Functional programming is a programming paradigm that aims to create software by building functions that produce outputs, rather than using objects and state changes. As a result, functional code is often easier to read, maintain, and test. In JavaScript, functional programming is supported by functional programming concepts such as pure functions, immutability, and higher-order functions.
A pure function is a function that always returns the same output for the same input, without any side effects. In other words, it does not modify any state or change any external variables. Here’s an example of a pure function in JavaScript:
function multiply(a, b) {
return a * b;
}
This function takes two arguments and returns their product. It does not modify any state or affect any external variables, making it a pure function.
Another concept of functional programming is immutability, which means that data should not be modified once it has been created. Instead, new data should be created from existing data. In JavaScript, this can be achieved by using methods such as map(), filter(), and reduce().
For example, let’s say we have an array of numbers and we want to multiply each number by 2 and store the result in a new array. Here’s how we can do it using map():
const numbers = [1, 2, 3, 4, 5];
const doubledNumbers = numbers.map((num) => num * 2);
console.log(doubledNumbers); // Output: [2, 4, 6, 8, 10]
In this example, we create a new array called doubledNumbers that contains the result of multiplying each number in the numbers array by 2. The original numbers array is not modified.
Finally, higher-order functions are functions that either take one or more functions as arguments or return a function as their result. Examples of higher-order functions in JavaScript include map(), filter(), and reduce().
Here’s an example of a higher-order function that takes a function as its argument:
function multiplyBy(factor) {
return function(num) {
return num * factor;
};
}
const double = multiplyBy(2);
console.log(double(5)); // Output: 10
In this example, we create a higher-order function called multiplyBy() that takes a factor as its argument and returns a new function that multiplies its argument by the factor. We then call multiplyBy(2) to create a new function called double that multiplies its argument by 2. Finally, we call double(5) to get the result of multiplying 5 by 2, which is 10.
Functions are first-class citizens, which means they can be assigned to variables, passed as arguments to other functions, and returned as values from functions.
Pure functions are the cornerstone of functional programming. They have no side effects and always return the same output for a given input, which makes them easier to test, reason about, and compose with other functions.
Higher-order functions are functions that operate on other functions or their result values. They can be used to abstract common patterns, such as map, filter, and reduce, or to create new functions based on existing ones.
Recursion is a common technique in functional programming, which allows a function to call itself with smaller inputs until it reaches a base case. It can be used to solve many problems, such as traversing trees or computing factorials.
Immutable data structures are preferred in functional programming, which means that once created, they cannot be modified. This makes them safer to use in concurrent or distributed environments, where multiple threads or processes can access the same data.
Composition is a powerful technique in functional programming, which allows developers to build complex systems from simple building blocks. By combining functions that perform specific tasks, developers can create new functions that solve more complex problems.
Higher-order types, such as functors, monads, and applicatives, are advanced concepts in functional programming that provide abstractions for common patterns, such as working with collections, handling errors, or dealing with side effects.
Functional programming emphasizes declarative programming over imperative programming. This means that developers focus on specifying what should be done, not how it should be done. This allows for more concise, readable, and maintainable code.
What is the difference between a pure function and an impure function in JS functional programming?
Answer: A pure function always returns the same output given the same inputs and does not have any side effects, while an impure function may have side effects and may not always return the same output given the same inputs.
What is a closure in JS functional programming?
Answer: A closure is a function that “closes over” variables from its outer lexical scope, allowing those variables to be accessed and modified even after the outer function has returned.
How can you compose multiple functions together in JS functional programming?
Answer: You can use the compose or pipe functions to combine multiple functions into a single function that applies them in a specific order.
What is currying in JS functional programming?
Answer: Currying is the process of transforming a function that takes multiple arguments into a series of functions that each take a single argument. This can be useful for function composition and creating reusable partial functions.
What is the purpose of the reduce function in JS functional programming?
Answer: The reduce function allows you to iterate over an array and accumulate a value by applying a function to each element and the previous accumulated value. This can be useful for performing complex transformations on arrays or calculating aggregate values.