# ReactiveVar

To use `ReactiveVar`, add the `reactive-var` package to your project by running
in your terminal:

```bash
meteor add reactive-var
```

<ApiBox name="ReactiveVar" />


A ReactiveVar holds a single value that can be get and set, such that calling
`set` will invalidate any Computations that called `get`, according to the
usual contract for reactive data sources.

A ReactiveVar is similar to a Session variable, with a few differences:

- ReactiveVars don't have global names, like the "foo" in `Session.get('foo')`.
  Instead, they may be created and used locally, for example attached to a
  template instance, as in: `this.foo.get()`.

- ReactiveVars are not automatically migrated across hot code pushes,
  whereas Session state is.

- ReactiveVars can hold any value, while Session variables are limited to
  JSON or EJSON.

An important property of ReactiveVars &mdash; which is sometimes a
reason for using one &mdash; is that setting the value to the same
value as before has no effect; it does not trigger any invalidations.
So if one autorun sets a ReactiveVar, and another autorun gets the
ReactiveVar, a re-run of the first autorun won't necessarily trigger
the second. By default, only primitive values are compared this way,
while calling `set` on an argument that is an _object_ (not a
primitive) always counts as a change. You can configure this behavior
using the `equalsFunc` argument.

<ApiBox name="ReactiveVar#get" instanceName="reactiveVar" />

<ApiBox name="ReactiveVar#set" instanceName="reactiveVar" />