Javascript "==" operator lies

Question

The following confuses me greatly. As noted in the comments, the compares seem to work by themselves, but when put together they don't

The while should run for all days in the same month, then increment i by one, then start over again.

I have laced the whole sequence up with console.log to try to figure it out, but it doesn't make any sense. Everything seems to equal one another, but still fails the "==" test in the while statement.

  var i=0;
  var currentdate = 0;
  var currentmonth = 0;
  var opensmonth = 0;
  var opens = [
  { "date":"3/30/2006","zip":"30038","latitude":"33.676358","longitude":"-84.15381"},
  { "date":"4/31/2006","zip":"30519","latitude":"34.089419","longitude":"-83.94701"}
  ];
  intid = setInterval("stepthrough()", 250);
  function stepthrough() {
    //figure out first date.
    if (currentdate == 0) { // we've not been run before
      currentdate = opens[0]["date"];
      currentmonth = currentdate.split("/", 1);
      console.log("Current Month: >" + currentmonth +"<");
    }
    console.log("Current month: " + currentmonth + " And opensdate: " + opens[i]["date"].split("/", 1));

    // 
    // TWILIGHT ZONE ENTERED.
    // 
    if (currentmonth == 3 ) { 
      console.log("Current month equals 3."); // PASSES
    }
    if (opens[i]["date"].split("/", 1) == 3) {
      console.log("Opens date equals 3."); // PASSES
    }
    // BOTH THE ABOVE TESTS PASS IN CHROME AND SAFARI WHAT THE F*$K JAVASCRIPT

    while(opens[i]["date"].split("/", 1) == currentmonth) { // WHY DOESNT THIS WORK I HATE COMPUTERS
      console.log("Trying to add a point one."); 
      addpoint(i);
      i++; 
      console.log("Trying to add a point."); 
    }

    //set the date for next iteration
    currentdate = opens[i]["date"];
    currentmonth = currentdate.split("/", 1);
    console.log ("Current date is now: " + currentdate + " and current month is now: " + currentmonth);
    jQuery('div#date').text(currentdate);

    //if (i>=5000) {
    if (!opens[i]["date"]) {
      console.log("Clearing interval");
      clearInterval(intid);
      //jQuery('div#date').text("Limited at 5000 records")
    }
  }

Answer 1

JavaScript typing is implicit. This means if it thinks you are trying to treat something as a number, it will do it's best to treat that object as a number, even if it is, say, a boolean, or a string.

When doing standard ==, JavaScript will use implicit conversions to try and match types. This often results in unexpected comparison results.

If you want to force strong comparisons, you must use the === operator.

That being said, if you are examining the 'number' representation of a string, e.g. "123", and want to use strong comparisons, you must convert it to a number using parseInt(str, 10);

For some examples on implicit typing in action, see the JavaScript truth table answer.

Answer 2

Here's the problem: ["1"] == 1 in Javascript, because of the implicit conversions @Matt describes. But ["1"] != ["1"] in Javascript, because you're comparing two arrays, and thus two objects, and object comparisons are only true if they point to the same object, not if they point to two identical objects.

When you assign with .split('/', 1), you're getting an array like ['3'], not the string "3" (as I think you might be assuming). So:

currentmonth = currentdate.split("/", 1); // currentmonth is ["3"]
currentmonth == 3; // true, as described above
opens[i]["date"].split("/", 1) == 3; // true, because left-hand evals to ["3"]
opens[i]["date"].split("/", 1) == currentmonth; 
// false, because you're comparing two arrays - ["3"] != ["3"]

To fix this with your current code, you can just get the string, not the array, like this:

currentmonth = currentdate.split("/")[0]; // currentmonth is "3"
opens[i]["date"].split("/")[0] == currentmonth; // true, both sides are "3"