CFP - Docs - negate

negate

```
(predicate: Predicate) => Predicate
```

Negate a predicate

For example, if you pass a function that always returns true, then negate gives you a function returning false.

const gt2 = n => n > 2
const lte2 = negate(gt2)

gt2(2) // is false
lte2(2) // is true
const gt2 = (n: number) => n > 2
const lte2 = negate(gt2)

gt2(2) // is false
lte2(2) // is true

Try It

const gt2 = n => n > 2
const lte2 = negate(gt2)

gt2(2) // is false
lte2(2) // is true
const gt2 = (n: number) => n > 2
const lte2 = negate(gt2)

gt2(2) // is false
lte2(2) // is true

Sometimes we want the opposite of a predicate. Below, we are writing a plugin for a chat API. We want to show a tooltip when a user cannot be messaged. Let's negate canMessage and update the users with our tooltip.

Try It

const pluginApi = {
  canMessage: user => user.status === 'active',
  updateUsers: (updateFn, predicate) => {
    pluginApi._users.filter(predicate).forEach(updateFn)
  },
  _users: [
    {
      email: 'matt@example.com',
      status: 'away',
    },
    {
      email: 'jason@example.com',
      status: 'active',
    },
  ],
}

const cannotMessage = negate(pluginApi.canMessage)
const addTooltip = mSet('tooltip', 'cannot message this user')

pluginApi.updateUsers(addTooltip, cannotMessage)

console.log(pluginApi._users)
// is [
//   {
//     email: matt@example.com
//     status: away
//     tooltip: cannot message this user
//   }
//   {
//     email: jason@example.com
//     status: active
//   }
// ]
type User = {
  email: string
  status: string
  tooltip?: string
}
type Predicate = (user: User) => boolean
type UpdateFn = (user: User) => void

const pluginApi = {
  canMessage: (user: User) => user.status === 'active',
  updateUsers: (updateFn: UpdateFn, predicate: Predicate) => {
    pluginApi._users.filter(predicate).forEach(updateFn)
  },
  _users: [
    {
      email: 'matt@example.com',
      status: 'away',
    },
    {
      email: 'jason@example.com',
      status: 'active',
    },
  ] as User[],
}

const cannotMessage = negate(pluginApi.canMessage)
const addTooltip = mSet('tooltip', 'cannot message this user')<User>

pluginApi.updateUsers(addTooltip, cannotMessage)

console.log(pluginApi._users)
// is [
//   {
//     email: matt@example.com
//     status: away
//     tooltip: cannot message this user
//   }
//   {
//     email: jason@example.com
//     status: active
//   }
// ]

Try It

const pluginApi = {
  canMessage: user => user.status === 'active',
  updateUsers: (updateFn, predicate) => {
    pluginApi._users
      .filter(predicate)
      .forEach(updateFn)
  },
  _users: [
    {
      email: 'matt@example.com',
      status: 'away',
    },
    {
      email: 'jason@example.com',
      status: 'active',
    },
  ],
}

const cannotMessage = negate(pluginApi.canMessage)
const addTooltip = mSet(
  'tooltip',
  'cannot message this user'
)

pluginApi.updateUsers(addTooltip, cannotMessage)

console.log(pluginApi._users)
// is [
//   {
//     email: matt@example.com
//     status: away
//     tooltip: cannot message this user
//   }
//   {
//     email: jason@example.com
//     status: active
//   }
// ]
type User = {
  email: string
  status: string
  tooltip?: string
}
type Predicate = (user: User) => boolean
type UpdateFn = (user: User) => void

const pluginApi = {
  canMessage: (user: User) =>
    user.status === 'active',
  updateUsers: (
    updateFn: UpdateFn,
    predicate: Predicate
  ) => {
    pluginApi._users
      .filter(predicate)
      .forEach(updateFn)
  },
  _users: [
    {
      email: 'matt@example.com',
      status: 'away',
    },
    {
      email: 'jason@example.com',
      status: 'active',
    },
  ] as User[],
}

const cannotMessage = negate(pluginApi.canMessage)
const addTooltip = mSet(
  'tooltip',
  'cannot message this user'
)<User>

pluginApi.updateUsers(addTooltip, cannotMessage)

console.log(pluginApi._users)
// is [
//   {
//     email: matt@example.com
//     status: away
//     tooltip: cannot message this user
//   }
//   {
//     email: jason@example.com
//     status: active
//   }
// ]