Issue with date comparison operator logic

Here's a simple JavaScript function I have:

function checkTime() {  
  var current = new Date();
  var target = new Date('April 10, 2017 12:11:00');

  if (current < target) {
    $('#modalnew').modal('show');
  } else {
    window.location.replace('https://www.example.php');
  }
}

The function works as expected, but the date comparison logic is puzzling me. Despite looking at similar examples online, I'm still unable to grasp it fully. For instance, consider this hypothetical situation without any code:

Current Time (now) = April 10, 2017 12:22:00
Target Time (date set) = April 10, 2017 12:11:00

Given that current seems to be 11 minutes ahead of target, why does the function trigger the window.location? It appears illogical based on the time values alone. What exactly is being compared in this scenario?

javascriptdatedate-comparison

Answer №1

When examining your scenario, it becomes apparent that d1 is actually greater than d2, contrary to what was expected. As a result, the code for window.location.replace is triggered.

During the comparison of the dates, the condition being evaluated is (d1.valueOf() < d2.valueOf()). The method valueOf() returns the time in milliseconds since the start of 

00:00:00 UTC Thursday 1, January 1970
, which is then used for the comparison.

Answer №2

When comparing two dates in JavaScript, it is recommended to utilize the Date.prototype.getTime() method. This method returns a numeric value representing the time for the specified date based on universal time.

The numerical result obtained from comparing the values of dates d1 and d2 will determine which date is earlier or later.

Example code:

function checkTime() {  
  var d1 = new Date();
  var d2 = new Date('April 10, 2017 12:11:00');

  if (d1.getTime() < d2.getTime()) {
    $('#modalnew').modal('show');
  } else {
    window.location.replace('https://www.example.php');
  }
}

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