https://www.educative.io/answers/function-composition-in-javascript
Function composition is an approach where the result of one function is passed on to the next function, which is passed to another until the final function is executed for the final result. Function compositions can be composed of any number of functions.
Traditional approach
Traditionally, function composition follows this format:
const double = x => x * 2
const square = x => x * x
// Tradition approach
var output1 = double(2);
var output2 = square(output1);
console.log(output2);
// variant two
var output_final = square(double(2));
console.log(output_final);
In the code above, we can see that we need to call the double function followed by the square function to square a term that has been doubled. We can do this by either assigning the values individually in variables and calling functions onto them (lines 4 to 5), or we could use a more direct approach like on line 8.
https://itnext.io/write-better-javascript-function-composition-with-pipe-and-compose-93cc39ab16ee
compose函数: 把层级嵌套的函数调用(一个函数的运行结果当作实参传给下一个函数的操作)扁平化
example:
1.调用compose([a, b, c])(param)和a(b(c(param)))效果相同
2.compose(f1,f2)(4)，执行结果是f1(f2(4))
3.
let x = fn1(6);
x = fn2(x);
x = fn1(x);
x = fn3(x);
用一个函数解决这种问题，形如：compose(fn1, fn2, fn1, fn3)(6);
4.
let fn1 = (x) => x + 10
let fn2 = (x) => x * 10
let fn3 = (x) => x / 10
console.log(fn3(fn1(fn2(fn1(6)))));
Alternate approach
Another approach is to use the compose and pipe functions.
compose function
compose function takes any number of functions and invokes them all one after the other:
// function composition of any number of functions
const compose = (...fns) => x => fns.reduceRight((y, f) => f(y), x);
const double = x => x * 2
const square = x => x * x
// function composition
var output_final = compose(square, double)(2);
console.log(output_final);
In the code above, we can see that the compose function is implemented using a general approach on line 2, so now it can take any number of functions. The output remains the same as before even with using this implementation.
pipe function
On the other hand, we can reverse the order of the function invocation by using the pipe function:
// function composition using pipe of any number of functions
const pipe = (...fns) => x => fns.reduce((y, f) => f(y), x);
const double = x => x * 2
const square = x => x * x
// function pipe
var output_final = pipe(square, double)(2);
console.log(output_final);
In the code above, we can see that the pipe function is implemented using a general approach on line 2, so it can now take any number of functions. This is similar to the previous compose function, except that it uses reduce instead of the reduceRight method. The output is different in this case because the square function is invoked before the double function while, in our compose function, it was in the opposite order.
Function composition enabling piping
// Building-blocks to use for composition
const double=x=>2*x
const triple=x=>3*x
const quadruple=x=>4*x
// Function composition enabling pipe functionality
const pipe=(...functions)=>input=>functions.reduce((acc,fn)=>fn(acc),input)
// Composed functions for multiplication of specific values
const multiply6=pipe(double,triple)
const multiply9=pipe(triple,triple)
const multiply16=pipe(quadruple,quadruple)
const multiply24=pipe(double,triple,quadruple)
// Usage
multiply6(6)// 36
multiply9(9)// 81
multiply16(16)// 256
multiply24(10)// 240
Libraries for function composition
Given the importance of function composition, we may use libraries that have already completed the implementation of both the compose and pipe functions. The following libraries might help.
Lodash: flowRight (compose) and flow (pipe)
These libraries can be used to avoid implementing either function so you can focus on the uses of it.
