Advanced applications of functional programming in JavaScript

JavaScript is a language that supports functional programming. The core idea of ​​functional programming is to separate data and functions, emphasizing the invariance and lack of side effects of the program. Functional programming has a concise syntax and has great advantages in code organization and testing. This article will introduce some advanced applications of functional programming in JavaScript.

1. Pure function

In functional programming, one of the most basic concepts is the purity of the function. A pure function refers to a function that has the same input at any time. parameters will get the same result without any side effects. The advantage of this kind of function is that it is easy to be cached in a concurrent environment, and it is also helpful for handling recursion and avoiding unnecessary repeated calculations. An addition can be implemented through the following function:

function add(a, b) {
  return a + b;
}

This function is a typical pure function. Because it will have output only under specific input. However, a common side effect is that a function affects the environment in which it is located, such as modifying the value of some variables or calling external functions. Therefore, in functional programming, these side effects should be avoided as much as possible, that is, pure functions should be written as much as possible.

2. Higher-order function

Higher-order function refers to a function that receives one or more functions as parameters or returns a function as a return value. This kind of function can realize requirements more flexibly and reduce code duplication. The following function is an example of a higher-order function. It can receive a function as a parameter and return a new function:

function compose(fn1, fn2) {
  return function(value) {
    return fn1(fn2(value));
  };
}

This function returns a new function. This function will first call fn2 and then pass the result Give fn1 as parameter. You can use this function to implement the combination of multiple functions, as shown below:

function add10(val) {
  return val + 10;
}

function square(val) {
  return val * val;
}

var result = compose(add10, square)(5);
console.log(result); // 35 (5 + 10) * (5 + 10)

In this example, we give add10 and square to the compose function. This function returns a new function. We will add this new function. The function is passed a value of 5. Finally, the results we expected were returned.

3. Currying

Currying is a technique that transforms a function into a form that accepts only one argument at a time. Converting the value of a multi-parameter function into a single-parameter function chain can also be said to be a kind of "preprocessing" of the function, so that they can be used more flexibly. The following function can implement the sum of multiple values:

function add(a, b, c) {
  return a + b + c;
}

add(1, 2, 3); // 6

And this function can be implemented step by step through currying:

function add(a) {
  return function(b) {
    return function(c) {
      return a + b + c;
    };
  };
}

add(1)(2)(3); // 6

As you can see, through currying, the add function After first receiving the first parameter, a function receiving the second parameter is returned. Now, we can pass in the parameters one by one, returning a new function each time, until finally the function is returned.

4. Function combination

Function combination is to combine two or more functions together to generate a new function. This new function will call the passed functions from left to right. input functions and then combine their results together. In functional programming, function composition is the most effective way to combine small functions into large functions. We can combine multiple functions using the compose function. The following is an example of function composition:

function add(a) {
  return a + 10;
}

function multiply(b) {
  return b * 2;
}

const addingAndMultiplying = compose(multiply, add);

addingAndMultiplying (10); // 40                                                        

In this example, we first define two simple functions add and multiply, and then use the combination function compose to combine them into a new function. This new function can call multiple functions at once and combine their results together. When we pass 10 to the addingAndMultiplying function, it first calls the add function, which adds 10 to 10, giving us 20. Next, it passes the result to the multiply function, which multiplies 20 by 2, giving us the final result of 40.

5. The combined application of function currying and composition

The combination of function currying and composition is often used in functional programming. For example, we can define and compound some functions to complete some operations:

function add(a) {
  return function(b) {
    return a + b;
  };
}

function multiply(a) {
  return function(b) {
    return a * b;
  };
}

function subtract(a) {
  return function(b) {
    return b - a;
  };
}

function divide(a) {
  return function(b) {
    return b / a;
  };
}

const discount = compose(subtract(10), multiply(0.1), add(20), divide(2));

discount(50); // 5

In this example, we first define four simple functions add, multiply, subtract and divide, and use function currying to They were transformed. Using the function combinator compose, we can associate all operations and get the final result. This example demonstrates how to combine multiple simple functions into a single function using currying and composition.

Conclusion

This article introduced some advanced applications for implementing functional programming in JavaScript, such as pure functions, higher-order functions, currying, function composition, and curried function composition. These concepts are based on some core ideas of functional programming, such as the purity and immutability of functions. These techniques can help write code that is more flexible, reusable, and easily testable. If you're interested in learning more about functional programming, check out the documentation for Ramda.js, a library that provides a wealth of functional programming tools.

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