# Startup Options

The startup component is responsible for creating the objects needed by MathJax to perform the mathematical typesetting of your pages, and for setting up the methods you may need to call in order to do that. It is configured using the `startup` block in your configuration object.

## The Configuration Block

In the example below, `Startup` represents the `MathJax.startup` object, for brevity.

    MathJax = {
      startup: {
        elements: null,          // The elements to typeset (default is document body)
        typeset: true,           // Perform initial typeset?
        ready: Startup.defaultReady.bind(Startup),          // Called when components are loaded
        pageReady: Startup.defaultPageReady.bind(Startup),  // Called when MathJax and page are ready
        document: document,      // The document (or fragment or string) to work in
        invalidOption: 'warn',   // Are invalid options fatal or produce an error?
        optionError: OPTIONS.optionError,  // Function used to report invalid options
        input: [],               // The names of the input jax to use from among those loaded
        output: null,            // The name for the output jax to use from among those loaded
        handler: null,           // The name of the handler to register from among those loaded
        adaptor: null            // The name for the DOM adaptor to use from among those loaded
      }
    };

## Option Descriptions

elements: null

  This is either `null` or an array of DOM elements whose contents should be typeset. The elements can either be actual DOM elements, or strings that give CSS selectors for the elements to typeset.

typeset: true

  This determines whether the initial typesetting action should be performed when the page is ready.

ready: MathJax.startup.defaultReady.bind(Startup)

  This is a function that is called when MathJax is loaded and ready to go. It is called by the loader when all the components are loaded. The default action is to create all the objects needed for MathJax, and set up the call to the `pageReady()` function below. You can override this function if you want to modify the setup process; see Performing Actions During Startup for more details. Note that this function may be called before the page is complete, so unless you are modifying the objects created by the startup module, replacing `pageReady()` may be the better choice.

pageReady: MathJax.startup.defaultPageReady.bind(Startup)

  This is a function that is called when MathJax is ready to go and the page is ready to be processed. The default action is to perform the initial typesetting of the page and return the promise that resolves what that is complete, but you can override it to do whatever you would like, though you should return the promise from the `MathJax.startup.defaultPageReady()` function if you call it. See Performing Actions During Startup for more details and examples of how to do this.

document: document

  This is the document (or fragment or string of serialized HTML) that you want to process. By default (for in-browser use) it is the browser document. When there is no global `document` variable, it is an empty HTML document.

invalidOption: 'warn' // or 'fatal'

  This determines whether an invalid option will cause a fatal error (when set to `'fatal'`) that stops MathJax from running, or a warning (when set to `'warn'`) that allows MathJax to go on. Prior to version 3.2, invalid options were fatal, but this option now allows control over that behavior.

optionError: OPTIONS.optionError

  This option gives a function that is called whenever there is an invalid option provided by the user. It takes two string arguments, the first being the message, and the second being the name of the invalid option. The default function looks at the `invalidOption` value and if it is `'fatal'` it throws an error using the given message, otherwise it logs the message to the browser console, allowing further options to be processed.

input: []

  This is an array of names of input processors that you want to use, from among the ones that have been loaded. So if you have loaded the code for several input jax, but only want to use the `tex` input jax, for example, set this to `['tex']`. If set to an empty array, then all loaded input jax are used.

output: null

  This is the name of the output processor that you want to use, from among the ones that have been loaded. So if you have loaded the code for several output jax, but only want to use the `svg` output jax, for example, set this to `'svg'`. If set to `null` or an empty string, then the first output jax that is loaded will be used.

handler: null

  This is the name of the document handler that you want to use, from among the ones that have been loaded. Currently, there is only one handler, the HTML handler, so unless you are creating your own handlers, leave this as `null`.

adaptor: null

  This is the name of the DOM adaptor that you want to use, from among the ones that have been loaded. By default the components load the `browser` adaptor, but you can load the `liteDOM` adaptor for use in node applications; if you do, it will set this value so that it will be used automatically.