let pets = 0;
obj.map( item => {
  if(item.pet) {
    pets += 1;
  }
})

使用pet属性和pet的color属性过滤数组，并执行数组计数。

let obj = [
    {
        "person": {
            "name": "a",
        },
        "pet": {
            "name": "1"
        }
    },

    {
        "person": {
            "name": "b",
        },
        "pet": {
            "name": "2",
            "color": "black",
        }
    },

    {
        "person": {
            "name": "c",
        },
        "pet": {
            "name": "3",
            "color": "red",
        }
    }
];
let petItemCount = obj.filter(x => x["pet"]).length;
console.log(petItemCount);

let petItemWithColorCount = obj.filter(x => x["pet"] && x["pet"]["color"]).length;
console.log(petItemWithColorCount);

在pet值可能为null或pet的color可能为空的情况下，

let petItemCount = obj.filter(x => x["pet"] != undefined).length;
console.log(petItemCount);

let petItemWithColorCount = obj.filter(x => x["pet"] && x["pet"]["color"] != undefined).length;
console.log(petItemWithColorCount);

听起来减少会是一个适合：

colors = obj.reduce(
 (sum, { pet }) => pet.color !== undefined ? ++sum : sum, 
0)); // -> initial count value