generator_1_fibonacci_1.js

function* fibonacciGenerator(){         //the * indicates it's a generator function
                                        //i.e. a function that returns a generator object
    yield 0;                            //first time next() is called
                                        //it'll return with value=0
    yield 1;                            //second time it'll return with value=1

    let penultimate = 0, ultimate = 1;
    let next = penultimate + ultimate;
    
    while (true){
        yield next;                     //on subsequent calls
        penultimate = ultimate;
        ultimate = next;
        next = penultimate + ultimate;  //it'll return sum of previous 
                                        //2 terms as value
    }
}

generator_1_fibonacci_2.js

const fibGen = fibonacciGenerator();
console.log(fibGen.next().value);    //0
console.log(fibGen.next().value);    //1
console.log(fibGen.next().value);    //1
console.log(fibGen.next().value);    //2