1var lab =["1","2","3"];
2var val = [42,55,51,22];
3var data = [];
4for(var i=0; i<4; i++)  {
5    data.push({label: lab[i], value: val[i]});
6}
7Or an other, less concise way, but closer to your original code:
8
9for(var i=0; i<4; i++)  {
10   data[i] = {};              // creates a new object
11   data[i].label = lab[i];
12   data[i].value = val[i];    
13}