Javascript - Weird issue with === and strings

Question

I have this code:

...
var id1 = playerTick.gameId;
var id2 = that.gameId;
if(id1 === id2)
 {}else{
throw "GameController instantiated with gameId '" + id2 + "' but tick has gameId '" + id1 + "'";
}

And when I run it, I get the message:

GameController instantiated with gameId 'game1' but tick has gameId 'game1'

How can === fail when it prints correctly 'game1' as the value of both properties? As a test, I have made this which works:

var g = "game1";
var g2 = "game1"
alert(g === g2); // Alerts true

Does anyone have an idea of ANY theoritical explanation of how my === can fail but the error message prints the same text? The type of both values are object.

Thankyou in advance.

UPDATE:

THE PROBLEM

It turned out, as all the replies pointed out (and thankyou for that), that the types were not identical. One of the was not a string resulting in === evaluating to false.

The problem occurred after using this function to retrieve a query parameter:

function querystring(key) {
       var re=new RegExp('(?:\\?|&)'+key+'=(.*?)(?=&|$)','gi');
       var r=[], m;
       while ((m=re.exec(document.location.search)) != null) r.push(m[1]);
       return r;
}

THE FIX

function querystring(key) {
           var re=new RegExp('(?:\\?|&)'+key+'=(.*?)(?=&|$)','gi');
           var r=[], m;
           while ((m=re.exec(document.location.search)) != null) r.push(m[1]);
           return r.toString(); // Ensures that a string is returned
    }

Are both variables of the same type? Because that's what === is there to check — Pekka
– Pekka, Commented Dec 22, 2011 at 17:35
Forgot a semicolon for g2. I would've just edited and added it in; but I don't know if you copied/pasted from original code. — Dave
– Dave, Commented Dec 22, 2011 at 17:36
Try opening your browser's console when those variables exist, and see what typeof id2 and typeof id1 tells you. — JesseBuesking
– JesseBuesking, Commented Dec 22, 2011 at 17:38

Esailija · Accepted Answer · 2011-12-22 17:39:08Z

6

You said they are objects so of course they are not the same:

var g = new String("game1"),
    g2 = new String("game1");

g===g2 //false
g==g2 //false

Variables that "hold objects" actually are just references to objects. Unless both the variables refer the exact same object in memory, they will not be equal no matter what the content unless you compare g.toString() === g2.toString() //true.

Note that when you do "hello"+var, var is automatically converted toString form:

null + "hello" === "nullhello"

answered Dec 22, 2011 at 17:39

Esailija

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1 Comment

Amadan Over a year ago

Heh, your example is simpler than my example :D +1

Amadan · Accepted Answer · 2011-12-22 17:38:57Z

3

Theoretically, if the type is object...

id1 = { toString: function() { return "game1" }, player: "monkey" };
id2 = { toString: function() { return "game1" }, player: "tiger" };

id1 === id2                        // false
"id1: " + id1 + "; id2: " + id2    // id1: game1; id2: game1
typeof(id1)                        // object

Basically, you have two things that both render to the same string, but are not the same.

answered Dec 22, 2011 at 17:38

Amadan

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Comments

Logan Serman · Accepted Answer · 2011-12-22 17:36:44Z

2

Triple equals means the type must be equal as well. So apparently, your variables id1 and id2 are not of the same type.

In your example, you are defining g and g2 to both be strings with value "game1", so the triple equals works in that case.

You need to check the type of playerTick.gameId and that.gameId. One of them is not a string.

answered Dec 22, 2011 at 17:36

Logan Serman

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2 Comments

Incognito Over a year ago

It means no such thing. That's not how JavaScript works.

Logan Serman Over a year ago

developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… Seems pretty accurate to me.

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