What is currying?

Currying is the process of splitting a function that takes multiple arguments into a series of functions that each take a single argument. The resulting function is a chain of functions, each function in turn accepting the arguments of the original function.

In other words, currying turns a function that takes multiple arguments into a function that takes a single argument and returns another function, and so on, until all arguments have been supplied and the final result is returned.

Currying can help create reusable code, you can create a function that is partially applied and then apply the remaining arguments later.

Currying in JavaScript

JavaScript is a functional programming language that supports currying. In JavaScript, functions are first-class citizens, which means they can be treated like any other variable. Functions in JavaScript can be defined, assigned to variables, and passed as arguments to another function.

Let's start with an example of a function that takes two arguments and returns their sum:

function add(x, y) {
  return x + y;
}

Now, suppose we want to convert this function into a curried function. We can do this by defining a new function that takes a first argument, returns another function that takes a second argument, and finally returns the sum of the two arguments.

function add(x) {
  return function(y) {
    return x + y;
  }
}

This is the curried version of the add() function. We can use it as follows:

const addCurried = add(5);
console.log(addCurried(10)); // 15

In this example, we created a curried version of the add() function by passing the first argument 5 to the add() function. It returns a new function that takes a second argument and returns the sum of the two arguments. We assign this new function to the addCurried variable, then call it with a second argument of 10, resulting in a final result of 15.

Benefits of Currying

Currying has several benefits, including:

Reusability: Currying enables you to create reusable functions that can be partially applied and used in different contexts. This reduces the need to repeat code and improves code readability.
Flexibility: Curried functions can be easily extended by passing additional parameters. This makes it easier to adapt functions to different use cases without modifying the original function.
Function Composition: With currying, function composition can be created by chaining functions together. This makes it easier to create complex functions by combining simple functions.

Currying using arrow functions

ES6 introduced arrow functions, providing a more concise way to define functions in JavaScript. An arrow function is an anonymous function expression that does not have its own this, arguments, and super bindings.

Currying with arrow functions works in the same way as with normal functions. Here is an example of currying using arrow functions:

const add = x => y => x + y;

const addCurried = add(5);
console.log(addCurried(10)); // 15

In this example, we define a curried add() function using an arrow function. The add() function takes a first parameter x and returns another arrow function that takes a second parameter y and returns the sum of the two parameters.

The difference between currying and partial application (bind function)

Currying is often confused with partial application, but they are not the same. Partial application is to fix one or more function parameters to create a new function that accepts fewer parameters.

Here is an example of a partial application:

function add(x, y) {
  return x + y;
}

// 部分应用
const addPartial = add.bind(null, 5); // 通过bind()方法固定第一个参数

// 调用部分应用的add()函数并传递第二个参数
const result = addPartial(10);

console.log(result); // 输出：15

In the above example, we defined a simple add() function that takes two arguments and returns their sum. We then created a partially applied add() function using the bind() method with the first parameter fixed to 5, and assigned it to the addPartial variable. Finally, we pass the second argument by calling addPartial(10) and get the final result 15.

Unlike currying, partial application fixes a portion of arguments to a function, rather than turning a function into a chain of functions that accept a single argument. It can be used to fix certain parameters of a function at runtime for more convenience in subsequent use.

Finish

Currying is a powerful technique in functional programming that allows you to transform a function that takes multiple arguments into a series of functions that each take a single argument. Currying makes your code more reusable, flexible, and composable. You can create curried functions in JavaScript using normal functions or arrow functions, or you can use the curry() function provided by Lodash. Currying should not be confused with partial application, which is a related but different technique.

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Briefly talk about what is function currying in JavaScript