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: stringfriends: ?Array<User>};
Getting to the friends of my first friend would resemble:
propsuser &&propsuserfriends &&propsuserfriends0 &&propsuserfriends0friends
idx allows us to safely write:
The second argument must be a function that returns one or more nested member expressions. Any other expression has undefined behavior.
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:
propsuser == null ? propsuser :propsuserfriends == null ? propsuserfriends :propsuserfriends0 == null ? propsuserfriends0 :propsuserfriends0friends