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.

Consider the following type:

const props: {
  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] &&
props.user.friends[0].friends

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.

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:

props.user == null ? props.user :
props.user.friends == null ? props.user.friends :
props.user.friends[0] == null ? props.user.friends[0] :
props.user.friends[0].friends

All this machinery exists due to the fact that an existential operator does not currently exist in JavaScript.

idx is BSD licensed. We also provide an additional patent grant.