const spans = [  

  { "begin": 870, "end": 889, "spanType": ["plan", "gt-plan"] },

  { "begin": 890, "end": 925, "spanType": ["plan", "gt-plan"] },

  { "begin": 926, "end": 938, "spanType": ["plan", "gt-plan"] },

  { "begin": 939, "end": 958, "spanType": ["plan", "gt-plan"] },

  { "begin": 7732, "end": 7790, "spanType": ["plan", "gt-plan"] },

  { "begin": 7791, "end": 7879, "spanType": ["plan", "gt-plan"] }

];

​

spans.sort((a,b) => a.begin - b.begin);

​

const result = [];

result.push(spans.reduce((prev,curr) => {

 if(prev.end < curr.begin - 1){

  result.push(prev);

  return Object.assign({},curr);

 }

 prev.end = curr.end;

 return prev;

}));

​

console.log(result);