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# The MathJax Startup Sequence

When you load `MathJax.js` into a web page, it configures itself and immediately begins loading the components it needs. As MathJax starts up, it uses its signaling mechanism to indicate the actions that it is taking so that MathJax extensions can tie into the initialization process, and so other applications within the page can synchronize their actions with MathJax.

The startup process performs the following actions:

- It creates the `MathJax` variable, and defines the following subsystems:
  - `MathJax.Object` (object-oriented programming model)
  - `MathJax.Callback` (callbacks, signals, and queues)
  - `MathJax.Ajax` (file-loading and style-creation code)
  - `MathJax.HTML` (support code for creating HTML elements)
  - `MathJax.Message` (manages the menu line in the lower left)
  - `MathJax.Hub` (the core MathJax functions)

- It then creates the base `MathJax.InputJax`, `MathJax.OutputJax`, and `MathJax.ElementJax` objects.

- MathJax sets up the default configuration, and creates the signal objects used for the startup and hub actions.

- MathJax locates the `<script>` tag that loaded the `MathJax.js` file, and sets the `MathJax.Hub.config.root` value to reflect the location of the MathJax root directory.

- MathJax determines the browser being used and its version. It sets up the `MathJax.Hub.Browser` object, which includes the browser name and version, plus `isMac`, `isPC`, `isMSIE`, and so on.

- MathJax set up the `MathJax.Hub.queue` command queue, and populates it with the commands MathJax runs at startup. This includes creating the `MathJax.Hub.Startup.onload` onload handler that is used to synchronize MathJax’s action with the loading of the page.

Once the `MathJax.Hub.queue` is created, the following actions are pushed into the queue:

1. Post the `Begin` startup signal

2. Perform the configuration actions:
   - Post the `Begin Config` startup signal
   - Load any configuration files specified via `config=` as a script parameter
   - Execute the content of the `<script>` that loaded MathJax, if it is not empty
   - Wait for the `delayStartupUntil` condition to be met, if one was specified
   - Execute any `text/x-mathjax-config` script blocks
   - load the files listed in the `MathJax.Hub.config.config` array
   - Post the `End Config` startup signal

3. Load the cookie values:
   - Post the `Begin Cookie` startup signal
   - Load the menu cookie values
   - Use the cookie to set the renderer, if it is set
   - Post the `End Cookie` startup signal

4. Define the MathJax styles:
   - Post the `Begin Styles` startup signal
   - Load the stylesheet files from the `MathJax.Hub.config.stylesheets` array
   - Define the stylesheet described in `MathJax.Hub.config.styles`
   - Post the `End Styles` startup signal

5. Load the jax configuration files:
   - Post the `Begin Jax` startup signal
   - Load the jax config files from the `MathJax.Hub.config.jax` array
     - The jax will register themselves when they are loaded
   - Post the `End Jax` startup signal

6. Load the extension files:
   - Post the `Begin Extension` startup signal
   - Load the files from the `MathJax.Hub.config.extensions` array
     - Most extensions will post a `Extension [name] Ready` startup message when they are loaded (where `[name]` is the name of the extension)
   - Post the `End Extension` startup signal

7. Set the MathJax menu’s renderer value based on the jax that have been loaded

8. Wait for the onload handler to fire

9. Set `MathJax.isReady` to `true`

10. Perform the typesetting pass (preprocessors and processors)
    - Post the `Begin Typeset` startup signal
    - Post the `Begin PreProcess` hub signal
    - Run the registered preprocessors
    - Post the `End PreProcess` hub signal
    - Clear the hub signal history
    - Post the `Begin Process` hub signal
    - Process the math script elements on the page
      - Each new math element generates a `New Math` hub signal with the math element’s ID
    - Post the `End Process` hub signal
    - Post the `End Typeset` startup signal

11. Post the `End` startup signal

The loading of the jax and extensions in steps 5 and 6 are now done in parallel, rather than sequentially. That is, all the jax and extensions are requested simultaneously, so they load concurrently. That means they can load in any order, and that the begin and end signals for the jax and extensions can be intermixed. (In general, you will get Begin Jax followed by Begin Extensions, but the order of End Jax and End Extensions will depend on the files being loaded.) Both 5 and 6 must complete, however, before 7 will be performed.

See the `test/sample-signals.html` file to see the signals in action.

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