# Lazy Typesetting

MathJax offers an extension that is designed to improve the performance of pages with large numbers of equations. It implements a “lazy typesetting” approach that only processes an expression when it comes into view. This means that expressions will not be typeset when they are not visible, and your readers will not have to wait for the entire document to typeset, speeding up their initial view of the page. Furthermore, any expressions that are never seen will not be typeset, saving the processing time that would normally have been spent on those expressions.

This also helps with the situation where you may link to a particular location in your page (via a URL with a hash); typesetting the material above that point can cause the browser to change the scroll position, and so the user may not end up at the proper location in the page. With the lazy extension, the material above that point is not typeset until the user scrolls upwards, and so there is no position change.

To use the lazy typesetting extension, simply add it to your configuration as follows:

    MathJax = {
      loader: {load: ['ui/lazy']}
    };

This will adjust the typesetting pipeline to implement the lazy-typesetting functionality.

Lazy typesetting works best with SVG output, but changes with the way the CommonHTML output handles its stylesheet updates make the CHTML output nearly as fast. With TeX input, the lazy extension makes sure that previous expressions are processed by TeX (though not output to the page) so that any macro definitions or automatic equation numbers are in place when the visible expressions are processed. Currently, documents that contain `\ref` or `\eqref` links may not yet work properly, since target equations may not have been typeset, and so the link location may not be marked in the document. In particular, forward references are unlikely to work, and backward references will work only if the target expression has already been typeset. We hope to improve this situation in a future release.

## Lazy Typesetting Options

Adding the ui/lazy extension to the loader.load array adds the following options to the MathJax configuration:

    MathJax = {
      options: {
        lazyMargin: '200px',
        lazyAlwaysTypeset: null
      }
    };

lazyMargin: '200px'

  This gives the extent of the typesetting margin outside the visible viewport. When mathematics appears within this range of the viewport, it will be typeset. This allows typesetting to occur slightly before the math appears in the window, for a smoother effect.

lazyAlwaysTypeset: null

  This gives an array of containers whose math expressions should always be typset during the initial typesetting pass, rather than waiting for them to scroll into view. This may be useful if MathJax output appears in diagrams or other layout that must be sized and placed during initial page layout.