# [Japt], <s>7</s> 6 bytes

    å+ ¬¬x

[Try it online!][TIO-j5yarcqt]

### Explanation

    å+ ¬¬x   Implicit: input = string
    å+       Cumulative reduce by concatenation. Gives the list of prefixes.
       ¬     Join into a single string of digits.
        ¬    Split back into digits.
         x   Sum.
             Implicit: output result of last expression



[Japt]: https://github.com/ETHproductions/japt
[TIO-j5yarcqt]: https://tio.run/##y0osKPn///BSbYVDaw6tqfj/X8nU2NzQTAkA "Japt – Try It Online"