Javascript Date comparison looks weird

Question

It seems to me that this should live on a golf code field, but the following behavior caused a strange error and is superficially incomprehensible:

(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)

** Change ** Where is this documented, please?

Answer 1

All your comparisons, other than ===, push dates to numbers (their time values, milliseconds from an era), and then compare these numbers. Comparison ===checks whether aand b to one and the same object . ( ==will do the same.)

( ) , . , , .

:

// Create two Date instances, check if a time value is less than b's
// (probably not, but it 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 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 time value is greater than b's
a > b    // false

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

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

, , ( , + ):

(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  

: , +a === +b ( +a == +b, — — +a == b a == +b), , .

, , , :

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 time is not < b's");
show("+a < +b:   ", +a < +b,   "a time is not < b's");
show("a < b:     ", a < b,     "a 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 time is not > b's");
show("a > b:     ", a > b,     "a time is not > b's");
show("+a <= +b:  ", +a <= +b,  "a time is equal to b's");
show("a <= b:    ", a <= b,    "a time is equal to b's");
show("+a >= +b:  ", +a >= +b,  "a time is equal to b   ");
show("a >= b:    ", a >= b,    "a time is equal to b   ");

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);
}

Hide result

Answer 2

, .getTime(). :

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