onmessage in SSE(Server Side Events) is not working. This is a potential solution. check simplest demo.

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The Problem is With ServerSideCode:
1-make sure you add correct headers such as : header("Content-Type: text/event-stream");.
2-make sure you set your response to data like this: 'data: your response text'.
3-make sure you add a line break to the end of response text twise like this: "\n\n";.
https://github.com/n4j1Br4ch1D