CFP - Docs - swapFirstTwoArgs

swapFirstTwoArgs

```
(fn: function) => function
```

Return a function that calls fn with the first two arguments swapped.

const returnArgs = (...args) => args

const returnSwappedArgs = swapFirstTwoArgs(returnArgs)

const result = returnSwappedArgs('a', 'b', 'c')
console.log(result)
// is [
//   b
//   a
//   c
// ]
const returnArgs = (a1: string, a2: number, a3: boolean) => [a1, a2, a3]

const returnSwappedArgs = swapFirstTwoArgs(returnArgs)

const result = returnSwappedArgs(1, 'a', true)
console.log(result)
// is [
//   a
//   1
//   true
// ]

Try It

const returnArgs = (...args) => args

const returnSwappedArgs =
  swapFirstTwoArgs(returnArgs)

const result = returnSwappedArgs('a', 'b', 'c')
console.log(result)
// is [
//   b
//   a
//   c
// ]
const returnArgs = (
  a1: string,
  a2: number,
  a3: boolean
) => [a1, a2, a3]

const returnSwappedArgs =
  swapFirstTwoArgs(returnArgs)

const result = returnSwappedArgs(1, 'a', true)
console.log(result)
// is [
//   a
//   1
//   true
// ]

Most iterating callbacks have the signature (value, key, collection) => any. For examples, see array.map, map.forEach, _.mapKeys etc. This makes sense, as we usually care about the value. Sometimes, however, we care more about the key.

This is where swapFirstTwoArgs is helpful. We can pass a function that expects a key as its first parameter, then use it as a callback. Because this is the primary use case, we can assign the alias 'byKey,' as seen below.

Onto the example

Below, we are writing library software that tracks checked-out books. Other systems require lowercase names, so let's map over our keys to be compatible.

Try It

import { mapKeys, swapFirstTwoArgs as byKey } from 'common-fp'

const checkedOutBooks = {
  Emma: ['The Very Hungry Caterpillar'],
  Meg: ['The Hobbit'],
}

const toLower = str => str.toLowerCase()
const lowerKey = byKey(toLower)
const lowerAllKeys = mapKeys(lowerKey)
const checkouts = lowerAllKeys(checkedOutBooks)

console.log(checkouts)
// is {
//   emma: [The Very Hungry Caterpillar]
//   meg: [The Hobbit]
// }
import { mapKeys, swapFirstTwoArgs as byKey } from 'common-fp'

type Checkouts = Record<string, string[]>
const checkouts: Checkouts = {
  Emma: ['The Very Hungry Caterpillar'],
  Meg: ['The Hobbit'],
}

const toLower = (str: string) => str.toLowerCase()
const lowerKey = byKey(toLower)
const lowerAllKeys = mapKeys(lowerKey)<Checkouts>
const updatedCheckouts = lowerAllKeys(checkouts)

console.log(updatedCheckouts)
// is {
//   emma: [The Very Hungry Caterpillar]
//   meg: [The Hobbit]
// }

Try It

import {
  mapKeys,
  swapFirstTwoArgs as byKey,
} from 'common-fp'

const checkedOutBooks = {
  Emma: ['The Very Hungry Caterpillar'],
  Meg: ['The Hobbit'],
}

const toLower = str => str.toLowerCase()
const lowerKey = byKey(toLower)
const lowerAllKeys = mapKeys(lowerKey)
const checkouts = lowerAllKeys(checkedOutBooks)

console.log(checkouts)
// is {
//   emma: [The Very Hungry Caterpillar]
//   meg: [The Hobbit]
// }
import {
  mapKeys,
  swapFirstTwoArgs as byKey,
} from 'common-fp'

type Checkouts = Record<string, string[]>
const checkouts: Checkouts = {
  Emma: ['The Very Hungry Caterpillar'],
  Meg: ['The Hobbit'],
}

const toLower = (str: string) => str.toLowerCase()
const lowerKey = byKey(toLower)
const lowerAllKeys = mapKeys(lowerKey)<Checkouts>
const updatedCheckouts = lowerAllKeys(checkouts)

console.log(updatedCheckouts)
// is {
//   emma: [The Very Hungry Caterpillar]
//   meg: [The Hobbit]
// }