# Composing functions to run in sequence

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We sometimes need to pass the result of one function call through to another function. When we do that with the same input many times in a row, we can compose these functions together instead.

Composing functions together gives us a single function made up of these functions. It takes one input, which it runs through the individual functions in sequence.

``````// these functions all take a string and do something with it
const reverse = name => [...name].reverse().join("")
const upcase = name => name.toUpperCase()
const greet = name => `Hi, \${name}!`

// we could store each result and pass it to the next function
const reversed = reverse("john")
const upcased = upcase(reversed)
greet(upcased)
// ⇒ "Hi, NHOJ!"

// if we nest them like this instead, they are run from inside to outside
greet(upcase(reverse("john")))
// ⇒ "Hi, NHOJ!"

// this takes several functions and runs an input through them in sequence
const compose = (...fns) => input => {
return fns.reduce((result, fn) => fn(result), input)
}

// with our `compose`, we can list the functions and call them like this
compose(reverse, upcase, greet)("john")
// ⇒ "Hi, NHOJ!"

// the order of functions defines the order the steps are executed in
compose(greet, reverse, upcase)("john")
// ⇒ "!NHOJ ,IH"

// we can also assign our composed function to a variable and reuse it
const sequence = compose(upcase, greet, reverse)
sequence("john")  // ⇒ "!NHOJ ,iH"
sequence("paul")  // ⇒ "!LUAP ,iH"``````
``````// these functions all take a string and do something with it
const reverse = name => [...name].reverse().join("")
const upcase = name => name.toUpperCase()
const greet = name => `Hi, \${name}!`

// we could store each result and pass it to the next function
const reversed = reverse("john")
const upcased = upcase(reversed)
greet(upcased)
// ⇒ "Hi, NHOJ!"

// if we nest them like this instead, they are run from inside to outside
greet(upcase(reverse("john")))
// ⇒ "Hi, NHOJ!"

// this takes several functions and runs an input through them in sequence
const compose = (...fns) => input => {
return fns.reduce((result, fn) => fn(result), input)
}

// with our `compose`, we can list the functions and call them like this
compose(reverse, upcase, greet)("john")
// ⇒ "Hi, NHOJ!"

// the order of functions defines the order the steps are executed in
compose(greet, reverse, upcase)("john")
// ⇒ "!NHOJ ,IH"

// we can also assign our composed function to a variable and reuse it
const sequence = compose(upcase, greet, reverse)
sequence("john")  // ⇒ "!NHOJ ,iH"
sequence("paul")  // ⇒ "!LUAP ,iH"``````

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