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<h1 style="letter-spacing: -2px">Implementing a Syntax-Highlighting<br/> JavaScript Editor, In JavaScript</h1>
<pre class="grey">
<img src="css/baboon.png" class="logo" alt="logo"/>/* A brutal odyssey to the dark
side of the DOM tree */
</pre>
<div class="clear"><div class="leftbig blk">
<p>In one of his (very informative) <a
href="http://www.learnwebdesignonline.com/videos/programming/javascript/yahoo-douglas-crockford">video
lectures</a>, Douglas Crockford remarks that writing JavaScript
for the web is 'programming in a hostile environment'. I had done
my fair share of weird workarounds, and even occasonally gave up
an on idea entirely because browsers just wouldn't support it, but
before this project I never really realized just how powerless a
programmer can be in the face of buggy, incompatible, and poorly
designed platforms.</p>
<p>The plan was not ridiculously ambitious. I wanted to 'enhance' a
textarea to the point where writing code in it is pleasant. This meant
automatic indentation and, if possible at all, syntax highlighting.</p>
<p>In this document I describe the story of implementing this, for your
education and amusement. A demonstration of the resulting program,
along with the source code, can be found at <a
href="http://codemirror.net/">the project website</a>.</p>
<p style="color: #811; font-size: 90%; font-style: italic">Note:
some of the details given here no longer apply to the current <a
href="http://codemirror.net/">CodeMirror</a> codebase, which has
evolved quite a bit in the meantime.</p>
<h2 id="indent">Take one: Only indentation</h2>
<p>The very first attempt merely added auto-indentation to a textarea
element. It would scan backwards through the content of the area,
starting from the cursor, until it had enough information to decide
how to indent the current line. It took me a while to figure out a
decent model for indenting JavaScript code, but in the end this seems
to work:</p>
<ul>
<li>Code that sits inside a block is indented one unit (generally two
spaces) more than the statement or brace that opened the block.</li>
<li>A statement that is continued to the next line is indented one unit
more than the line that starts the statement.</li>
<li>When dealing with lists of arguments or the content of array and
object literals there are two possible models. If there is any text
directly after the opening brace, bracket, or parenthesis,
subsequent lines are aligned with this opening character. If the
opening character is followed by a newline (optionally with whitespace
or comments before it), the next line is indented one unit further
than the line that started the list.</li>
<li>And, obviously, if a statement follows another statement it is
indented the same amount as the one before it.</li>
</ul>
<p>When scanning backwards through code one has to take string values,
comments, and regular expressions (which are delimited by slashes)
into account, because braces and semicolons and such are not
significant when they appear inside them. Single-line ('//') comments
turned out to be rather inefficient to check for when doing a
backwards scan, since every time you encounter a newline you have to
go on to the next newline to determine whether this line ends in a
comment or not. Regular expressions are even worse ― without
contextual information they are impossible to distinguish from the
division operator, and I didn't get them working in this first
version.</p>
<p>To find out which line to indent, and to make sure that adding or
removing whitespace doesn't cause the cursor to jump in strange ways,
it is necessary to determine which text the user has selected. Even
though I was working with just a simple textarea at this point, this
was already a bit of a headache.</p>
<p>On W3C-standards-respecting browsers, textarea nodes have
<code>selectionStart</code> and <code>selectionEnd</code>
properties which nicely give you the amount of characters before
the start and end of the selection. Great!</p>
<p>Then, there is Internet Explorer. Internet Explorer also has an API
for looking at and manipulating selections. It gives you information
such as a detailed map of the space the selected lines take up on the
screen, in pixels, and of course the text inside the selection. It
does, however, not give you much of a clue on where the selection is
located in the document.</p>
<p>After some experimentation I managed to work out an elaborate
method for getting something similar to the
<code>selectionStart</code> and <code>selectionEnd</code> values
in other browsers. It worked like this:</p>
<ul>
<li>Get the <code>TextRange</code> object corresponding to the selection.</li>
<li>Record the length of the text inside it.</li>
<li>Make another <code>TextRange</code> that covers the whole textarea element.</li>
<li>Set the start of the first <code>TextRange</code> to the start of the second one.</li>
<li>Again get the length of the text in the first object.</li>
<li>Now <code>selectionEnd</code> is the second length, and <code>selectionStart</code> is
the second minus the first one.</li>
</ul>
<p>That seemed to work, but when resetting the selection after modifying
the content of the textarea I ran into another interesting feature of
these <code>TextRange</code>s: You can move their endpoints by a given number of
characters, which is useful when trying to set a cursor at the Nth
character of a textarea, but in this context, newlines are <em>not</em>
considered to be characters, so you'll always end up one character too
far for every newline you passed. Of course, you can count newlines
and compensate for this (though it is still not possible to position
the cursor right in front of a newline). Sheesh.</p>
<p>After ragging on Internet Explorer for a while, let us move on and rag
on Firefox a bit. It turns out that, in Firefox, getting and setting
the text content of a DOM element is unexplainably expensive,
especially when there is a lot of text involved. As soon as I tried to
use my indentation code to indent itself (some 400 lines), I found
myself waiting for over four seconds every time I pressed enter. That
seemed a little slow.</p>
<h2 id="designmode">designMode it is</h2>
<p>The solution was obvious: Since the text inside a textarea can only be
manipulated as one single big string, I had to spread it out over
multiple nodes. How do you spread editable content over multiple
nodes? Right! <code>designMode</code> or <code>contentEditable</code>.</p>
<p>Now I wasn't entirely naive about <code>designMode</code>, I had been looking
into writing a non-messy WYSIWYG editor before, and at that time I had
concluded two things:</p>
<ul>
<li>It is impossible to prevent the user from inserting whichever HTML
junk he wants into the document.</li>
<li>In Internet Explorer, it is extemely hard to get a good view
on what nodes the user has selected.</li>
</ul>
<p>Basically, the good folks at Microsoft designed a really bad interface
for putting editable documents in pages, and the other browsers, not
wanting to be left behind, more or less copied that. And there isn't
much hope for a better way to do this appearing anytime soon. Wise
people probably use a Flash movie or (God forbid) a Java applet for
these kind of things, though those are not without drawbacks either.</p>
<p>Anyway, seeing how using an editable document would also make syntax
highlighting possible, I foolishly went ahead. There is something
perversely fascinating about trying to build a complicated system on a
lousy, unsuitable platform.</p>
<h2 id="parser">A parser</h2>
<p>How does one do decent syntax highlighting? A very simple scanning can
tell the difference between strings, comments, keywords, and other
code. But this time I wanted to actually be able to recognize regular
expressions, so that I didn't have any blatant incorrect behaviour
anymore.</p>
<p>That brought me to the idea of doing a serious parse on the code. This
would not only make detecting regular expressions much easier, it
would also give me detailed information about the code, which can be
used to determine proper indentation levels, and to make subtle
distinctions in colouring, for example the difference between variable
names and property names.</p>
<p>And hey, when we're parsing the whole thing, it would even be possible
to make a distinction between local and global variables, and colour
them differently. If you've ever programmed JavaScript you can
probably imagine how useful this would be ― it is ridiculously easy
to accidentally create global instead of local variables. I don't
consider myself a JavaScript rookie anymore, but it was (embarrasingly
enough) only this week that I realized that my habit of typing <code>for
(name in object) ...</code> was creating a global variable <code>name</code>, and that
I should be typing <code>for (var name in object) ...</code> instead.</p>
<p>Re-parsing all the code the user has typed in every time he hits a key
is obviously not feasible. So how does one combine on-the-fly
highlighting with a serious parser? One option would be to split the
code into top-level statements (functions, variable definitions, etc.)
and parse these separately. This is horribly clunky though, especially
considering the fact that modern JavaScripters often put all the code
in a file in a single big object or function to prevent namespace
pollution.</p>
<p>I have always liked continuation-passing style and generators. So the
idea I came up with is this: An interruptable, resumable parser. This
is a parser that does not run through a whole document at once, but
parses on-demand, a little bit at a time. At any moment you can create
a copy of its current state, which can be resumed later. You start
parsing at the top of the code, and keep going as long as you like,
but throughout the document, for example at every end of line, you
store a copy of the current parser state. Later on, when line 106
changes, you grab the interrupted parser that was stored at the end of
line 105, and use it to re-parse line 106. It still knows exactly what
the context was at that point, which local variables were defined,
which unfinished statements were encountered, and so on.</p>
<p>But that, unfortunately, turned out to be not quite as easy as it
sounds.</p>
<h2 id="dom">The DOM nodes underfoot</h2>
<p>Of course, when working inside an editable frame we don't just
have to deal with text. The code will be represented by some kind
of DOM tree. My first idea was to set the <code>white-space:
pre</code> style for the frame and try to work with mostly text,
with the occasional coloured <code>span</code> element. It turned
out that support for <code>white-space: pre</code> in browsers,
especially in editable frames, is so hopelessly glitchy that this
was unworkable.</p>
<p>Next I tried a series of <code>div</code> elements, one per
line, with <code>span</code> elements inside them. This seemed to
nicely reflect the structure of the code in a shallowly
hierarchical way. I soon realized, however, that my code would be
much more straightfoward when using no hierarchy whatsoever
― a series of <code>span</code>s, with <code>br</code> tags
at the end of every line. This way, the DOM nodes form a flat
sequence that corresponds to the sequence of the text ―
just extract text from <code>span</code> nodes and substitute
newlines for <code>br</code> nodes.</p>
<p>It would be a shame if the editor would fall apart as soon as
someone pastes some complicated HTML into it. I wanted it to be
able to deal with whatever mess it finds. This means using some
kind of HTML-normalizer that takes arbitrary HTML and flattens it
into a series of <code>br</code>s and <code>span</code> elements
that contain a single text node. Just like the parsing process, it
would be best if this did not have to done to the entire buffer
every time something changes.</p>
<p>It took some banging my head against my keyboard, but I found a very
nice way to model this. It makes heavy use of generators, for which I
used <a href="http://www.mochikit.com">MochiKit</a>'s iterator
framework. Bob Ippolito explains the concepts in this library very
well in his <a
href="http://bob.pythonmac.org/archives/2005/07/06/iteration-in-javascript/">blog
post</a> about it. (Also notice some of the dismissive comments at the
bottom of that post. They say "I don't think I really want to learn
this, so I'll make up some silly reason to condemn it.")</p>
<p>The highlighting process consists of the following elements:
normalizing the DOM tree, extracting the text from the DOM tree,
tokenizing this text, parsing the tokens, and finally adjusting the
DOM nodes to reflect the structure of the code.</p>
<p>The first two, I put into a single generator. It scans the DOM
tree, fixing anything that is not a simple top-level
<code>span</code> or <code>br</code>, and it produces the text
content of the nodes (or a newline in case of a <code>br</code>)
as its output ― each time it is called, it yields a string.
Continuation passing style was a good way to model this process in
an iterator, which has to be processed one step at a time. Look at
this simplified version:</p>
<pre class="code" style="width: 110%"><span class="js-keyword">function</span> <span class="js-variable">traverseDOM</span>(<span class="js-variabledef">start</span>){
<span class="js-keyword">var</span> <span class="js-variabledef">cc</span> = <span class="js-keyword">function</span>(){<span class="js-keyword">return</span> <span class="js-variable">scanNode</span>(<span class="js-localvariable">start</span>, <span class="js-variable">stop</span>);};
<span class="js-keyword">function</span> <span class="js-variabledef">stop</span>(){
<span class="js-localvariable">cc</span> = <span class="js-localvariable">stop</span>;
<span class="js-keyword">throw</span> <span class="js-variable">StopIteration</span>;
}
<span class="js-keyword">function</span> <span class="js-variabledef">yield</span>(<span class="js-variabledef">value</span>, <span class="js-variabledef">c</span>){
<span class="js-localvariable">cc</span> = <span class="js-localvariable">c</span>;
<span class="js-keyword">return</span> <span class="js-localvariable">value</span>;
}
<span class="js-keyword">function</span> <span class="js-variabledef">scanNode</span>(<span class="js-variabledef">node</span>, <span class="js-variabledef">c</span>){
<span class="js-keyword">if</span> (<span class="js-localvariable">node</span>.<span class="js-property">nextSibling</span>)
<span class="js-keyword">var</span> <span class="js-variabledef">nextc</span> = <span class="js-keyword">function</span>(){<span class="js-keyword">return</span> <span class="js-localvariable">scanNode</span>(<span class="js-localvariable">node</span>.<span class="js-property">nextSibling</span>, <span class="js-localvariable">c</span>);};
<span class="js-keyword">else</span>
<span class="js-keyword">var</span> <span class="js-variabledef">nextc</span> = <span class="js-localvariable">c</span>;
<span class="js-keyword">if</span> (<span class="js-comment">/* node is proper span element */</span>)
<span class="js-keyword">return</span> <span class="js-localvariable">yield</span>(<span class="js-localvariable">node</span>.<span class="js-property">firstChild</span>.<span class="js-property">nodeValue</span>, <span class="js-localvariable">nextc</span>);
<span class="js-keyword">else</span> <span class="js-keyword">if</span> (<span class="js-comment">/* node is proper br element */</span>)
<span class="js-keyword">return</span> <span class="js-localvariable">yield</span>(<span class="js-string">"\n"</span>, <span class="js-localvariable">nextc</span>);
<span class="js-keyword">else</span>
<span class="js-comment">/* flatten node, yield its textual content */</span>;
}
<span class="js-keyword">return</span> {<span class="js-property">next</span>: <span class="js-keyword">function</span>(){<span class="js-keyword">return</span> <span class="js-localvariable">cc</span>();}};
}</pre>
<p>The variable <code>c</code> stands for 'continuation', and <code>cc</code> for 'current
continuation' ― that last variable is used to store the function to
continue with, when yielding a value to the outside world. Every time
control leaves this function, it has to make sure that <code>cc</code> is set to
a suitable value, which is what <code>yield</code> and <code>stop</code> take care of.</p>
<p>The object that is returned contains a <code>next</code> method, which is
MochiKit's idea of an iterator, and the initial continuation just
throws a <code>StopIteration</code>, which is how MochiKit signals that an
iterator has reached its end.</p>
<p>The first lines of <code>scanNode</code> extend the continuation with the task of
scanning the next node, if there is a next node. The rest of the
function decides what kind of value to <code>yield</code>. Note that this is a
rather trivial example of this technique, since the process of going
through these nodes is basically linear (it was much, much more
complex in earlier versions), but still the trick with the
continuations makes the code shorter and, for those in the know,
clearer than the equivalent 'storing the iterator state in variables'
approach.</p>
<p>The next iterator that the input passes through is the
tokenizer. Well, actually, there is another iterator in between
that isolates the tokenizer from the fact that the DOM traversal
yields a bunch of separate strings, and presents them as a single
character stream (with a convenient <code>peek</code> operation),
but this is not a very interesting one. What the tokenizer returns
is a stream of token objects, each of which has a
<code>value</code>, its textual content, a <code>type</code>, like
<code>"variable"</code>, <code>"operator"</code>, or just itself,
<code>"{"</code> for example, in the case of significant
punctuation or special keywords. They also have a
<code>style</code>, which is used later by the highlighter to give
their <code>span</code> elements a class name (the parser will
still adjust this in some cases).</p>
<p>At first I assumed the parser would have to talk back to the
tokenizer about the current context, in order to be able to
distinguish those accursed regular expressions from divisions, but
it seems that regular expressions are only allowed if the previous
(non-whitespace, non-comment) token was either an operator, a
keyword like <code>new</code> or <code>throw</code>, or a specific
kind of punctuation (<code>"[{}(,;:"</code>) that indicates a new
expression can be started here. This made things considerably
easier, since the 'regexp or no regexp' question could stay
entirely within the tokenizer.</p>
<p>The next step, then, is the parser. It does not do a very
thorough job because, firstly, it has to be fast, and secondly, it
should not go to pieces when fed an incorrect program. So only
superficial constructs are recognized, keywords that resemble each
other in syntax, such as <code>while</code> and <code>if</code>,
are treated in precisely the same way, as are <code>try</code> and
<code>else</code> ― the parser doesn't mind if an
<code>else</code> appears without an <code>if</code>. Stuff that
binds variables, <code>var</code>, <code>function</code>, and
<code>catch</code> to be precise, is treated with more care,
because the parser wants to know about local variables.</p>
<p>Inside the parser, three kinds of context are stored. Firstly, a set
of known local variables, which is used to adjust the style of
variable tokens. Every time the parser enters a function, a new set of
variables is created. If there was already such a set (entering an
inner function), a pointer to the old one is stored in the new one. At
the end of the function, the current variable set is 'popped' off and
the previous one is restored.</p>
<p>The second kind of context is the lexical context, this keeps track of
whether we are inside a statement, block, or list. Like the variable
context, it also forms a stack of contexts, with each one containing a
pointer to the previous ones so that they can be popped off again when
they are finished. This information is used for indentation. Every
time the parser encounters a newline token, it attaches the current
lexical context and a 'copy' of itself (more about that later) to this
token.</p>
<p>The third context is a continuation context. This parser does not use
straight continuation style, instead it uses a stack of actions that
have to be performed. These actions are simple functions, a kind of
minilanguage, they act on tokens, and decide what kind of new actions
should be pushed onto the stack. Here are some examples:</p>
<pre class="code" style="width: 110%"><span class="js-keyword">function</span> <span class="js-variable">expression</span>(<span class="js-variabledef">type</span>){
<span class="js-keyword">if</span> (<span class="js-localvariable">type</span> in <span class="js-variable">atomicTypes</span>)
<span class="js-variable">cont</span>(<span class="js-variable">maybeoperator</span>);
<span class="js-keyword">else</span> <span class="js-keyword">if</span> (<span class="js-localvariable">type</span> == <span class="js-string">"function"</span>)
<span class="js-variable">cont</span>(<span class="js-variable">functiondef</span>);
<span class="js-keyword">else</span> <span class="js-keyword">if</span> (<span class="js-localvariable">type</span> == <span class="js-string">"("</span>)
<span class="js-variable">cont</span>(<span class="js-variable">pushlex</span>(<span class="js-string">"list"</span>), <span class="js-variable">expression</span>, <span class="js-variable">expect</span>(<span class="js-string">")"</span>), <span class="js-variable">poplex</span>);
<span class="js-keyword">else</span> <span class="js-keyword">if</span> (<span class="js-localvariable">type</span> == <span class="js-string">"operator"</span>)
<span class="js-variable">cont</span>(<span class="js-variable">expression</span>);
<span class="js-keyword">else</span> <span class="js-keyword">if</span> (<span class="js-localvariable">type</span> == <span class="js-string">"["</span>)
<span class="js-variable">cont</span>(<span class="js-variable">pushlex</span>(<span class="js-string">"list"</span>), <span class="js-variable">commasep</span>(<span class="js-variable">expression</span>), <span class="js-variable">expect</span>(<span class="js-string">"]"</span>), <span class="js-variable">poplex</span>);
<span class="js-keyword">else</span> <span class="js-keyword">if</span> (<span class="js-localvariable">type</span> == <span class="js-string">"{"</span>)
<span class="js-variable">cont</span>(<span class="js-variable">pushlex</span>(<span class="js-string">"list"</span>), <span class="js-variable">commasep</span>(<span class="js-variable">objprop</span>), <span class="js-variable">expect</span>(<span class="js-string">"}"</span>), <span class="js-variable">poplex</span>);
<span class="js-keyword">else</span> <span class="js-keyword">if</span> (<span class="js-localvariable">type</span> == <span class="js-string">"keyword c"</span>)
<span class="js-variable">cont</span>(<span class="js-variable">expression</span>);
}
<span class="js-keyword">function</span> <span class="js-variable">block</span>(<span class="js-variabledef">type</span>){
<span class="js-keyword">if</span> (<span class="js-localvariable">type</span> == <span class="js-string">"}"</span>) <span class="js-variable">cont</span>();
<span class="js-keyword">else</span> <span class="js-variable">pass</span>(<span class="js-variable">statement</span>, <span class="js-variable">block</span>);
}</pre>
<p>The function <code>cont</code> (for continue), will push the actions it is given
onto the stack (in reverse order, so that the first one will be popped
first). Actions such as <code>pushlex</code> and <code>poplex</code> merely adjust the
lexical environment, while others, such as <code>expression</code> itself, do
actual parsing. <code>pass</code>, as seen in <code>block</code>, is similar to <code>cont</code>, but
it does not 'consume' the current token, so the next action will again
see this same token. In <code>block</code>, this happens when the function
determines that we are not at the end of the block yet, so it pushes
the <code>statement</code> function which will interpret the current token as the
start of a statement.</p>
<p>These actions are called by a 'driver' function, which filters out the
whitespace and comments, so that the parser actions do not have to
think about those, and keeps track of some things like the indentation
of the current line and the column at which the current token ends,
which are stored in the lexical context and used for indentation.
After calling an action, if the action called <code>cont</code>, this driver
function will return the current token, if <code>pass</code> (or nothing) was
called, it will immediately continue with the next action.</p>
<p>This goes to show that it is viable to write a quite elaborate
minilanguage in a macro-less language like JavaScript. I don't think
it would be possible to do something like this without closures (or
similarly powerful abstraction) though, I've certainly never seen
anything like it in Java code.</p>
<p>The way a 'copy' of the parser was produced shows a nice usage
of closures. Like with the DOM transformer shown above, most of
the local state of the parser is held in a closure produced by
calling <code>parse(stream)</code>. The function
<code>copy</code>, which is local to the parser function, produces
a new closure, with copies of all the relevant variables:</p>
<pre class="code"><span class="js-keyword">function</span> <span class="js-variable">copy</span>(){
<span class="js-keyword">var</span> <span class="js-variabledef">_context</span> = <span class="js-variable">context</span>, <span class="js-variabledef">_lexical</span> = <span class="js-variable">lexical</span>,
<span class="js-variabledef">_actions</span> = <span class="js-variable">copyArray</span>(<span class="js-variable">actions</span>);
<span class="js-keyword">return</span> <span class="js-keyword">function</span>(<span class="js-variabledef">_tokens</span>){
<span class="js-variable">context</span> = <span class="js-localvariable">_context</span>;
<span class="js-variable">lexical</span> = <span class="js-localvariable">_lexical</span>;
<span class="js-variable">actions</span> = <span class="js-variable">copyArray</span>(<span class="js-localvariable">_actions</span>);
<span class="js-variable">tokens</span> = <span class="js-localvariable">_tokens</span>;
<span class="js-keyword">return</span> <span class="js-variable">parser</span>;
};
}</pre>
<p>Where <code>parser</code> is the object that contains the <code>next</code> (driver)
function, and a reference to this <code>copy</code> function. When the function
that <code>copy</code> produces is called with a token stream as argument, it
updates the local variables in the parser closure, and returns the
corresponding iterator object.</p>
<p>Moving on, we get to the last stop in this chain of generators, the
actual highlighter. You can view this one as taking two streams as
input, on the one hand there is the stream of tokens from the parser,
and on the other hand there is the DOM tree as left by the DOM
transformer. If everything went correctly, these two should be
synchronized. The highlighter can look at the current token, see if
the <code>span</code> in the DOM tree corresponds to it (has the same text
content, and the correct class), and if not it can chop up the DOM
nodes to conform to the tokens.</p>
<p>Every time the parser yields a newline token, the highligher
encounters a <code>br</code> element in the DOM stream. It takes the copy of the
parser and the lexical context from this token and attaches them to
the DOM node. This way, a new highlighting process can be started from
that node by re-starting the copy of the parser with a new token
stream, which reads tokens from the DOM nodes starting at that <code>br</code>
element, and the indentation code can use the lexical context
information to determine the correct indentation at that point.</p>
<h2 id="selection">Selection woes</h2>
<p>All the above can be done using the DOM interface that all major
browsers have in common, and which is relatively free of weird bugs
and abberrations. However, when the user is typing in new code, this
must also be highlighted. For this to happen, the program must know
where the cursor currently is, and because it mucks up the DOM tree,
it has to restore this cursor position after doing the highlighting.</p>
<p>Re-highlighting always happens per line, because the copy of the
parser is stored only at the end of lines. Doing this every time the
user presses a key is terribly slow and obnoxious, so what I did was
keep a list of 'dirty' nodes, and as soon as the user didn't type
anyting for 300 milliseconds the program starts re-highlighting these
nodes. If it finds more than ten lines must be re-parsed, it does only
ten and waits another 300 milliseconds before it continues, this way
the browser never freezes up entirely.</p>
<p>As mentioned earlier, Internet Explorer's selection model is not the
most practical one. My attempts to build a wrapper that makes it look
like the W3C model all stranded. In the end I came to the conclusion
that I only needed two operations:</p>
<ul>
<li>Creating a selection 'snapshot' that can be restored after
highlighting, in such a way that it still works if some of the nodes
that were selected are replaced by other nodes with the same
size but a different structure.</li>
<li>Finding the top-level node around or before the cursor, to mark it
dirty or to insert indentation whitespace at the start of that line.</li>
</ul>
<p>It turns out that the pixel-based selection model that Internet
Explorer uses, which always seemed completely ludricrous to me, is
perfect for the first case. Since the DOM transformation (generally)
does not change the position of things, storing the pixel offsets of
the selection makes it possible to restore that same selection, never
mind what happened to the underlying DOM structure.</p>
<p>[Later addition: Note that this, due to the very random design
of the <a
href="http://msdn2.microsoft.com/en-us/library/ms535872(VS.85).aspx#">TextRange
interface</a>, only really works when the whole selection falls
within the visible part of the document.]</p>
<p>Doing the same with the W3C selection model is a lot harder. What I
ended up with was this:</p>
<ul>
<li>Create an object pointing to the nodes at the start and end of the
selection, and the offset within those nodes. This is basically the
information that the <code>Range</code> object gives you.</li>
<li>Make references from these nodes back to that object.</li>
<li>When replacing (part of) a node with another one, check for such a
reference, and when it is present, check whether this new node will
get the selection. If it does, move the reference from the old to the
new node, if it does not, adjust the offset in the selection object to
reflect the fact that part of the old node has been replaced.</li>
</ul>
<p>Now in the second case (getting the top-level node at the
cursor) the Internet Explorer cheat does not work. In the W3C
model this is rather easy, you have to do some creative parent-
and sibling-pointer following to arrive at the correct top-level
node, but nothing weird. In Internet Explorer, all we have to go
on is the <code>parentElement</code> method on a
<code>TextRange</code>, which gives the first element that
completely envelops the selection. If the cursor is inside a text
node, this is good, that text node tells us where we are. If the
cursor is between nodes, for example between two <code>br</code>
nodes, you get to top-level node itself back, which is remarkably
useless. In cases like this I stoop to a rather ugly hack (which
fortunately turned out to be acceptably fast) ― I create a
temporary empty <code>span</code> with an ID inside the selection,
get a reference to this <code>span</code> by ID, take its
<code>previousSibling</code>, and remove it again.</p>
<p>Unfortunately, Opera's selection implementation is buggy, and it
will give wildly incorrect <code>Range</code> objects when the cursor
is between two nodes. This is a bit of a showstopper, and until I find
a workaround for that or it gets fixed, the highlighter doesn't work
properly in Opera.</p>
<p>Also, when one presses enter in a <code>designMode</code>
document in Firefox or Opera, a <code>br</code> tag is inserted.
In Internet Explorer, pressing enter causes some maniacal gnome to
come out and start wrapping all the content before and after the
cursor in <code>p</code> tags. I suppose there is something to be
said for that, in principle, though if you saw the tag soup of
<code>font</code>s and nested paragraphs Internet Explorer
generates you would soon enough forget all about principle.
Anyway, getting unwanted <code>p</code> tags slowed the
highlighter down terribly ― it had to overhaul the whole
DOM tree to remove them again, every time the user pressed enter.
Fortunately I could fix this by capturing the enter presses and
manually inserting a <code>br</code> tag at the cursor.</p>
<p>On the subject of Internet Explorer's tag soup, here is an interesting
anecdote: One time, when testing the effect that modifying the content
of a selection had, I inspected the DOM tree and found a <code>"/B"</code>
element. This was not a closing tag, there are no closing tags in the
DOM tree, just elements. The <code>nodeName</code> of this element was actually
<code>"/B"</code>. That was when I gave up any notions of ever understanding the
profound mystery that is Internet Explorer.</p>
<h2 id="closing">Closing thoughts</h2>
<p>Well, I despaired at times, but I did end up with a working JavaScript
editor. I did not keep track of the amount of time I wasted on this,
but I would estimate it to be around fifty hours. Finding workarounds
for browser bugs can be a terribly nonlinear process. I just spent
half a day working on a weird glitch in Firefox that caused the cursor
in the editable frame to be displayed 3/4 line too high when it was at
the very end of the document. Then I found out that setting the
style.display of the iframe to "block" fixed this (why not?). I'm
amazed how often issues that seem hopeless do turn out to be
avoidable, even if it takes hours of screwing around and some truly
non-obvious ideas.</p>
<p>For a lot of things, JavaScript + DOM elements are a surprisingly
powerful platform. Simple interactive documents and forms can be
written in browsers with very little effort, generally less than with
most 'traditional' platforms (Java, Win32, things like WxWidgets).
Libraries like Dojo (and a similar monster I once wrote myself) even
make complex, composite widgets workable. However, when applications
go sufficiently beyond the things that browsers were designed for, the
available APIs do not give enough control, are nonstandard and buggy,
and are often poorly designed. Because of this, writing such
applications, when it is even possible, is <em>painful</em> process.</p>
<p>And who likes pain? Sure, when finding that crazy workaround,
subdueing the damn browser, and getting everything to work, there
is a certain macho thrill. But one can't help wondering how much
easier things like preventing the user from pasting pictures in
his source code would be on another platform. Maybe something like
Silverlight or whatever other new browser plugin gizmos people are
pushing these days will become the way to solve things like this
in the future. But, personally, I would prefer for those browser
companies to put some real effort into things like cleaning up and
standardising shady things like <code>designMode</code>, fixing
their bugs, and getting serious about ECMAScript 4.</p>
<p>Which is probably not realistically going to happen anytime soon.</p>
<hr/>
<p>Some interesting projects similar to this:</p>
<ul>
<li><a href="http://www.ymacs.org">Ymacs</a></li>
<li><a href="http://gpl.internetconnection.net/vi/">vi clone</a></li>
<li><a href="http://robrohan.com/projects/9ne/">Emacs clone</a></li>
<li><a href="http://codepress.sourceforge.net/">CodePress</a></li>
<li><a href="http://www.codeide.com">CodeIDE</a></li>
<li><a href="http://www.cdolivet.net/editarea">EditArea</a></li>
</ul>
<hr/>
<p>If you have any remarks, criticism, or hints related to the
above, drop me an e-mail at <a
href="mailto:marijnh@gmail.com">marijnh@gmail.com</a>. If you say
something generally interesting, I'll include your reaction here
at the bottom of this page.</p>
</div><div class="rightsmall blk">
<p style="font-size: 80%">
<b>Topics</b>: JavaScript, advanced browser weirdness, cool programming techniques<br/>
<b>Audience</b>: Programmers, especially JavaScript programmers<br/>
<b>Author</b>: Marijn Haverbeke<br/>
<b>Date</b>: May 24th 2007
</p>
<h2>Contents</h2>
<ul>
<li><a href="#indent">Only Indentation</a></li>
<li><a href="#designmode"><code>designMode</code></a></li>
<li><a href="#parser">A Parser</a></li>
<li><a href="#dom">DOM Nodes</a></li>
<li><a href="#selection">Selection Woes</a></li>
<li><a href="#closing">Closing Thoughts</a></li>
</ul>
<h2>Site</h2>
<ul>
<li><a href="index.html">Front Page</a></li>
<li><a href="manual.html">User Manual</a></li>
<li><a href="faq.html">FAQ</a></li>
<li><a href="http://groups.google.com/group/codemirror">Google Group</a></li>
<li><a href="compress.html">Compression Helper</a></li>
</ul>
</div></div>
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