<script>
      // 只把a中b的值，替换成f中d的值，c值不变
	  var a = [
	  {b:1,c:5},
	  {b:2,c:6},
	  {b:3,c:7},
	  {b:4,c:8},
	  ]

     var f =[
      {b:12},
      {b:13},
      {b:14},
      {b:15},
     ]

     a.forEach((item, index) => {
        item.b = f[index].d;
     });
	 console.log(a);
	</script>