Trying to reassign a reactiveVal to the name of an existing reactiveVal doesn't overwrite the latter

[BajczA475]

System details

R version 4.6.1 (2026-06-24 ucrt)
Platform: x86_64-w64-mingw32/x64
Running under: Windows 11 x64 (build 26200)

Matrix products: default
  LAPACK version 3.12.1

locale:
[1] LC_COLLATE=English_United States.utf8  LC_CTYPE=English_United States.utf8    LC_MONETARY=English_United States.utf8
[4] LC_NUMERIC=C                           LC_TIME=English_United States.utf8    

time zone: America/Chicago
tzcode source: internal

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
 [1] ggplotplus_0.5.7      shinyvalidate_0.1.3   patchwork_1.3.2       tibble_3.3.1          DT_0.34.0             ggraph_2.2.2         
 [7] ggplot2_4.0.3         igraph_2.3.3          data.table_1.18.4     scales_1.4.0          Matrix_1.7-5          shinyWidgets_0.9.1   
[13] shinydisconnect_0.1.1 waiter_0.2.5.1        shinyjs_2.1.1         stringr_1.6.0         tidyr_1.3.2           dplyr_1.2.1          
[19] shiny_1.14.0         

loaded via a namespace (and not attached):
 [1] gtable_0.3.6       httr2_1.3.0        bslib_0.12.0       htmlwidgets_1.6.4  ggrepel_0.9.8      lattice_0.22-9    
 [7] vctrs_0.7.3        tools_4.6.1        crosstalk_1.2.2    generics_0.1.4     curl_7.1.0         pkgconfig_2.0.3   
[13] checkmate_2.3.4    RColorBrewer_1.1-3 S7_0.2.2           lifecycle_1.0.5    compiler_4.6.1     farver_2.1.2      
[19] textshaping_1.0.5  ggforce_0.5.0      graphlayouts_1.2.5 httpuv_1.6.17      htmltools_0.5.9    sass_0.4.10       
[25] yaml_2.3.12        later_1.4.8        pillar_1.11.1      jquerylib_0.1.4    MASS_7.3-65        openssl_2.4.2     
[31] rsconnect_1.10.1   cachem_1.1.0       viridis_0.6.5      mime_0.13          tidyselect_1.2.1   digest_0.6.39     
[37] stringi_1.8.9      purrr_1.2.2        labeling_0.4.3     polyclip_1.10-7    fastmap_1.2.0      grid_4.6.1        
[43] cli_3.6.6          magrittr_2.0.5     tidygraph_1.3.1    withr_3.0.3        promises_1.5.0     backports_1.5.1   
[49] otel_0.2.0         gridExtra_2.3.1    askpass_1.2.1      ragg_1.5.2         memoise_2.0.1      viridisLite_0.4.3 
[55] rlang_1.3.0        Rcpp_1.1.2         xtable_1.8-8       glue_1.8.1         tweenr_2.0.3       pkgload_1.5.3     
[61] rstudioapi_0.19.0  jsonlite_2.0.0     R6_2.6.1           systemfonts_1.3.2 

Example application or steps to reproduce the problem

library(shiny)

ui <- fluidPage(
  actionButton("reset", "Recreate x")
)

server <- function(input, output, session) {
  x <- reactiveVal(0)

  observe({
    cat("Observer saw:", x(), "\n")
  })

  observeEvent(input$reset, {
    x <<- reactiveVal(0)  # Rebind x to a new reactiveVal
    x(1)                  # Update the new reactiveVal
  })
}

shinyApp(ui, server)

Describe the problem in detail

I don't think this is actually a bug; I think it's the code working as intended. However, I found it unintuitive enough to warrant a mention in the help docs for reactiveVal().

Basically, in "plain R," if you ran x = 15 and then later ran x = 12, you'd overwrite the original object with the new one. However, if you try this with reactiveVal()s, the same doesn't hold true. It seems like instead of overwriting the original, it creates a new one with the same name, creating a second version that observers and other entities could seemingly point at. Perhaps this has something to do with how assignment works in JS versus in R? I dunno.

However, it means that if I am trying to code, say, a reset button, and I try to reset a reactiveVal by essentially just re-running the same command I used to create it, that won't "reset it," but it also won't error, and observers may still find the un-updated reactiveVal and access it instead, so it creates "silent failure states."

It's obvious, in retrospect, that I need to use the reactiveVal as a function to reset it once it's made, but it wasn't obvious before. Just a note in the Details that says something to the effect of "Assigning a reactiveVal to the name of an existing reactiveVal will not overwrite the latter entity; use the reactiveVal as a function to set or reset its value" would have been instructive for me!

[gadenbuie]

I'm going to try to explain what's happening here, but it's a bit nuanced. I'll do my best.

Reactivity in Shiny is implemented as, essentially, a set of signals. On the first pass through the code, each observer/reactive tracks which reactive variables are called inside its code block. The observer/reactive then subscribes to changes -- it waits for a signal that a reactive value that it relied on has changed and when that happens it marks itself as "ready to run again".

So on the first pass, this is what happens:

1. You create a reactive value (id: 6e78) and store it in x.
2. The observer runs, prints the value of x and waits for a signal that 6e78 has updated.
3. When the input$reset observer runs, you replace x with a new reactive value (id: f0e0)
4. When you update x(1), you're updating reactive value f0e0

As a result, the original reactive value never receives as signal that 6e78 was invalidated. The easiest way to fix this is to do something to tell the observer that 6e78 is invalid. Here are a few options.

Quick and Dirty

The fastest way is to just change the value of 6e78 (the old reactive value) before you create the new reactive value.

  observeEvent(input$reset, {
    # Update old `x` so all downstream dependencies are invalidated
    x(0)
    x <<- reactiveVal(0)  # Rebind x to a new  reactiveVal
    x(1)                  # Update the new reactiveVal
  })

The first x(0) will mark the original reactive value as invalidated, which will cause the observe() code to run again... after this observeEvent() is done. But by the time the observe() code runs, x will be fully replaced.

This works, but leaks reactive values. (I mean, they'll get cleaned up eventually, so it's not a big deal.)

Correct and Dirty

The more "correct" way to do this would be to destroy the old reactive (6e78) before creating a new one (f0e0). To do that, you'd need to reach into Shiny internals, so this is still a little "dirty".

  observeEvent(input$reset, {
    # Destroy old `x` so all downstream dependencies are invalidated
    attr(x, ".impl")$destroy()
    x <<- reactiveVal(0)  # Rebind x to a new  reactiveVal
    x(1)                  # Update the new reactiveVal
  })

This is correct but slightly sketchy. That said, there's a note in the source code that we want to have a user-facing front door for this action and I think it's relatively safe to use.

Correct and Long-Winded

We recently added the ability for modules to self-destruct, which is a helpful way to tear down all of the reactives inside a module scope. You could create a small server module that holds the reactive and use session$destroy() inside the module to tear down all of its reactive values. This is the most "approved APIs only" approach available right now.

First, create a small module to hold the reactive value and to return the session$destroy method

xValServer <- function(id) {
  moduleServer(id, function(input, output, session) {
    x <- reactiveVal(0)
    list(value = x, destroy = session$destroy)
  })
}

Then create an instance of the reactive value. Before creating a new copy of the server module, call the $destroy() method to tear down the old version, which also invalidates any dependent calculations.

server <- function(input, output, session) {
  x <- xValServer("xval")

  observe({
    cat("Observer saw:", x$value(), "\n")
  })

  observeEvent(input$reset, {
    # Destroy the old module scope (and its reactiveVal), then re-create
    x$destroy()
    x <<- xValServer("xval")
    x$value(1)
  })
}

Notice that because I tore down the old module, we don't need to worry about creating a new module ID each time we re-create x. The other downside is that everywhere that x is used we need to call x$value() rather than x directly.

This probably isn't the pattern I'd reach for in this simple example, but if you're already using modules or if there's any way to organize the reactive into a module -- it probably has additional logic and isn't fully standing on its own -- then I'd consider refactoring to better fit this approach.

Good luck and I hope this helps!

[BajczA475]

I think the underlying mechanics of this makes sense to me, especially given your nice explanation! But while I've been developing in Shiny for ~5 years now, I still didn't anticipate this behavior; I tried to "reset" a reactiveVal() "incorrectly," and had to use AI to figure out why it wasn't working because the help documentation didn't set me sufficiently straight. I just trusted that what I knew to be true for R would also be true for Shiny in this way. That leads me to believe that, for those with even less Shiny experience than myself, that suggests the documentation could address this (or at least its applied implications) explicitly somehow, even if briefly. So, I'd make an accessibility argument to bolster the docs on this issue so it doesn't trip up others like it tripped me up.

Also, to be clear, in terms of intent, I wasn't trying to "destroy" the original reactiveVal; I was trying to replace its value with the same value I initialized it with so as to bring the app back to its "starting place" with respect to this specific tracker. In base R, the two would be equivalent; in this instance, they aren't, and I think that's maybe neat but also confusing.

[gadenbuie]

Also, to be clear, in terms of intent, I wasn't trying to "destroy" the original reactiveVal; I was trying to replace its value with the same value I initialized it with so as to bring the app back to its "starting place" with respect to this specific tracker.

Hmmm, that's interesting. I'm not sure I understand why you were creating a new reactive value then. Can you share a little more context about the problem you were trying to solve? Or are you saying that instead of calling x(initial_value) you had written x <<- reactiveVal(initial_value) and it took a while to realize that introduce a bug?

[BajczA475]

Hmm--more or less, but not exactly. Let me try to explain more clearly: Say I have an app that tracks user actions and creates a "living record" of them that accumulates over a session. Let's say one of these trackers is a reactiveVal called x that I initialize on app launch via ```r x = reactiveVal(0) ``` at the top of my server code. I then have a variety of operations within observers that will maybe increment that value like ` x(x() + 1)` as the user performs actions. Then, I also have a "reset app" button that will basically perform a hard reset on the app and return it to more or less its initialization state. This should include resetting my 'trackers.' Well, I tried to do this: ```r observeEvent(input$reset, { x = reactiveVal(0) }) ``` Instead of ```r observeEvent(input$reset, { x(0) }) ``` and, yeah, that didn't have the same effect--It had the appearance of my reset button just not doing anything, and I found that unintuitive, since the former would work in base R just fine to "reset" a variable x. The `<<`- was just something AI put in there when I asked it to create a minimal reprex of this specific issue--it's not an operator I use in my apps. Hope this clears up the confusion!

[gadenbuie]

Oh okay, I see. Yeah, there's definitely ambiguity in code like

observeEvent(input$reset, {
  x = reactiveVal(0)
})

which could be

1. You were trying to set the value of the reactive value x to 0
2. You really were intending to set x outside of the observeEvent() scope

and the AI chose to help you "fix" the issue by picking one of two possible ambiguous intentions and correcting that behavior (the second choice).

I think it's just an unfortunate accident that it made that choice! Glad we've cleared it up.

I'm not sure that there's much we can do to mitigate this specific error type, but if you have a proposal for a place in the docs that you looked and felt like you were led astray or could have been better advised, please let me know!

[BajczA475]

Under Details, I'd probably say something like this:

Once a reactiveVal has been initialized—for example, x <- reactiveVal(0)—executing a command later such as x <- reactiveVal(TRUE) will not change the value of the original reactiveVal. Instead, it will create a new, separate reactiveVal and make x refer to it instead. Any reactive expressions and observers that were already dependent on the original will remain dependent on it and will not automatically switch their dependencies to the new reactiveVal with the same name. To change the value of an existing reactiveVal, instead call it as a function with the new value as the sole argument (e.g., x(TRUE)).