不用 reduce 时实现

统计一个数组中有多少个不重复的元素 以对象形式输出

let arr = ["a", "b", "c", "a", "a", "c"];
function getObjNum() {
    let obj = {};
    for (let i = 0, l = arr.length; i < l; i++) {
        let item = arr[i];
        obj[item] = (obj[item] + 1) || 1;
    }
    return obj;
}
console.log(getObjNum());  // 输出：{ a: 3, b: 1, c: 2 }

// 以数组形式输出
let object = getObjNum();
let newArr = [];
for (key in obj) {
    newArr.push({
        type: key,
        count: obj[key]
    })
};
console.log(getObjNum());  
// 输出如下：
[ 
    { type: "a",  count: 3 },
    { type: "b",  count: 1 },
    { type: "c", count: 2 }
]

使用 reduce 时实现

统计一个数组中有多少个不重复的元素 以对象形式输出

let arr = ["a", "b", "c", "a", "a", "c"];
function getObjNum() {
    return arr.reduce(function(prev,next){ 
            prev[next] = (prev[next] + 1) || 1; 
            return prev; 
        },{}); 
}
console.log(getObjNum());  // 输出：{ a: 3, b: 1, c: 2 }

// 以数组形式输出
let object = getObjNum();
let newArr = [];
for (key in obj) {
    newArr.push({
        type: key,
        count: obj[key]
    })
};
console.log(getObjNum());  
// 输出如下：
[ 
    { type: "a",  count: 3 },
    { type: "b",  count: 1 },
    { type: "c", count: 2 }
]