generators.js

function* fibonacci( ){
  let n1 = 0;
  let n2 = 1;
  while ( true ) {  
    const current = n1;
    n1 = n2;
    n2 = current + n1;
    yield current;
  }
}

let iterator = fibonacci( );
iterator.next( ); // 0
iterator.next( ); // 1
iterator.next( ); // 1
iterator.next( ); // 2
iterator.next( ); // 3
iterator.next( ); // 5