Add Get type

[mmkal]

Gets a deeply-nested property from an object, like lodash's get function.

Use-case: retrieve a property from deep inside an API response or other complex object.

I couldn't find anything like this, so thought I'd put up a proposal since I've written shoddy versions of it a few times now.

import { Get } from 'type-fest'

interface ApiResponse {
  hits: {
    hits: Array<{
      _id: string
      _source: {
        name: Array<{
          given: string[]
          family: string
        }>
        birthDate: string
      }
    }>
  }
}
type Name = Get<ApiResponse, 'hits.hits[0]._source.name'> // Array<{ given: string[]; family: string }>

This works by first splitting the path based on . and [...] characters into a tuple of string keys.
Then it recursively uses the head key to get the next property of the current object, until there are no keys
left. Number keys extract the item type from arrays, or are converted to strings to extract types from tuples
and dictionaries with number keys.

Todo:

• propagate nullable props
• add tests for nullable props (likely not blocked by Run tsd in test script #162 but I don't think tests are being run by CI properly right now so maybe)
• make bad paths return unknown, not undefined
• add test for readonly props/arrays
• rename Numbers to StringDigit or similar

Follow-ups:

• Extract KeyPath type like in Add KeyPath and PathValue types #158
• Set type - Set<{ foo: number }, 'bar.baz', string> = { foo: number; bar: { baz: string } }
• (maybe) support foo\\.bar.baz to allow for dots within keys, similar to dot-prop.

---

Fixes #147

[sindresorhus]

Yes, we definitely want this. Thanks for working on it. We actually have an issue for it: #147

Did you see https://www.typescriptlang.org/play?ts=4.1.0-pr-40336-88#code/FAFwngDgpgBACgQxACwJIFsIBsA8AVAGhgGkowYoAPEKAOwBMBnGAazIHsAzGPAPhgC8wGCTIVqdJjEYgATgEtaAc2EwA-DwDapMAF1xNBswBKUAMbtZ9HDIXKiCWmF6qRGgD4wABgBIA3joAvgB0-ogoGNj42mS6RACilGZYAK70UDhsYFxaOnGsHNyOYJq6vPwAZNJyikqBXq4iMJ6+AWQh-onJaRlZOXgxekR9RU6lldV2dQ1NMABcMLRQAG5QsqoLS6uyANzAoJCw4WiYWABM+PwC8EgnUYQF2dx8zY-9ewfQNyiXgt9351+VEMUlstVeIx46n+kUBLwWkLwH3AX2OADUEKkMg84AZJMxjpcrqpccD8d5-IpOGtRGAOn4qTTTDJ6qoNDo8UY3s9GhpmSBOVJCQM8i5Zm5-hisdE8kR+WLxZsVmtGkrtht4ILmIjeVo4LpVYtlbt9ulkghZLBOClaGYQPJ2LQYEooCB8ERSRIucLygAKdgAIwAVgsHhBbgs4ABKSO3KUpbEe3gfCy0GQwQNB8wC65+VSceSyGQAOQQ6CgCwARAAReRQJTsSsEVRYBAlssVmCVgASCAAXk3VAgXQsAMwABmbIggsnYWbtjAWmkafkWHarpgQLHkICbMFTNQDKRAlkXMAAbAAOGCBKdNVe0dddgDCjpkSCge4PCiPJ6LCwARjOG87wNQIYDbfc3xAD4XRAf1g2zIhKxnOds0YYJx2CR9y0rKMdiAA ? Maybe there's some inspiration in there.

[sindresorhus]

Make sure you adhere to https://github.com/sindresorhus/type-fest/blob/master/.github/contributing.md

[mmkal]

@sindresorhus thanks for reviewing, have updated based on comments. One note - in the latest revision I changed the nevers to undefineds since that's the standard behaviour - never would imply "throwing"

get({ foo: 'bar' }, 'some.nonsense.string') // returns undefined, rather than throws

Edit: hadn't seen the issue/playground - will take a look and see if there are any simplifications/improvements to be made.

[sindresorhus]

Can you add some more tests? And fix the lint errors.

For example, https://github.com/sindresorhus/type-fest/pull/158/files#diff-c4a707129eadc5349a83e23c9bd5a1fc2f05e0c88b1365e6fb7efe9961ae95ddR13-R21

[sindresorhus]

@mmkal This looks good to me when #153 (comment) is fixed.

@voxpelli LGTY?

[mmkal]

Sounds good - will be able to work on it in a couple of days. Happy new year!