# Modifying Math on the Page

If you are writing a dynamic web page where content containing
mathematics may appear after MathJax has already typeset the rest of
the page, then you will need to tell MathJax to look for mathematics
in the page again when that new content is produced. To do that, you
need to use the MathJax.Hub.Typeset() method. This will cause
the preprocessors (if any were loaded) to run over the page again, and
then MathJax will look for unprocessed mathematics on the page and
typeset it, leaving unchanged any math that has already been typeset.

You should not simply call this method directly, however. Because
MathJax operates asynchonously (see for details), you need to be sure that
your call to MathJax.Hub.Typeset() is synchronized with the
other actions that MathJax is taking. For example, it may already be
typesetting portions of the page, or it may be waiting for an output
jax to load, etc., and so you need to queue the typeset action to be
performed after MathJax has finished whatever else it may be doing.
That may be immediately, but it may not, and there is no way to tell.

To queue the typeset action, use the command

MathJax.Hub.Queue(["Typeset",MathJax.Hub]);

This will cause MathJax to typeset the page when it is next able to do
so. It guarantees that the typesetting will synchronize properly
asynchronous activity, and is the only truly safe way to ask MathJax

The MathJax.Hub.Typeset() command also accepts a parameter
that is a DOM element whose content is to be typeset. That could be
a paragraph, or a <div> element, or even a MathJax math
<script> tag. It could also be the DOM id of such an object, in
which case, MathJax will look up the DOM element for you. So

MathJax.Hub.Queue(["Typeset",MathJax.Hub,"MathExample"]);

would typeset the mathematics contained in the element whose id is
MathExample. This is equivalent to

var math = document.getElementById("MathExample");
MathJax.Hub.Queue(["Typeset",MathJax.Hub,math]);

If no element or element id is provided, the whole document is
typeset.

Note that the MathJax.Hub.Queue() method will return
immediately, regardless of whether the typesetting has taken place or
not, so you can not assume that the mathematics is visible after you
make this call. That means that things like the size of the container
for the mathematics may not yet reflect the size of the typeset
mathematics. If you need to perform actions that depend on the
mathematics being typeset, you should push those actions onto the
MathJax.Hub.queue as well.

This can be quite subtle, so you have to think carefully about the
structure of your code that works with the typeset mathematics. Also,
the things you push onto the queue should be Callback objects that
perform the actions you want when they are called, not the results
of calling the functions that do what you want.

## Manipulating Individual Math Elements

If you are not changing a complete DOM structure, but simply want to
update the contents of a single mathematical equation, you do not need
to use innerHTML and MathJax.Hub.Typeset() to preprocess
and process an element’s new content. Instead, you can ask MathJax to
find the element jax for the math element on the page, and use its
methods to modify and update the mathematics that it displays.

For example, suppose you have the following HTML in your document

<div id="MathDiv">
The answer you provided is: ${}$.
</div>

and MathJax has already preprocessed and typeset the mathematics
within the div. A student has typed something elsewhere on the page,
and you want to typeset their answer in the location of the
mathematics that is already there. You could replace the entire
contents of the MathDiv element and call
MathJax.Hub.Typeset() as described above, but there is a more
efficient approach, which is to ask MathJax for the element jax for
the mathematics, and call its method for replacing the formula shown
by that element. For example:

var math = MathJax.Hub.getAllJax("MathDiv")[0];
MathJax.Hub.Queue(["Text",math,"x+1"]);

This looks up the list of math elements in the MathDiv element
(there is only one) and takes the first one (element 0) and stores it
in math. This is an element jax object (see the Element
Jax
specification for details), which has a
method that can be used to set the input text of the
math element, and retypeset it.

Again, since the typesetting should be synchronized with other actions
of MathJax, the call should be pushed onto the MathJax processing
queue using MathJax.Hub.Queue(), as shown above, rather than
called directly. The example above performs the equivalent of
math.Text("x+1") as soon as MathJax is able to do so. Any
additional actions that rely on the expression x+1 actually
showing on screen should also be pushed onto the queue so that they
will not occur before the math is typeset.

The actions you can perform on an element jax include:

Text(newmath)

to set the math text of the element to newmath and typeset.

Rerender()

to remove the output and reproduce it again (for example, if
CSS has changed that would alter the spacing of the
mathematics). Note that the internal representation isn’t
regenerated; only the output is.

Reprocess()

to remove the output and then retranslate the input into the
internal MathML and rerender the output.

Remove()

to remove the output for this math element (but not
the original <script> tag).

needsUpdate()

to find out if the mathematics has changed so that its output
needs to be updated.

SourceElement()

to obtain a reference to the original
<script> object that is associated with this element jax.

Note that once you have located an element jax, you can keep using it
and don’t have to look it up again. So for the example above, if the
student is going to be able to type several different answers that you
will want to typeset, you can look up the element jax once at the
beginning after MathJax has processed the page the first time, and
then use that result each time you adjust the mathematics to be
displayed.

To get the element jax the first time, you need to be sure that you
ask MathJax for it after MathJax has processed the page the first
time. This is another situation where you want to use the MathJax
queue. If your startup code performs the commands

var studentDisplay = null;
MathJax.Hub.Queue(function () {
studentDisplay = MathJax.Hub.getAllJax("MathDiv")[0];
});

then you can use

MathJax.Hub.Queue(["Text",studentDisplay,studentAnswer])

to change the student’s answer to be the typeset version of whatever

Here is a complete example that illustrates this approach. Note,
however, that Internet Explorer does not fire the onchange event
when you press RETURN, so this example does not work as expected in
IE. A more full-featured version that addresses this problem is
available in test/sample-dynamic.html.

<html>
<title>MathJax Dynamic Math Test Page</title>

<script type="text/x-mathjax-config">
MathJax.Hub.Config({
tex2jax: {
inlineMath: [["$","$"],["\$","\$"]]
}
});
</script>
<script type="text/javascript"
src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS_HTML-full">
</script>

<body>

<script>
//
//  Use a closure to hide the local variables from the
//  global namespace
//
(function () {
var QUEUE = MathJax.Hub.queue;  // shorthand for the queue
var math = null;                // the element jax for the math output.

//
//  Get the element jax when MathJax has produced it.
//
QUEUE.Push(function () {
math = MathJax.Hub.getAllJax("MathOutput")[0];
});

//
//  The onchange event handler that typesets the
//  math entered by the user
//
window.UpdateMath = function (TeX) {
QUEUE.Push(["Text",math,"\\displaystyle{"+TeX+"}"]);
}
})();
</script>

Type some TeX code:
<input id="MathInput" size="50" onchange="UpdateMath(this.value)" />
<p>

<div id="MathOutput">
You typed: ${}$
</div>

</body>
</html>

There are a number of additional example pages at test/examples.html that
illustrate how to call MathJax dynamically or perform other actions
with MathJax.