ES6 added this: https://262.ecma-international.org/6.0/#sec-tail-position-calls https://262.ecma-international.org/6.0/#sec-tail-position-ca... So TCO is part

ES6 added this: https://262.ecma-international.org/6.0/#sec-tail-position-calls https://262.ecma-international.org/6.0/#sec-tail-position-ca... So TCO is part of the language standard but not universally implemented. You are correct about the ones that support it.