In computer science, Peter Landin's J operator is a programming construct that post-composes a lambda expression with the continuation to the current lambda-context.
The J operator was created to make labels and jumps a first class value.
It was designed to work with the SECD machine with the following extra transitions: The J operator originally created what was called a "program closure", consisting of a function called the body and a SECD state called the dump.
This is because λx.x is the identity function, so when it gets applied it will do nothing to the value given and returns it straight away.
λv.J(λx.x) initially returns the J of λx.x, but that could be used in a surrounding expression to make it re-return a different value.