多個數組對象求和

var arr=[{a:1,b:2,c:3},
    	{a:2,b:2,c:2},
    	{a:1,b:2,c:7},
    	{a:2,b:2,c:3}];
    var obj={};
    var numa_i=0;
    var numb_i=0;
    var numc_i=0;
    for(var  i in arr){
    for(var  j in arr[i]){
    	if(j=="a"){
    		numa_i=arr[i][j]+=numa_i;
    	}
    	if(j=="b"){
    		numb_i=arr[i][j]+=numb_i;
    	}
    	if(j=="c"){
    		numc_i=arr[i][j]+=numc_i;
    	}
    }
    }
    obj.a=numa_i;
    obj.b=numb_i;
    obj.c=numc_i;
    console.log(obj)