The Ecmascript 4 proposal included the `yield` keyword, but the proposal as a whole was abandoned in 2008. That’s why the version names jump from ES3 to ES5. So

The Ecmascript 4 proposal included the `yield` keyword, but the proposal as a whole was abandoned in 2008. That’s why the version names jump from ES3 to ES5. Some of the features then returned in later versions, including generators with ES6 in 2015.