function*
The function* declaration ( function keyword followed by an asterisk) defines a generator function, which returns a object. You can also define generator functions using the GeneratorFunction constructor, or the function expression syntax. Generators are functions that can be exited and later re-entered. Their context (variable bindings) will be saved across re-entrances.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function*