let prices = [
  {date:'2000-01-01', IBM:100, MSFT:145},
  {date:'2000-01-02', IBM:101, MSFT:144},
];
let newPrices = [];
prices.forEach(p=>{
  for(let key in p){
    if(key !== 'date'){
      let item = {
        date:p.date
      };
      item.price = p[key];
      newPrices.push(item);
    } 
  }
});