: 1130
Example
HOW REDUCE WORKS:
Initial: sum = 0
Step 1: sum = 0 + 250 = 250
Step 2: sum = 250 + 30 = 280
Step 3: sum = 280 + 850 = 1130
Returns: 1130
4. Array.prototype.forEach() – Execute for Each (no return value)
Example
const names = ["Riya", "Arjun", "Priya"];
names.forEach((name, index) => {
console.log(`${index + 1}. ${name}`);
});
Output
1. Riya 2. Arjun 3. Priya
5. Array.prototype.find() – Find First Match
Example
const users = [
{ id: 1, name: "Riya" },
{ id: 2, name: "Arjun" },
{ id: 3, name: "Priya" }
];
const user = users.find(u => u.id === 2);
console.log(user);
Output
{ id: 2, name: 'Arjun' }6. Array.prototype.sort() – Custom Sorting
Example
const students = [
{ name: "Riya", marks: 88 },
{ name: "Arjun", marks: 95 },
{ name: "Priya", marks: 76 }
];
// Sort by marks descending
students.sort((a, b) => b.marks - a.marks);
students.forEach(s => console.log(`${s.name}: ${s.marks}`));
Output
Arjun: 95 Riya: 88 Priya: 76
Functions That Return Functions (HOF Pattern 2)
Higher-order functions can also return a new function.
js example
function multiplier(factor) {
// returns a function — this is a HOF
return function (number) {
return number * factor;
};
}
const double = multiplier(2);
const triple = multiplier(3);
const tenX = multiplier(10);
console.log(double(5));
console.log(triple(5));
console.log(tenX(5));Output
10 15 50
Building Custom Higher-Order Functions
Custom map Implementation:
Example
function myMap(arr, transform) {
const result = [];
for (const item of arr) {
result.push(transform(item));
}
return result;
}
console.log(myMap([1, 2, 3, 4], n => n ** 2));
Output
[1, 4, 9, 16]
Custom filter Implementation:
Example
function myFilter(arr, test) {
const result = [];
for (const item of arr) {
if (test(item)) result.push(item);
}
return result;
}
console.log(myFilter([1, 2, 3, 4, 5, 6], n => n % 2 === 0));
Output
[2, 4, 6]
Custom compose (Function Composition):
Example
function compose(...fns) {
return function (value) {
return fns.reduceRight((acc, fn) => fn(acc), value);
};
}
const double = n => n * 2;
const addTen = n => n + 10;
const square = n => n * n;
const transform = compose(square, addTen, double);
// applies: double → addTen → square
// 5 → 10 → 20 → 400
console.log(transform(5));
Output
400
Step-by-Step Trace: Chaining HOFs
Example
const students = [
{ name: "Riya", score: 88, passed: true },
{ name: "Arjun", score: 45, passed: false },
{ name: "Priya", score: 92, passed: true },
{ name: "Sam", score: 38, passed: false }
];
const topStudentNames = students
.filter(s => s.passed) // keep passed students
.filter(s => s.score >= 85) // keep top scorers
.map(s => s.name) // extract names
.sort(); // sort alphabetically
console.log(topStudentNames);
Example
Step 1: filter(passed) → [Riya(88), Priya(92)]
Step 2: filter(score≥85) → [Riya(88), Priya(92)]
Step 3: map(name) → ["Riya", "Priya"]
Step 4: sort() → ["Priya", "Riya"]
Output
['Priya', 'Riya']
Common Mistakes
❌ Mistake 1: Mutating the original array inside map
Example
const arr = [1, 2, 3];
// WRONG – map should return new value, not mutate
arr.map(n => { arr.push(n * 10); });
console.log(arr);
Output
[1, 2, 3, 10, 20, 30] ← unexpected mutation!
Example
// CORRECT – return transformed value
const doubled = arr.map(n => n * 2);
console.log(doubled);
console.log(arr); // original unchanged
Output
[2, 4, 6] [1, 2, 3]
❌ Mistake 2: Using forEach when you need the result
Example
// WRONG – forEach returns undefined
const doubled = [1, 2, 3].forEach(n => n * 2);
console.log(doubled); // undefined
Example
// CORRECT – use map when you need a transformed array
const doubled = [1, 2, 3].map(n => n * 2);
console.log(doubled);
Output
[2, 4, 6]
❌ Mistake 3: Forgetting reduce's initial value
Example
// RISKY – no initial value, breaks on empty array
const sum = [1, 2, 3].reduce((acc, n) => acc + n);
console.log(sum); // 6 (works here)
[].reduce((acc, n) => acc + n); // TypeError!
Example
// SAFE – always provide initial value
const sum = [1, 2, 3].reduce((acc, n) => acc + n, 0);
console.log(sum);
Output
6
❌ Mistake 4: Confusing filter and find
Example
const arr = [1, 2, 3, 4, 2];
// filter – returns ALL matching items (array)
console.log(arr.filter(n => n === 2)); // [2, 2]
// find – returns FIRST matching item (single value)
console.log(arr.find(n => n === 2)); // 2
Output
[2, 2] 2
HOFs vs Imperative Loops – Comparison
Example
const numbers = [1, 2, 3, 4, 5];
// IMPERATIVE (manual loop)
const squaredImperative = [];
for (let i = 0; i < numbers.length; i++) {
if (numbers[i] % 2 !== 0) {
squaredImperative.push(numbers[i] ** 2);
}
}
console.log(squaredImperative);
// DECLARATIVE (HOFs)
const squaredDeclarative = numbers
.filter(n => n % 2 !== 0)
.map(n => n ** 2);
console.log(squaredDeclarative);
Output
[1, 9, 25] [1, 9, 25]
Same result — but the HOF version is shorter, more readable, and expresses what to do rather than how.
Interview Questions
Q1. What is a higher-order function?
A higher-order function is a function that either takes a function as an argument, returns a function, or both.
Q2. What are the most common built-in higher-order functions in JavaScript?
map, filter, reduce, forEach, find, findIndex, sort, every, some, flatMap.
Q3. What is the difference between map and forEach?
map returns a new transformed array. forEach returns undefined and is used for side effects only.
Q4. What is the difference between filter and find?
filter returns an array of all matching elements. find returns the first matching element (or undefined).
Q5. How does reduce work?
reduce(callback, initialValue) applies the callback to an accumulator and each element, returning one final value. The callback takes (accumulator, currentValue).
Q6. What makes JavaScript functions "first-class"?
In JavaScript, functions are first-class values — they can be stored in variables, passed as arguments, returned from functions, and stored in arrays/objects. This is what enables higher-order functions.
Q7. How do you build a custom higher-order function?
Accept a function as a parameter and call it at the right time:
Example
function repeat(n, action) {
for (let i = 0; i < n; i++) action(i);
}
repeat(3, i => console.log(`Step ${i + 1}`));
Output
Step 1 Step 2 Step 3
Q8. What is function composition and how do HOFs enable it?
Function composition chains functions together so the output of one becomes the input of the next. HOFs like compose and pipe implement this pattern by returning a new function that applies each step in sequence.
Key Takeaways
- ✅ A HOF either takes or returns a function (or both)
- ✅
maptransforms each element into a new array (non-mutating) - ✅
filterkeeps only elements that pass a test - ✅
reduceaccumulates elements into a single value - ✅
forEachis for side effects only — returnsundefined - ✅ Always provide an initial value to
reduce - ✅ HOFs make code declarative — describe what to do, not how
- ✅ Functions are first-class values in JS — HOFs are possible because of this
- ✅ You can build custom HOFs by accepting functions as parameters
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for JavaScript Higher Order Functions Tutorial – map, filter, reduce & Custom HOFs.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this JavaScript Master Course topic.
Search Terms
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