pick

266 bytes

Usage

Given an object and a list of keys in the object, returns a new object with only the given keys.

import * as _ from 'radashi'

const fish = {
  name: 'Bass',
  weight: 8,
  source: 'lake',
  brackish: false,
}

_.pick(fish, ['name', 'source']) // => { name, source }

Predicate function

The pick function can also accept a predicate function as the filter argument. This allows for more complex filtering logic beyond simple key inclusion or exclusion.

import * as _ from 'radashi'

const source = { a: 1, b: 2, c: 3, d: 4 }

_.pick(source, (value, key) => {
  return value % 2 === 0 // Include only even values
})
// => { b: 2, d: 4 }

Unsafe predicate function

Caution

When used with a predicate function, pick is potentially unsafe, because of partial type matching performed by TypeScript. If you pass an object with more properties than its TypeScript type has listed, the value and key parameter types of your callback will be inaccurate.

// Example demonstrating potential inaccuracy in `key` and `value` types within `_.pick` callback
import * as _ from 'radashi'

interface User {
  name: string
  age: number
}

function getUserDetails(user: User) {
  return _.pick(user, (value, key) => {
    // TypeScript believes `key` is 'name' | 'age', but at runtime
    // it could be 'email'
    if (key === 'name' || key === 'age') {
      console.log(key, '=', value)
    } else {
      // TypeScript believes this will never run, but it does.
      console.log('Unexpected key:', key)
    }
  })
}

// At runtime, the function may receive an object with more properties
const runtimeUser = {
  name: 'John',
  age: 30,
  // This property is not listed in the User type:
  email: 'john@example.com',
}

getUserDetails(runtimeUser)
// Logs the following:
//     name = John
//     age = 30
//     Unexpected key: email