x === x在没有NaN的情况下返回false是否有任何值？ [英] Is there any value for what x === x returns false without NaN?

问题描述

x === x 是否有任何值返回 false 没有NaN？

Is there any value for what x === x returns false without NaN?

例如：

> x = 1
1
> x === x
true
> x = {}
{}
> x === x
true
> x = new Date()
Wed Nov 13 2013 15:44:22 GMT+0200 (EET)
> x === x
true
> x = NaN
NaN
> x === x
false

我看到唯一值 x === x 返回 false 是 isNaN（x）=== true 。

是否还有 x 的另一个值 x === x 返回 false ？我们欢迎官方参考！

Is there another value of x for what x === x returns false? An official reference would be welcome!

推荐答案

两个相等的非 - NaN 值将永远为真（ SLaks的回答正确引用规范）。但是，表达式 x 可以在评估相等性时更改其值。使用访问者属性描述符（即属性获取者）时，属性访问可能会发生这种情况：

The strict comparison between two equal non-NaN values will always be true (SLaks's answer correctly quotes the spec). However, it's possible for the expression x to change its value during the evaluation of the the equality. This can happen with property access when using accessor property descriptors (i.e., property getters):

foo = {};
Object.defineProperty(foo, "bar", {
    get: function() {
        return Math.random();
    }
})

foo.bar === foo.bar; // false

如果对全局对象执行此操作窗口（或节点中的全局），然后您可以看到 x === x 比较失败全局范围变量：

If you do this for the global object window (or global in Node), then you can see the x === x comparison fail for a global-scope variable:

Object.defineProperty(window, "bar", {
    get: function() {
        return Math.random();
    }
})

bar === bar; // false

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