JavaScript中扩展数组推入另一数组：按值还是按引用？含示例疑问

示例1 #

先看这段代码：

const hobbies = ['Sports', 'Cooking'];
const activeHobbies = ['Hiking'];

activeHobbies.push(...hobbies);
console.log(activeHobbies); //['Hiking', 'Sports', 'Cooking']

hobbies.push('Skiing');
console.log(hobbies); //['Sports', 'Cooking', 'Skiing']
console.log(activeHobbies); //['Hiking', 'Sports', 'Cooking']

解答 #

• 用扩展语法activeHobbies.push(...hobbies)完全等同于activeHobbies.push(hobbies[0], hobbies[1])，是按值传递数组元素。
• 扩展语法会把hobbies的元素逐个提取出来，作为独立参数传给push方法。这里的元素是字符串（原始值），原始值在传递时会复制值本身，所以activeHobbies里存的是'Sports'和'Cooking'的副本，和原hobbies数组后续的修改无关。这就是为什么给hobbies添加'Skiing'后，activeHobbies不会跟着更新。

示例2 #

再看这段代码：

const hobbies = ['Sports', 'Cooking'];
const activeHobbies = ['Hiking'];

activeHobbies.push(hobbies);
console.log(activeHobbies); //['Hiking', ['Sports', 'Cooking', 'Skiing']], why not ['Hiking', ['Sports', 'Cooking']] ?

hobbies.push('Skiing');
console.log(hobbies); //['Sports', 'Cooking', 'Skiing']
console.log(activeHobbies); //['Hiking', ['Sports', 'Cooking', 'Skiing']]

解答 #

• 你的理解完全正确：直接执行activeHobbies.push(hobbies)时，是按引用传递数组。数组属于引用类型，传递的是指向内存中数组对象的引用，所以activeHobbies的第二个元素和hobbies指向同一个数组对象，后续修改hobbies时，activeHobbies里的数组也会同步变化。
• 至于第一个console.log的注释是错误的：这行代码在hobbies.push('Skiing')之前执行，实际输出应该是['Hiking', ['Sports', 'Cooking']]，注释里写的是执行完push('Skiing')之后的结果，属于注释失误。

内容的提问来源于stack exchange，提问作者madfcat