I suppose the key question is what came first - was code that the creator was exposed to whilst in employment incorporated into Chart.js, or was code created by

I suppose the key question is what came first - was code that the creator was exposed to whilst in employment incorporated into Chart.js, or was code created by the creator incorporated into projects at work. Clearly in the former case, the employer would have a more legitimate reason to express concern than the latter. However, in any event, as other posters have identified, there could be very positive PR benefits from supporting the open sourcing of the project. In any event, the main justification for retaining the secrecy of a project would be to maintain a competitive advantage however, the extent of the competitive advantage is dependent on the complexity of the code and the ease with which equivalent functionality could be reproduced independently. That's a subject which I would be interested to hear more on.