CODEWITHSUNDEEP
javascriptmeteor
Marcus Riemer
Marcus Riemer

Reputation: 7688

Javascript arrays and Meteor session

I have made an interesting observation. When trying to update an array that is stored in the Meteor session storage, the following code will not propagate the changes:

var tags = Session.get("Tags");
tags.push("a");
Session.set("Tags", tags);

But if I change the first line to use Session.get("Tags").slice(), everything depending on the session will update accordingly. I guess this is due to the fact that Meteor tests some references for equality and therefore does not update anything.

Is there a better way to manage lists stored in the meteor session store?

If I now try to remove an element from the collection (using array.remove() from here), the behavior turns out to be a bit ... of ... I am doing this inside a Meteor template event, the code looks like this:

"click .taglist li" : function(e) {
  var tags = Session.get("Tags").slice();
  var index = cardTags.indexOf(this);

  Meteor._debug(Session.get("Tags").slice().indexOf("a"));
  Meteor._debug("Removing tag \"" + this + "\", index: " + index, ", typeof(this) = " + typeof(this).toString());

  tags.remove(index);
  Session.set("Tags", tags);
}

This outputs:

1
Removing tag "a", index: -1, typeof(this) = string

So somehow, the cardTags.indexOf(this); statement seems to return -1 for almost any case. I guess I am doing something fundamentally wrong, as I am quite now to javascript, but somehow I can not figure out whats going on here.

Why will those two calls to indexOf() behave different?

Upvotes: 11

Views: 6141

Answers (2)

Petrov
Petrov

Reputation: 4250

I believe this is the same as this situation in Backbone.js. In order for the change event to be triggered, Meteor needs to have a new reference for the array, not just an updated copy of the old one.

In brief, in order to have the 'correct' behaviour, you'll need to clone the array, make the changes you want, and then do Session.set('foo', myCopiedArray).

Upvotes: 10

Lars Knickrehm
Lars Knickrehm

Reputation: 747

In short: Use var index = cardTags.indexOf(this.toString()); instead.

Long version:

When using strings in JavaScript, those are strings, whereas typeof 'test' returns string.

Let's take a look at the following code in order to get find out another way to represent strings in JavaScript:

var func = function () {
    return this;
};
console.log(func.call('test'));

The console (at least FireBug) won't show us "test", but instead it shows String {0="t", 1="e", 2="s", 3="t" }. typeof would return "object".

The content of the this statement seems to need to be an object. In order to convert a string into a "String" object we can do console.log(new String('test'));, which is the same as the previously logged value.

To convert a string object into a string (data type), just use its prototype toString.

Upvotes: -3

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