Javascript日期比较很奇怪

Question

我觉得这应该生活在一个电码高尔夫球杆上，但以下行为引起了一个奇怪的错误，表面上看是无法理解的：

(a = new Date()) < (b = new Date())
a  // Thu Feb 11 2016 12:24:19 GMT+0000 (GMT)
b  // Thu Feb 11 2016 12:24:19 GMT+0000 (GMT)
a < b    // false (correct)
a === b  // false (incorrect)  <<<
a > b    // false (correct)
a <= b   // true (correct)
a >= b   // true (correct)

** 编辑 **请问这些文件在哪里？

Answer 1

除了=== 1之外，您的所有比较都是将日期强制限定为数字(它们的时间值，自纪元以来的毫秒)，然后比较这些数字。===比较是检查a和b是否引用了同一个对象。(==也会做同样的事情。)

时间(在您的示例中)完全相同，但仍然有两个不同的对象。所以你所看到的结果。

附加说明：

// Create two Date instances, check if a's time value is less than b's
// (probably not, but it's just feasible this could cross a millisecond
// threshold)
(a = new Date()) < (b = new Date())

// Show the value of a and b
a  // Thu Feb 11 2016 12:24:19 GMT+0000 (GMT)
b  // Thu Feb 11 2016 12:24:19 GMT+0000 (GMT)

// See if a's time value is less than b's
a < b    // false

// See if a and b refer to the same object (they don't)
a === b  // false

// See if a's time value is greater than b's
a > b    // false

// See if a's time value is less than or equal to b's
a <= b   // true

// See if a's time value is greater than or equal to b's
a >= b  

或者换个说法，您的代码基本上就是这样做的(请注意，+标志在哪里，而不是在哪里)：

(a = new Date()) < (b = new Date())
a  // Thu Feb 11 2016 12:24:19 GMT+0000 (GMT)
b  // Thu Feb 11 2016 12:24:19 GMT+0000 (GMT)
+a < +b    // false
a === b    // false
+a > +b    // false
+a <= +b   // true
+a >= +b  

请注意，如果时间是完全相同的，+a === +b (或+a == +b，或相当棘手- +a == b或a == +b)都将是真实的，因为我们将比较时间值。

为了好玩，下面是一个我们知道底层时间值完全相同的例子：

​

var a, b;
var now = Date.now(); // = +new Date(); on old browsers
show("create and compare: ", (a = new Date(now)) < (b = new Date(now)), "a's time is not < b's");
show("+a < +b:   ", +a < +b,   "a's time is not < b's");
show("a < b:     ", a < b,     "a's time is not < b's");
show("a === b:   ", a === b,   "a and b don't refer to the same object");
show("a == b:    ", a == b,    "a and b don't refer to the same object");
show("+a === +b: ", +a === +b, "their time values are the same");
show("+a == +b:  ", +a == +b,  "their time values are the same");
show("+a > +b:   ", +a > +b,   "a's time is not > b's");
show("a > b:     ", a > b,     "a's time is not > b's");
show("+a <= +b:  ", +a <= +b,  "a's time is equal to b's");
show("a <= b:    ", a <= b,    "a's time is equal to b's");
show("+a >= +b:  ", +a >= +b,  "a's time is equal to b's   ");
show("a >= b:    ", a >= b,    "a's time is equal to b's   ");

function show(prefix, result, because) {
    var msg = prefix + result + ", " + because;
    var pre = document.createElement('pre');
    pre.innerHTML = msg.replace(/&/g, "&amp;").replace(/</g, "&lt;");
    document.body.appendChild(pre);
}

​

Answer 2

要比较日期，您需要使用.getTime()。因此，正确的比较应该是：

a.getTime() === b.getTime() //true
a.getTime() > b.getTime()   //false
...