How do I delete or remove Session variables?

Question

Meteor has a Session that provides a global object on the client that you can use to store an arbitrary set of key-value pairs. Use it to store things like the currently selected item in a list.

It supports Session.set, Session.get and Session.equals.

How do I delete a Session name, value pair? I can't find a Session.delete(name) ?

Geosyncline

Answer 1

[note: this answer is for Meteor 0.6.6.2 through at least 1.1.0.2]

[edit: updated to also explain how to do this while not breaking reactivity. Thanks to @DeanRadcliffe, @AdnanY, @TomWijsman, and @MikeGraf !]

The data is stored inside Session.keys, which is simply an object, so you can manually delete keys:

Session.set('foo', 'bar')
delete Session.keys['foo']

console.log(Session.get('foo')) // will be `undefined`

To delete all the keys, you can simply assign an empty object to Session.keys:

Session.set('foo', 'bar')
Session.set('baz', 'ooka!')
Session.keys = {}

console.log(Session.get('foo')) // will be `undefined`
console.log(Session.get('baz')) // will be `undefined`

That's the simplest way. If you want to make sure that any reactive dependencies are processed correctly, make sure you also do something like what @dean-radcliffe suggests in the first comment. Use Session.set() to set keys to undefined first, then manually delete them. Like this:

// Reset one value
Session.set('foo', undefined)
delete Session.keys.foo

// Clear all keys
Object.keys(Session.keys).forEach(function(key){ Session.set(key, undefined); })
Session.keys = {}

There will still be some remnants of the thing in Session.keyDeps.foo and Session.keyValueDeps.foo, but that shouldn't get in the way. 

Shortbread

It's not advisable to go under the covers of Session and manipulate keys directly. You can however use it for each key name:Object.keys(Session.keys).forEach(function(key){ Session.set(key, undefined); }) – Sech

@DeanRadcliffe, many thanks for your contribution. I was actually following this anwser (which I edited now), and couldn't found why reactivity was gone. Without your contribution, I would have wandered for ages. You rock. Big thanks! – Queenqueena

Answer 2

Session.set('name', undefined) or Session.set('name', null) should work.

Webb

Does that actually delete it or just set the value to undefined or null? – Geosyncline

@SteeveCannon: No, it doesn't actually delete it; but it would make Session.get('name') return undefined or null. I think that when Session.get('name') has not been used yet it would return undefined as well. I don't see any need to actually delete the variable, at least not in a reactive context. For non-reactive storage you should most likely use something like amplify (persistent) or just some variable (temporary). – Webb

it works if you set it to undefined. I have not tested setting it to null. The Session key/value pair does not immediately disappear but moments later it does. Must be related to the gc. Thanks – Geosyncline

Since we can't delete them, I wonder how long it takes for them to disappear when set to undefined? – Geosyncline

This conversation continued on IRC. Summary: GC deletion times are browser dependent, that is, if a GC is running. Session.delete() ought to be implemented... – Webb

This solution fails because of how Session.setDefault() works. Session.set('a', 'a'); Session.set('a', undefined); Session.setDefault('a', 'b'); Session.get('a'); => undefined The delete Session.keys.a solution does not suffer this same issue. (presently testing with release 7.2.x ) – Matless

Answer 3

The disadvantage with using delete Session.keys['foo'] is that your template will not hot reload if the session key holds an array. For instance, if you are doing

Template.mytempl.helpers({
    categories: function() {
        return Session.get('srch-categories')
    }
})

and in your template

{{#if categories}}
    {{#each categories}}
        {{this}}
    {{/each}}
{{/if}}

And categories is an array, if you delete the session key, your template will continue to display the last value of categories.

Conglomeration