undefined: <variable name>

What causes the error?

This error is caused when a variable isn’t defined before attempting to assign it a value. This can commonly happen when you forget to use the short declaration form’s colon (:), or when you previously defined a variable and then later removed that declaration. For instance, imagine you had this code:

err := doStuff()
if err != nil {
  // handle the err
}
err = doMoreStuff()
if err != nil {
  // handle the err
}

And you later updated the code to remove the call to doStuff():

err = doMoreStuff()
if err != nil {
  // handle the err
}

Because you no longer declare the variable before calling doMoreStuff() you now need to update your code to declare the err variable.

How can I fix it?

To fix this error you need to declare the undefined variable. Often you can do this by adding a : to the very first declaration of it:

err := doMoreStuff()
if err != nil {
  // handle the err
}

Or by adding a var err error before the first usage of the variable:

err := doMoreStuff()
if err != nil {
  // handle the err
}