let invoices = [{

    id: 1,

    date: '07-29-2016',

    customerId: 4711,

    positions: [

      { id: 1, title: 'Angular 2 for beginners' },

      { id: 2, title: 'Angular 2 for pros' }

    ]

  },{

    id: 2,

    date: '07-29-2016',

    customerId: 4712,

    positions: [

      { id: 1, title: 'React for beginners' },

      { id: 2, title: 'React for pros' }

    ]

  }

];

​

let positions = [];

​

for (let inv of invoices) {

  positions.push(inv.positions);

}

​

console.log(positions);