What is “object of type ‘closure’ is not subsettable” error in Shiny?

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See also this question which covers this error in a non-Shiny context.

How to fix this:

This is a very common error in shiny apps. This most typically appears when you create an object such as a list, data.frame or vector using the reactive() function – that is, your object reacts to some kind of input. If you do this, when you refer to your object afterwards, you must include parentheses.

For example, let’s say you make a reactive data.frame like so:

MyDF<-reactive({ code that makes a data.frame with a column called “X” })

If you then wish to refer to the data.frame and you call it MyDF or MyDF$X you will get the error. Instead it should be MyDF() or MyDF()$X You need to use this naming convention with any object you create using reactive().

Why this happens:

When you make a reactive object, such as a data.frame, using reactive() it is tempting to think of it as just like any other non-reactive data.frame and write your code accordingly. However, what you have created is not really a data.frame. Rather, what you have made is instructions, in the form of a function, which tell shiny how to make the data.frame when it is needed. When you wish to actually use this function to get the data.frame you have to use the parenthesis, just like you would any other function in R. If you forget to use the parenthesis, R thinks you are trying to use part of a function and gives you the error. Try typing:

plot$x

at the command line and you will get the same error.

You may not see this error right when your app starts. Reactive objects have what is called “lazy” evaluation. They are not evaluated until they are needed for some output. So if your data.frame is only used to make a plot, the data.frame will not exist until the user sees the plot for the first time. If when the app starts up the user is required to click a button or change tabs to see the plot, the code for the data.frame will not be evaluated until that happens. Once that happens, then and only then will shiny use the current values of the inputs to run the function that constructs the data.frame needed to make the plot. If you have forgotten to use the parentheses, this is when shiny will give you the error. Note that if the inputs change, but the user is not looking at the plot, the function that makes the data.frame will not be re-run until the user looks at the plot again.

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