写一个能遍历数组和对象的forEach函数

forEach函数遍历数组：

var arr = [1,2,3];
arr.forEach (function (item, index) {
    console.log (index,item)
})

forEach函数遍历对象：

var obj = {
    x: 100,
    y: 200,
    z: 300
}
var key;
for (key in obj){
    if (obj.hasOwnProperty (key)) {
        console.log (key,obj[key])
    }
}

能遍历二者的forEach函数

function forEach (obj, fn) {
    var key;
    if (obj instanceof Array) {
        obj.forEach (function (item, index) {
            fn (item,index)
        })
    } else {
        for (key in obj) {
            if (obj.hasOwnProperty (key)){
                fn (key, obj[key])
            }
        }
    }
}
var arr = [1, 2, 3];
var obj = {
    x: 100,
    y: 200,
    z: 300
}
forEach (arr, function (item, index) {
    console.log (index,item)
})
forEach (obj, function (key, val) {
    console.log (key, val)
})