更改为原始副本时，为什么分配给变量的数组和对象会发生变化？

Question

在这段代码中，我将array1的副本分配给变量array2，但是当我更改array1时，它会更改array2。为什么会这样？

var array1 = [1, 2, 3, 4, 5];
var array2 = array1;
array1.pop();
console.log("array1: " + array1 + "\narray2: " + array2);

//output: array1: 1,2,3,4
//        array2: 1,2,3,4

由于数组是对象，我用一个对象测试了相同的概念并得到了相同的结果。我从原始对象中删除了一个属性，并从另一个对象的副本中删除了相同的属性。引擎盖下发生了什么？

var obj1 = {name: "object", day: "Tuesday", color: "yellow"};
var obj2 = obj1;

console.log("BEFORE DELETE:");
console.log("obj1 properties:");

for(var prop in obj1) {
    console.log(prop);
}
console.log("obj2 properties:");

for(var prop in obj2) {
    console.log(prop);
}

delete obj1.color; //delete only from obj1

console.log("AFTER DELETE:");
console.log("obj1 properties:");

for(var prop in obj1) {
    console.log(prop);
}
console.log("obj2 properties:");

for(var prop in obj2) {
    console.log(prop);
}

//output: BEFORE DELETE:
//        obj1 properties:
//        name
//        day
//        color
//        obj2 properties:
//        name
//        day
//        color
//        AFTER DELETE:
//        obj1 properties:
//        name
//        day
//        obj2 properties:
//        name
//        day

令人困惑，因为我希望它的行为与此相同：

var cow = "moo";
var calf = cow;

console.log("BEFORE CHANGE:");
console.log("cow: " + cow);  //prints "moo"
console.log("calf: " + calf); //prints "moo"

cow = "oink";
console.log("AFTER CHANGE:");
console.log("cow: " + cow); //prints "oink"
console.log("calf: " + calf); //still prints "moo"