Getting error interface conversion: interface {} is map[string]interface {}, not map[string]string - Why does it fail

Why does the following statement fail:

// type of fileData --> type PackageJson map[string]interface{}

dep, fDep := fileData["dependencies"]
if fDep {
    // Following Line Fails
    version, fVer := dep.(map[string]string)[packageName]
}

The above statement throws an error:

 interface conversion: interface {} is map[string]interface {}, not map[string]string [recovered]

Here:

• dep is of type interface{}
• With the statement dep.(map[string]string)[packageName], am I not telling that dep is a map with key as string and value as also the string? Does it mean different?

An interface value is two things: a data type, and a data value of that type. Type assertions like dep.(X) returns the value stored in the interface dep if that value is of type X.

So, based on the error, you can see that the value stored in dep is a map[string]interface{}, and not map[string]string. That's why type assertion fails.

Based on the code, I believe there is a tacit expectation that you want to get a string value from that map. As I said before, an interface contains a value in it, so once you recovered the value contained in dep, then you can access the value of a key, which is also an interface{}, and then access the value in that. So:

k,ok:=dep.(map[string]interface{})[packageName]
if ok {
   str, ok:=k.(string)
   if ok {
      // k is a string, and str is the string contained in `packageName` key in the map
   } else {
      // k is not a string
   }
}