SOURCE

var button = document.getElementsByTagName('button')[0];
var value1  = document.getElementById('number1');
var value2  = document.getElementById('number2');
var span = document.getElementsByTagName('span')[0];
button.addEventListener('click',()=>{
  plus(value1.value,value2.value);
},false);

function plus(value1,value2){
  var temp,res = [];
  if(value1.length < value2.length){
    temp = value1;
    value1 = value2;
    value2 = temp;
  }
  
  value1 = dx2(value1);
  value2 = dx2(value2);
  
  var jw = 0,bw = 0;
  for(var i in value1){
    if(value2[i] == undefined){
      value2[i] = 0;
    }
    jw = parseInt((value1[i] + value2[i]) / 10);
    bw = parseInt((value1[i] + value2[i]) % 10);
    res[parseInt(i)] = parseInt(res[i]?res[i]:0) + bw;
    res[parseInt(i+1)] = jw;
  }
  
  res = dx2(res.join(''));
  console.log(res);
  span.textContent = res.join('');
}

function dx2(value){
  var shang,ys,res=[],type;
  if(value[0] == '-'){
    shang = value.substr(1);
    type = '-'
  }else{
    shang = value;
  }
  do{
    ys = shang % 10;
    shang = parseInt(shang / 10);
    res.push(ys);
  }while(shang != 0);
  
  for(var i in res){
    if(res[i] == 0){
      res.shift()
    }else{
      break;
    }
  }
  if(type){
    res.unshift(type)
  }
  return res;
}

function type(value1,value2){
  var type1 = value1[0] == '-'? '-' : '+';
  var type2 = value2[0] == '-'? '-' : '+';
  
  return 
}

<input type="text" id='number1'/>
<input type="text" id='number2'/>
<button>相加</button>
<span></span>