javascript - 为什么 object[key] 不等于 key，如果 key 是一个对象，在 JavaScript 中？

Question

var a = new Object;
var b = new Object;
var c = new Object;

c[a] = a;
c[b] = b;

console.log(c[a] === a);

我测试了上面的代码并得到了false。如果我尝试 console.log(c[a] === b)，则会打印 true。

为什么？

Answer 1

这里的问题与 Object 的键如何设置有关。来自 MDN :

Parameters

nameValuePair1, nameValuePair2, ... nameValuePairN

• Pairs of names (strings) and values (any value) where the name is separated from the value by a colon.

value

• Any value.

可以通过三种方式(通过适当的键)访问对象的值:

var o = {};
var key = "fun";

// method 1:
o[key]    = "the key will be equal to `key.toString()"
// method 2:
o.key     = "the key will be equal to 'key'"
// method 3:
o["key2"] = "the key will be equal to `key2`"
/*
{
    "fun" : "the key will be...",    // method 1
    "key" : "the key will be...",    // method 2
    "key2": "the key will be..."     // method 3
}
*/

使用括号表示法时，您需要注意括号之间的空隙!对象使用 toString 设置它们的键和值方法，除非向它们传递了一个字符串(那么 toString 就没有意义了)。当使用点符号时，他们使用 .key 作为键。

让我们看看你的情况:

var a = {}
  , b = {}
  , c = {}
  ;

c[a] = a;
// `a` is not a string, and we're using brackets, so the key
// will be equal to `key.toString()`:
// a.toString() === "[object Object]"
// Try the following in your console: `{}.toString()`
// Note how this is different from console.log({}), since
// the console exposes your object (that's why the dev console is useful)
// c is now: `{ "[object Object]" : a }`

c[b] = b;
// b is also an object, so `b.toString()` is the same as `a.toString()`
// that means c is now `{ "[object Object]" : b }`

assert c[a] === a
// a.toString() == b.toString() == "[object Object]"
// and we just noted that c was `{ "[object Object]" : b }`
// so of course this is false
assert c[b] === b
// true because c[b] == b;
assert c["[object Object]"] === b;
// also true
assert c.b === b
// false, since `c` has no "b" key (c.b is `undefined`)