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+# Full Stack Open Notes
+
+My notes and other stuff for this course: https://fullstackopen.com
diff --git a/parts/0/exercise0.4. b/parts/0/exercise0.4.
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+> Writing HTML amid the code is of course not smart, but for old-school PHP programmers, it was a normal practice.
+
+Lol
+
+I need to do the exercises 0.1.-0.6. here: https://fullstackopen.com/en/part0/fundamentals_of_web_apps 
+* [X] 0.1 
+* [X] 0.2
+* [X] 0.3 
+* [X] 0.4 
+* [X] 0.5 
+* [X] 0.6 
+
+## exercise 0.4
+
+```mermaid
+sequenceDiagram
+    participant browser
+    participant server
+
+    Note left of browser: User fills in form and clicks on submit button.
+
+    browser->>server: POST https://studies.cs.helsinki.fi/exampleapp/new_note
+    activate server
+    server->>browser: 302 Redirect to https://studies.cs.helsinki.fi/exampleapp/notes
+    deactivate server
+
+    browser->>server: GET https://studies.cs.helsinki.fi/exampleapp/notes
+    activate server
+    server-->>browser: HTML document
+    deactivate server
+
+    browser->>server: GET https://studies.cs.helsinki.fi/exampleapp/main.css
+    activate server
+    server-->>browser: the css file
+    deactivate server
+
+    browser->>server: GET https://studies.cs.helsinki.fi/exampleapp/main.js
+    activate server
+    server-->>browser: the JavaScript file
+    deactivate server
+
+    Note right of browser: The browser starts executing the JavaScript code that fetches the JSON from the server
+
+    browser->>server: GET https://studies.cs.helsinki.fi/exampleapp/data.json
+    activate server
+    server-->>browser: [{ "content": "HTML is easy", "date": "2023-1-1" }, ... ]
+    deactivate server
+
+    Note right of browser: The browser executes the callback function that renders the notes, including the new note.
+```
+
+## exercise 0.5
+
+```mermaid
+sequenceDiagram
+    participant browser
+    participant server
+
+    browser->>server: GET https://studies.cs.helsinki.fi/exampleapp/spa
+    activate server
+    server-->>browser: HTML document
+    deactivate server
+
+    browser->>server: GET https://studies.cs.helsinki.fi/exampleapp/main.css
+    activate server
+    server-->>browser: the css file
+    deactivate server
+
+    browser->>server: GET https://studies.cs.helsinki.fi/exampleapp/spa.js
+    activate server
+    server-->>browser: the SPA JavaScript file
+    deactivate server
+
+    Note right of browser: The browser starts executing the JavaScript code that fetches the JSON from the server
+
+    browser->>server: GET https://studies.cs.helsinki.fi/exampleapp/data.json
+    activate server
+    server-->>browser: [{ "content": "HTML is easy", "date": "2023-1-1" }, ... ]
+    deactivate server
+
+    Note right of browser: The browser executes the callback function that renders the notes.
+```
+
+## exercise 0.6
+
+* browser fills in data and triggers form submission
+* browser renders the new note without talking to the server
+* browser sends POST request to `/new_note_spa`
+* server returns 201 created
+* browser logs the response to console
+
+```mermaid
+sequenceDiagram
+    participant browser
+    participant server
+
+    Note right of browser: Form gets filled and submission button gets triggered.
+
+    Note right of browser: The browser updates the in-memory notes array and renders again all notes, before talking to the server at all.
+
+    browser->>server: POST https://studies.cs.helsinki.fi/exampleapp/new_note_spa
+    activate server
+    server-->>browser: 201 Created with JSON success message.
+    deactivate server
+
+    Note right of browser: browser logs to console the server's response text.
+```