Concatenating string with JSX expression (which in fact is javascript object tree of virtual nodes) in

prevState.content + <TemplateComp>Added Content</TemplateComp>

results in that you have described. You have to convert virtual DOM nodes to HTML, e.g. in following way:

prevState.content + ReactDOMServer.renderToStaticMarkup(<TemplateComp>Added Content</TemplateComp>)

this will work, albeit it is one way route how to statically render React component to Tiny MCE editor. If you really want to place React component with its lifecycle to rich text editor consider using Draft.js or a component based on it instead of Tiny MCE.

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