为什么（new Date（）== new Date（））为假，而（Date（）== Date（））为真？ [英] Why is (new Date() == new Date()) false, but (Date() == Date()) is true?

问题描述

我一直在与JSFiddle纠缠在一起，以解决FreeCodeCamp中的此问题。当我使用Date作为字符串（即没有 new）时：

I have been messing around with JSFiddle to solve this problem in FreeCodeCamp. When I use Date as a string (i.e., no "new"):

情况1：

function isSameDay (dtFrom, dtTo) {
    return dtFrom == dtTo
  }

  let today = Date()
  let tomorrow = Date()

  console.log(today)
  console.log(tomorrow)
  console.log(isSameDay(today, tomorrow))

isSameDay返回 true 。但是，当我使用Date作为构造函数（带有 new）时：

isSameDay returns true. However when I use Date as a constructor (with "new"):

情况2：

function isSameDay (dtFrom, dtTo) {
    return dtFrom == dtTo
  }

  let today = new Date()
  let tomorrow = new Date()

  console.log(today)
  console.log(tomorrow)

  console.log(isSameDay(today, tomorrow))

isSameDay返回 false 。但是（！），当我添加一元运算符 +时：

isSameDay returns false. However(!), when I add the unary operator "+":

情况3：

function isSameDay (dtFrom, dtTo) {
    return dtFrom == dtTo
  }

  let today = + new Date()
  let tomorrow = + new Date()

  console.log(today)
  console.log(tomorrow)

  console.log(isSameDay(today, tomorrow))

isSameDay返回 true 。我了解情况1和情况3返回true，因为它们只是相同的字符串和相同的毫秒值。

isSameDay returns true. I understand case 1 and case 3 returning true because they are just the same strings and the same millisecond values.

为什么情况2返回 false ？

推荐答案

使用 Date（），JavaScript Date对象只能可以通过调用JavaScript Date作为构造函数实例化：作为常规函数调用（即不使用new运算符）将返回字符串，而不是Date对象。 MDN参考。

Using Date(), the JavaScript Date objects can only be instantiated by calling JavaScript Date as a constructor: calling it as a regular function (i.e. without the new operator) will return a string rather than a Date object. MDN Reference.

typeof Date()    //"string"
Date() == Date() //true

使用构造函数作为 new Date（），每个实例都是唯一的（两个实例同一构造函数的实例彼此之间仍然是不同的），这就是为什么它们在比较时不相等的原因。

Using instead a constructor as new Date(), each instance is unique (the two instances of the same constructor are still different to each-other), this is the reason why they are not equal when compared.

typeof new Date();        //"object"
new Date() === new Date() //false

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