# Guide to Higher Order Functions in Javascript🧑💻

## Maximize Your Code's Efficiency with Higher Order Functions

In this article, we will dive into the world of higher-order functions, a fundamental concept in functional programming. We will explore what higher-order functions are, how they are used, and their many benefits. We will also look at examples of higher-order functions in action, including the popular `reduce,`

`map`

and `filter`

functions, and how to use closures in conjunction with higher-order functions. By the end of this article, you will have a deeper understanding of how to use higher-order functions in your own code and how they can improve the quality and maintainability of your codebase.

# What is Higher-Order Function?🤔

An operation on another function is known as a higher-order function. This means that it either takes one or more functions as arguments or returns a function as its result. These functions are called higher-order functions because they take other functions as inputs or outputs.

Higher-order functions are a key concept in functional programming and allow for abstraction over actions, such as iteration, and for passing behaviour as data. They allow for the creation of expressive and composable APIs and are commonly used in functional libraries and frameworks.

Higher-order functions can also return a function as their result, such as the `partial`

function, which takes a function and some arguments and returns a new function with those arguments pre-filled.

## Example📝

A basic example of a higher-order function is a function that takes another function as an argument and returns a new function. Here's an example of such a function:

```
function multiplication(x) {
return function(y) {
return x * y;
}
}
const double = multiplication(2);
console.log(double(5)); // 10
```

In this example, the `multiplication`

function is a higher-order function because it takes a function as an argument and returns a new function (the `return function(y)`

). The `multiplication`

function takes a single argument `x`

and returns a new function that multiplies its input by `x`

.

We can also see `multiplication`

as a factory function that creates new functions with different behaviour. The example above `multiplication(2)`

returns a function that multiplies its input by 2, and `multiplication(3)`

returns a function that multiplies its input by 3. This way, we can create different functions with different behaviour.

It demonstrates how a higher-order function can be used to create new functions with different behavior by abstracting over the action of multiplication.

# Solving real-world problem🗺️

For example, let's say we have a large dataset of transactions and we want to calculate the total amount of money spent by each customer and the average amount spent by all customers. We could use a combination of the `reduce`

, `filter`

, and `map`

higher-order functions to achieve this.

Here's an example of how this could be done:

```
const transactions = [
{ customer: "Binod", amount: 50 },
{ customer: "kumar", amount: 75 },
{ customer: "Roxx", amount: 25 },
{ customer: "Vishnu", amount: 100 },
{ customer: "Bob", amount: 150 },
{ customer: "Binod", amount: 200 },
{ customer: "Roxx", amount: 75 },
{ customer: "Vishnu", amount: 120 },
];
function getCustomerSpend(transactions, customer) {
return transactions
.filter(transaction => transaction.customer === customer)
.reduce((total, transaction) => total + transaction.amount, 0);
}
function getAverageSpend(transactions) {
return transactions
.map(transaction => transaction.amount)
.reduce((total, amount) => total + amount, 0) / transactions.length;
}
const customers = Array.from(new Set(transactions.map(transaction => transaction.customer)));
const customerSpends = customers.map(customer => ({customer,
spend: getCustomerSpend(transactions, customer)
}));
const averageSpend = getAverageSpend(transactions);
console.log(customerSpends);
/* Output of customerSpends:
[
{ customer: 'Binod', spend: 250 },
{ customer: 'kumar', spend: 75 },
{ customer: 'Roxx', spend: 100 },
{ customer: 'Vishnu', spend: 220 },
{ customer: 'Bob', spend: 150 }
] */
console.log(averageSpend); // 99.375
```

In this example, the `getCustomerSpend`

function is used to calculate the total amount spent by a specific customer, while the `getAverageSpend`

function calculates the average amount spent by all customers.

The `getCustomerSpend`

function uses the `filter`

higher-order function to select only the transactions made by the specified customer and the `reduce`

higher-order function to sum up the amounts of the selected transactions.

The `getAverageSpend`

function uses the `map`

higher-order function to extract the amounts from the transactions, and the `reduce`

higher-order function to sum up the amounts and divide by the number of transactions.

This example demonstrates how a combination of higher-order functions can be used to process and analyze large amounts of financial data in a concise and readable way. It shows how the functions can be used to filter, map, and reduce large datasets to provide meaningful insights, such as the total amount spent by each customer and the average amount spent by all customers.

# Best practices for Higher-order function📈

Here are some best practices for using higher-order functions in your code:

**Keep it Simple**: Higher-order functions should be simple and easy to understand. Use them to perform one specific task and keep the code concise and readable.**Use Higher-Order Functions for Common Tasks**: Higher-order functions are useful for tasks that are commonly performed, such as filtering, mapping, or reducing data. When you find yourself repeating the same logic multiple times, consider using a higher-order function to encapsulate the logic.**Avoid Side Effects**: Higher-order functions should not have any side effects, meaning they should not modify the input data. Instead, they should return a new value based on the input data.**Test Higher-Order Functions**: As with any other code, higher-order functions should be thoroughly tested to ensure they work as expected. Test the function's input and output, as well as its behaviour in different scenarios.**Document Higher-Order Functions**: Document the purpose, input, and output of higher-order functions. This makes it easier for other developers to understand how the function works and how to use it.

By following these best practices, we can ensure that our higher-order functions are simple, easy to understand, and perform as expected.

# Conclusion👨🏫

In conclusion, a higher-order function is a function that either takes one or more functions as arguments, or returns a function as its result. Higher-order functions are a fundamental concept in functional programming, providing abstraction over actions and passing behaviour as data. They are used to create expressive and composable APIs and are commonly used in functional libraries and frameworks. Higher-order functions can be used to solve real-world problems, such as processing and analyzing large amounts of data, by using functions such as reduce, map, and filter. This way, they allow solving complex problems in a concise and readable way.