Tail call optimization

only return call() either implicitly such as in arrow function or explicitly, can be a tail call statment
function foo(){ return bar(); } // the call to bar is a tail call
function foo(){ bar(); }// bar is not a tail call. The function returns undefined when no return is given
const foo = () => bar(); // bar() is a tail call
const foo = () => (poo(),bar()); // poo is not a tail call, bar is a tail call
const foo = () => poo() && bar(); // poo is not a tail call, bar is a tail call
const foo = () => bar() + 1; // bar is not a tail call as it requires context to return + 1

TCO is also known as PTC (Proper Tail Call) as it is referred to in the ES2015 specifications.

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