JavaScript Higher Order Functions

A higher-order function is a function that does one of the following:

• Takes another function as an argument.
• Returns another function as its result.

Higher-order functions help make your code more reusable and modular by allowing you to work with functions like any other value.

function fun() {
    console.log("Hello, World!");
}
function fun2(action) {
    action();
    action();
}

fun2(fun);

In this example

• fun2 is a higher-order function because it takes another function (action) as an argument.
• It calls the action function twice.

Popular Higher Order Functions in JavaScript

1. map

The map function is used to transform an array by applying a callback function to each element. It returns a new array.

const n = [1, 2, 3, 4, 5];
const square = n.map((num) => num * num);
console.log(square);

• map applies the callback (num) => num * num to each element of numbers.
• A new array is returned where each element is the square of the original

2. filter

The filter function is used to create a new array containing elements that satisfy a given condition.

const n = [1, 2, 3, 4, 5];
const even = n.filter((num) => num % 2 === 0);
console.log(even);

• The callback (num) => num % 2 === 0 filters out elements not divisible by 2.
• The resulting array contains only even numbers.

3. reduce

The reduce function accumulates array elements into a single value based on a callback function.

const n = [1, 2, 3, 4, 5];
const sum = n.reduce((acc, curr) => acc + curr, 0);
console.log(sum); 

• The callback (acc, curr) => acc + curr adds all elements.
• 0 is the initial value of the acc.

4. forEach

The forEach function executes a provided function once for each array element.

const n = [1, 2, 3];
n.forEach((num) => console.log(num * 2));

• forEach performs the side effect of printing each element multiplied by 2.
• It does not return a new array like map.

5. find

The find function returns the first element in the array that satisfies a given condition.

const n = [1, 2, 3, 4, 5];
const fEven = n.find((num) => num % 2 === 0);
console.log(fEven);

• The callback (num) => num % 2 === 0 finds the first even number.
• If no element satisfies the condition, it returns undefined.

6. some

The some function checks if at least one array element satisfies a condition.

const n = [1, 2, 3, 4, 5];
const hasNeg = n.some((num) => num < 0);
console.log(hasNeg);

• The callback (num) => num < 0 checks for negative numbers.
• It returns true if any element passes the condition, false otherwise.

7. every

The every function checks if all array elements satisfy a condition.

const n = [1, 2, 3, 4, 5];
const allPos = n.every((num) => num > 0);
console.log(allPos)

• The callback (num) => num > 0 checks if all numbers are positive.
• It returns true only if all elements pass the condition.

Advanced Techniques with Higher Order Functions

1. Function Composition

Function composition is the process of combining multiple functions to create a new function. The composed function applies multiple operations in sequence.

function add(x) {
    return x + 2;
}
function mul(x) {
    return x * 3;
}

function compose(f, g) {
    return function(x) {
        return f(g(x));
  };
}
var res = compose(add, mul)(4);
console.log(res);

• compose combines add and multiply, so the output of multiply is passed as input to add.
• The result of compose(add, mul)(4) is 14 because 4 is first multiplied by 3 and then 2 is added.

2. Currying

Currying transforms a function that takes multiple arguments into a series of functions that each take one argument. This allows partial application of the function.

function mul(x) {
    return function(y) {
        return x * y;
  };
}
var mul = mul(2);
console.log(mul(5));

• The multiply function is curried, returning a new function each time it is called with an argument.
• multiplyBy2 is a partially applied function that multiplies any given number by 2.

3. Memoization

Memoization is a technique where function results are cached so that repeated calls with the same arguments return faster. This is particularly useful for expensive function calls.

function memoize(func) {
    var cache = {};
    return function (arg) {
        if (cache[arg]) {
            return cache[arg];
        } else {
            var res = func(arg);
            cache[arg] = res;
            return res;
        }
    };
}
function slow(num) {
    console.log("Computing...");
    return num * 2;
}

var fast = memoize(slow);
console.log(fast(5)); // Computing... 10
console.log(fast(5)); // 10 (cached)

• memoize caches the results of slowFunction calls. The second time fast(5) is called, the result is fetched from the cache, avoiding recomputation.
• This optimization improves performance by saving on redundant calculations.

Use case's of higher order functions

1. Passing Functions as Arguments

In the following example, we define a Higher-Order Function called greet that accepts a callback function as an argument and executes it

function greet(name, callback) {
    console.log("Hello, " + name);
    callback();
}

function sayGoodbye() {
    console.log("Goodbye!");
}


//Driver Code Starts{
greet("Ajay", sayGoodbye);

//Driver Code Ends }

• Function as Argument: greet accepts another function (e.g., sayGoodbye) as a callback, demonstrating the ability to pass functions as arguments.
• Sequence Control: It first logs a greeting message and then executes the callback, showing how actions can be performed in a specific order.
• Modularity and Reusability: By separating the greeting and goodbye actions, the pattern allows flexibility and reusability, enabling different callbacks to be passed as needed.

2. Returning Functions from Functions

Higher-order functions can also return a function. This enables the creation of more dynamic behavior

function mul(factor) {
    return function(num) {
        return num * factor;
    };
}


//Driver Code Starts{
const mul2 = mul(2);
console.log(mul2(5));
const mul3 = mul(3);
console.log(mul3(5)); 
//Driver Code Ends }

• Function Factory: mulBy returns a new function based on the provided factor, demonstrating the ability to create dynamic, parameterized functions.
• Closure in Action: The returned function uses the captured factor to perform multiplication, showcasing the power of closures to retain access to external variables.
• Reusability and Customization: This pattern simplifies creating reusable multipliers (e.g., mul2, mul3), enabling efficient and customizable solutions with minimal effort.

3. Array Method map() as a Higher-Order Function

JavaScript array methods such as map(), filter(), and reduce() are excellent examples of higher-order functions. These methods take callback functions as arguments and provide powerful ways to manipulate arrays.

const a = [1, 2, 3, 4, 5];
const double = a.map(function(n) {
    return n * 2;
});

console.log(double); 

• Array Transformation: map() applies a callback function to each array element, returning a new array with transformed values while keeping the original array unchanged
• Immutability: By not mutating the original array, map() supports immutable data handling, which is key to predictable and safer code.
• Declarative Iteration: It abstracts the iteration logic, promoting a declarative programming style that focuses on what should be done rather than how.

4. Array Method filter() as a Higher-Order Function

The filter() method is another array function that is a higher-order function. It filters the elements of an array based on a condition provided by the callback function.

const a = [1, 2, 3, 4, 5];
const even = a.filter(function(n) {
    return n % 2 === 0;
});

console.log(even);

• Conditional Filtering: filter() applies a callback function to test each element, returning a new array containing only those that meet the specified condition.
• Immutability: It leaves the original array unchanged, ensuring the integrity of the source data while providing filtered results.
• Customizable and Reusable: filter() is highly flexible, allowing easy customization for different conditions to extract specific subsets of data.

5. Array Method reduce() as a Higher-Order Function

The reduce() method is a powerful higher-order function used to reduce an array to a single value.

const n = [1, 2, 3, 4, 5];
const sum = n.reduce(function(acc, curr) {
    return acc + curr;
}, 0);
console.log(sum);

• Accumulation: reduce() processes each element of the array, accumulating a single value (e.g., sum, product) based on the provided callback function.
• Initial Value and Flexibility: The second argument (e.g., 0) sets the initial value for the accumulator, ensuring consistent results and allowing for flexible aggregation.
• Versatility: It can be used for a wide range of tasks, such as summing values, calculating products, or even more complex operations like flattening arrays.