ES6 different generator expressions

generator.js

var g1 = function* (){ yield 1; yield 2; yield 3; return 'hola'};
var g2 = function* (){ x = yield* g1(); yield 4; return 'bye' }

// yield: 1,2,3,4 ( x becomes 'hola'), bye, done

//else

var g1 = function* (){ yield 1; yield 2; yield 3; return 'hola'};
var g2 = function* (){ yield* g1() };

// yield: 1,2,3, done

// but

var g1 = function* (){ yield 1; yield 2; yield 3; return 'hola'};
var g2 = function* (){ return yield* g1() };

//yield: 1,2,3, 'hola', done

yield* is an operator that is only available inside generators. It yields all items iterated over by an iterable.

function* yieldAllValuesOf(iterable) {
yield* iterable;
}
The most important use case for yield* is to recursively call a generator (which produces something iterable).

Source: http://exploringjs.com/es6/ch_iteration.html