var input = [

 {x: Date(1234), y: 0},

 {x: Date(1235), y: 0},

 {x: Date(1236), y: 300},

 {x: Date(1237), y: 300},

 {x: Date(1238), y: 300},

 {x: Date(1239), y: 300},

 {x: Date(1240), y: 300},

 {x: Date(1241), y: 0},

 {x: Date(1242), y: 0},

 {x: Date(1243), y: 0},

];

​

var output = input.reduce(function (acc, val) {

  var lastIndex = acc.length - 1;

  if (val.y <= 0 || lastIndex < 0 || acc[lastIndex].y <= 0) {

    acc.push(val);

  } else {

    acc[lastIndex].y += val.y;

  }

  return acc;

}, []);