ITEC 325 2017spring ibarland

home—lects—hws
D2L—breeze (snow day)

js-dom-surgery
case study: refactoring javascript

We'll look at using javascript to modify the DOM in response to events. Also, we'll consider an extended example of refactoring some javascript (in the process, illustrating some standard ways of modifying the DOM).

2017-Mar-14 distance section (57m05s): introduction to the DOM and javascript to modify it.

HTML vs. DOM

You have already talked about javascript code and the DOM in WebI; we'll review those concepts in the context of improving an actual web page.

We think of HTML as being a tree. (Or, a way of representing a document's tree-structure as a flat string full of angle-brackets.) The HTML gives content-information associated with nodes of the tree; CSS further provides presentation-information associated with nodes. And the browser even tracks more information with each node: all its default-styles and spacing, and more we'll see below.

Barland's definition:DOM (“Document Object Model”): The tree, as it exists in-memory, representing the web-page; it can be modified after the page is loaded.

Do not attempt to write any significant javascript without a decent debugging tool like Firebug or Chrome Tools' javascript console window help. At least, not if you value your sanity. In particular:

• Chrome » ⋮ » More Tools » Developer Tools, and
• From that same panel Elements » Properties.
• When you select a part of the HTML on the left pane, on the right pane you'll see the corresponding DOM object's class (e.g. “em”), as well as its superclass-ancestors (e.g. “HTMLElement”, “Node”, etc.). We'll focus on just the most specific class (we can use the others to figure out where it's inheriting an attribute from).

Javascript in a nutshell:

• Objects have fields and methods, accessed via “.” (just like Java).
• When running javascript inside a browser, the browser gives you a global-variable “document” which refers to the DOM (the tree).
• The method getElementById is the preferred way to access an element inside document.
  (This means that if you are going to access a node with javascript, then your HTML should include an id attribute for it.)

Note that all HTML attributes become DOM corresponding attributes: if you have “<table lang="en" id='sample' contentEditable='true'>…<p>” then the corresponding DOM node will have those attributes:

    var someNode = document.getElementById("sample");
    var isBarlandSpeakingTruth = (someNode.lang==="en" && someNode.innerText==="magic-blue");
    someNode.style.color = (isBarlandSpeakingTruth ? "blue" : "orange");
  

not green!

Weirdnesses/inconsistencies:

• Every HTML attribute turns into a javascript attribute for the corresponding node of the DOM.
  1. But, hyphenated style-attributes turn into camelcase (HTML <span id="blah" style="border-width: 2px;"> turns into an javascript attribute document.getAttributeById("blah").style.borderWidth.)
  2. And one attribute changes its name entirely: HTML attribute “class” becomes DOM-attribute “className”¹.
• If you provide an invalid node-attribute in the HTML, it doesn't get attached to the DOM. However, once the DOM is created, its nodes (like any javascript object) can have any new attribute you like assigned to them.
• The attribute “contentEditable" can have value "true","false","" or (in html5 but not in xhtml) no-value (which gives it the boolean true, in the DOM?). Beware javascript that just tests if (someNode.contentEditable) …, since in javascript the non-empty string "false" is true-ish!
• On the other hand, for html "boolean" attributes like "selected", the html selected='selected' turns into a DOM node where selected is the boolean value true.

A Case Study

We'll show-source, and discuss evolution of the javascript program. For each version: discuss how much work it would be to add another category (animal)? (And: what is the least conceivable amount of work/info required?)

1. js-hiding-parts-v0.html (An initial version, chock full of gross repeated code; I found this the web, and modified to be about animals. C-)
   youtube (14m11s):
2. js-hiding-parts-v1.html (A small tweak, greatly reduces length of code. B)
   (This version v1 discussed in the video for v2 below.)
3. js-hiding-parts-v2.html (Generalize -- much nicer. A)
   youtube (11m01s):
4. js-hiding-parts-v2-oops.html: Debugging your javascript.
   youtube (10m10s):
   Take note that javascript's for-in construct loops over indices, not contents, so you still need to use square-brackets inside the loop. (This is different from java's foreach and php's foreach).
5. js-hiding-parts-v3a.html Generalize the entire navbar. We do tree surgery: create new nodes which we splice onto the DOM entirely at run-time. A+.
   youtube (14m31s):

   As seen in the video, the javascript methods-of-interest are createElement (and its friend createTextNode) to create a element(tag), appendChild to add that node into the existing DOM, and setAttribute to add attributes to the element(tag).

6. Pitfall: js-hiding-parts-v3b.html As before, but placing the paragraph in a different place suddenly breaks the code?!
   youtube (09m33s):
   It breaks because the processing happens ASAP, before the named id has been parsed by the browser. An error message like “no such id” would have been very nice!² Had my first attempt suffered from this bug, I would have given up assuming my whole approach didn't work.
   See also: attribute defer="defer" for tag script, which waits for the entire page to load before running.
   (Or, less helpfully, the onload attribute which can be placed in body and just a few other tags).
7. js-hiding-parts-v3c.html, Minor, final touch: Pulling the javascript into its own file, so that it can now be re-used between many pages.
   Reminder (see preceding video): When writing a script tag, never use the self-closing notation (“<script src="…" />”), since the browser might ignore it entirely. Grrr!

Applying the above: How can/should we generalize a Bingo card javscript?

Note: here's an on-line javascript interpreter. We'll use it to note that: arrays are objects; any object can have properties accessed via square-brackets; but you should still iterate over arrays by indexing over [0,data.count). You can also use “for…in” to iterate over a general object's properties. For example, .childnodes returns a list, which can be iterated over via “for…in”.

arrays vs. objects, in javascript

Arrays in javascript are not associative -- the indices are only numeric.
However: any object (array or string or DOM-node or whatever) can have properties; properties are key/value pairs. So I guess every object could be used as an asssociate array incidentally. This is discouraged, because other code might add properties to your object.

Just to muddle things thoroughly, properties can be assigned/retrieve either through dot-notation (“a.color = "true"”) or through array-brackets (“a['color'] = "true"”) even though they are not arrays.

Arrays have a property named “length”. To demo, paste the below into this on-line javascript interpreter.

var a;
a = [71,72,73];


a.foo = "huh";

writeln( "The length of a: " + a.length );
writeln( "The length of a: " + a['length'] );



writeln( "Using a regular for-loop, to loop over numeric indices:" );
for (var i = 0;   i < a.length;  ++i ) {
  writeln("at index i=" + i + ", a[i]=" + a[i] );
}


writeln( "Using for-in, to loop over all *enumerable* properties:" );
for (var i in a) {
  writeln("at index i=" + i + ", a[i]=" + a[i] );
}
writeln( "Note: the for-in loop will also include properties from the object's "
       + "parent object, and it's parent, etc." );
writeln( "Note: `length` is a property of the object, but the `for` loop doesn't "
       + "enumerate over it.  That's because some properties can be tagged as "
       + "'non-enumerable' -- a concept invented just for for-each loops?" );

Tips for minimizing the difference between php and javascript code:

• You are allowed to have javascript variables start with "$" (weird, but allowed³)
• Grabbing input from a form:
  • in php: $_POST['fname']
  • in javascript: document.getElementById('fname')
  You can abstract these into a function

  getDocElement('fname')

  and then make getDocElement a (1-line wrapper) in both php and in javascript.

  This has the advantage that you can provide a default argument, to use when the element doesn't exist [especially helpful in php handling checkboxes, where if no checkboxes were selected the lookup $_POST would give a warning, and you can have your wrapper avoid the warning with isset, and instead have that function allow an optional second input: a value to use when the item isn't in the array: getDocElement('dietaryRestrictions', 'no restrictions'); or even getDocElement('billingAddr', getDocElement('shippingAddr') ).

home—lects—hws
D2L—breeze (snow day)

©2017, Ian Barland, Radford University Last modified 2017.May.01 (Mon)

Please mail any suggestions (incl. typos, broken links) to ibarlandradford.edu