Can `json.Marshal` Fail When Encoding a `map[string]string` in Go?

Can Marshalling a map[string]string to JSON Return an Error?

Consider the following code:

m := map[string]string{}
//... do stuff to the map
b, err := json.Marshal(m)

Can the json.Marshal call return an error in this case?

Answer:

Theoretically, json.Marshal will not return any errors when marshalling a map[string]string to JSON. This is because any valid string can be used as a key or value in JSON. However, there are a few exceptions:

• Out-of-memory errors: If the JSON representation of the map is too large to allocate in memory, json.Marshal will not return and the app will terminate with an out-of-memory error code.
• Invalid UTF-8 strings: Although JSON allows any Unicode string as a value, Go stores strings as UTF-8 encoded byte sequences. If a string contains invalid UTF-8 encoded bytes, Go will substitute them with the Unicode replacement character U FFFD. Despite this substitution, json.Marshal will not return an error.
• Concurrent map modification: Since Go 1.6, the runtime detects and prevents concurrent modification of maps between multiple goroutines. If the map is modified while json.Marshal is iterating over it, the runtime will crash the app with a "concurrent map iteration and map write" error.

It's important to note that even though json.Marshal is unlikely to return an error when marshalling a map[string]string, it's good practice to check for errors in all cases. This is because the standard library may contain errors, or a future update could introduce behavior changes.

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