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idx Circle Status

idx is a utility function for traversing properties on objects and arrays.

If an intermediate property is either null or undefined, it is instead returned. The purpose of this function is to simplify extracting properties from a chain of maybe-typed properties.

This module exists as a stop-gap solution because JavaScript does not currently support optional chaining.


Consider the following type for props:

type User = {
  user: ?{
    name: string,
    friends: ?Array<User>,

Getting to the friends of my first friend would resemble:

props.user &&
props.user.friends &&
props.user.friends[0] &&

Instead, idx allows us to safely write:

idx(props, _ => _.user.friends[0].friends)

The second argument must be a function that returns one or more nested member expressions. Any other expression has undefined behavior.

Flow Type

Flow understands the idx idiom:

// @flow
import idx from 'idx';
function getName(props: User): ?string {
  return idx(props, _ =>;

Babel Transform

The idx runtime function exists for the purpose of illustrating the expected behavior and is not meant to be executed. The idx function is used in conjunction with a Babel plugin that replaces it with better performing code.

This babel plugin searches for requires or imports to the idx module and replaces all its usages, so this code:

import idx from 'idx';
function getFriends() {
  return idx(props, _ => _.user.friends[0].friends)

gets transformed to something like:

function getFriends() {
  props.user == null ? props.user :
  props.user.friends == null ? props.user.friends :
  props.user.friends[0] == null ? props.user.friends[0] :
  return props.user.friends[0].friends

(note that the original import gets also removed).

It's possible to customize the name of the import/require, so code that is not directly requiring the idx npm package can also get transformed:

  plugins: [
    ["babel-plugin-idx", {
      importName: './idx',


idx is MIT licensed.