Difference between revisions of "Code snippets"

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#REDIRECT [[:Category:Coding Tips]]
= Shortcut to document.getElementById =
function $(id) {
  return document.getElementById(id);
Example usage:
$("header").innerHTML = "Halloa!";
= XPath helper =
Run a particular [[XPath]] expression <code>p</code> against the context node <code>context</code> (or the document, if not provided).
Returns the results as an array.
function $x(p, context) {
  if (!context) context = document;
  var i, arr = [], xpr = document.evaluate(p, context, null, XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE, null);
  for (i = 0; item = xpr.snapshotItem(i); i++) arr.push(item);
  return arr;
Example usage (with [http://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Reference:Objects:Array:forEach Array.forEach]):
var i, paragraphs = $x("//p");
paragraphs.forEach(function(paragraph) {  // Loop over every paragraph
  paragraph.innerHTML = "Halloa!";
= Conditional logging =
Used to easily toggle sending debug messages to the console. Passes all arguments on to [[console.log]], but only if <code>console</code> is defined (for backward compatibility) and <code>DEBUG</code> is <code>true</code>.
Code and example usage:
const DEBUG = true;
var links = document.links;
debug("Links: %o", links);
function debug() {
  if (DEBUG && console) {
    console.log.apply(this, arguments);
= Serialize/deserialize for GM_getValue =
Used to store and retrieve multiple values (typically as a serialized hash) in a single [[GM_getValue]] slot.
function deserialize(name, def) {
  return eval(GM_getValue(name, (def || '({})')));
function serialize(name, val) {
  GM_setValue(name, uneval(val));
Example usage:
var settings = {a: 1, b: 2, c: 3};
serialize('test', settings);
var _settings = deserialize('test');
// now "settings == _settings" should be true
= Log only when debugging =
function debug(message) {
  if (DEBUG) GM_log(message);
Example usage:
var DEBUG = true;
debug("Attempting to load wubbles...");
= DOM node manipulation =
== Use .innerHTML to create DOM structure ==
The non W3 standard setter function [http://developer.mozilla.org/en/docs/DOM:element.innerHTML <code>.innerHTML</code>] can be used to concisely create DOM structure in trivial userscripts. The following is a complete branding script to show that Greasemonkey has run on a host.
host = document.location.host;
dummyDiv = document.createElement('div');
dummyDiv.innerHTML = '&lt;div>&lt;span style="color: red">Greased: ' + host + '&lt;/span>&lt;/div>';
document.body.insertBefore(dummyDiv.firstChild, document.body.firstChild);
Note above that <code>dummyDiv</code> is only needed as a holder for its <code>.firstChild</code>.
Here is a helper function that reuses the <code>dummyDiv</code>:
function firstNodeOf(html){
  firstNodeOf.dummyDiv.innerHTML = html;
  return firstNodeOf.dummyDiv.firstChild;
firstNodeOf.dummyDiv = documentCreateElement('div');
With this helper, one can write the following trivial script:
document.body.appendChild(firstNodeOf('&lt;div>&lt;span style="color: red">END OF PAGE&lt;/span>&lt;/div>');
'''STRONGLY NOTE:''' <code>.innerHTML</code> seems to be sensitive to <code>document.contentType</code>. When the type is <code>text/plain</code> the <code>.innerHTML</code> setter does not parse its argument into DOM nodes, but instead returns #text nodes. The setter seems to work fine for types such as <code>text/html</code> or <code>application/xhtml+xml</code> but where and how it works is undocumented.
'''Advantages:''' Concise and and probably more efficient than any hand coded approach.
'''Disadvantages:''' Other than the <code>text/plain</code> probem, <code>.innerHTML</code> is not a W3 standard and many people don't like non-standard code.
'''Upshot:''' Until [http://developer.mozilla.org/en/docs/E4X ECMAScript for XML (E4X)] becomes availble in user scripts, this is a useful hack for non <code>text/plain</code> pages.
=== Notes ===
* <code>.innerHTML</code> [http://developer.mozilla.org/en/docs/DOM:element.innerHTML#Notes cannot be used to create parts of a <code>TABLE</code>]. E.g.
tab = firstNodeOf('&lt;table>&lt;tr>&lt;td> ... &lt;/td>&lt;/tr>&lt;/table>'); // WORKS
tr  = firstNodeOf('&lt;tr>&lt;td> ... &lt;/td>&lt;/tr>');                // DOESN'T WORK
td  = firstNodeOf('&lt;td> ... &lt;/td>');                        // DOESN'T WORK
td.innerHTML = ' ... ';                                      // WORKS
So, if a script builds a table from a HTML string, it has to be done as a whole rather than in parts.
== Build a DOM node with attributes ==
Creates a new DOM node with the given attributes.
function createElement(type, attributes){
  var node = document.createElement(type);
  for (var attr in attributes){
  node.setAttribute(attr, attributes[attr]);
  return node;
Example usage:
link = createElement('link', {rel: 'stylesheet', type: 'text/css', href: basedir + 'style.css'});
== Remove DOM node ==
function remove(element) {
== Insert node after node ==
function insertAfter(newNode, node) {
  return node.parentNode.insertBefore(newNode, node.nextSibling);
This works because even if <code>node</code> is the last node, <code>nextSibling</code> returns <code>null</code> so <code>insertBefore</code> puts the new node at the end.
Example usage:
var link = document.getElementById("the_link");
var icon = document.createElement("img");
icon.src = "…";
insertAfter(icon, link);
== Hijacking browser properties ==
Sometimes you want to cook your own browser constants, for instance change the value of navigator.userAgent. Getters are good for that:
var real = window.navigator.userAgent;
var lie = function() { return real + " Macintosh"; };
unsafeWindow.navigator.__defineGetter__("userAgent", lie);
== Extending the DOM with missing functions ==
Other times you might want to emulate proprietary functionality of another browser, for instance when a site uses such features:
var getter = function() { return this.textContent; };
var setter = function(t) { return this.textContent = t; };
unsafeWindow.HTMLElement.prototype.__defineGetter__("innerText", getter);
unsafeWindow.HTMLElement.prototype.__defineSetter__("innerText", setter);
== Advanced createElement for creating hierarchies of elements ==
Creates an element with attributes as well as child elements with their own attributes and children. Function should be called with arguments in the form of the following hash (note that "child1", "child2" should be hashes of the same structure):
createEl({n: nodename, a: {attr1: val, attr2: val}, c: [child1, child2], evl: {type: eventlistener_type, f: eventlistener_function, bubble: bool}}, appendTo)
function createEl(elObj, parent) {
  var el;
  if (typeof elObj == 'string') {
      el = document.createTextNode(elObj);
  else {
      el = document.createElement(elObj.n);
      if (elObj.a) {
        attributes = elObj.a;
        for (var key in attributes) {
            if (key.charAt(0) == '@')
              el.setAttribute(key.substring(1), attributes[key]);
              el[key] = attributes[key];
      if (elObj.evl) {
        el.addEventListener(elObj.evl.type, elObj.evl.f, elObj.evl.bubble);
      if (elObj.c) {
        elObj.c.forEach(function (v, i, a) { createEl(v, el); });
  if (parent)
  return el;
Example usage:
    createEl({n: 'ol', a: {'@class': 'some_list', '@id': 'my_list'}, c: [
    {n: 'li', a: {textContent: 'first point'}, evl: {type: 'click', f: function() {alert('first point');}, bubble: true}},
    {n: 'li', a: {textContent: 'second point'}},
    {n: 'li', a: {textContent: 'third point'}}
    ]}, document.body);
= GET an URL with callback function =
Retrieves <code>url</code> using HTTP GET, then calls the function <code>cb</code> with the response text as its single argument.
function get(url, cb) {
    method: "GET",
      url: url,
      onload: function(xhr) { cb(xhr.responseText); }
Example usage:
function inform(text) {
  alert("The HTML of the page: " + text);
get("&#104;ttp://www.google.com", inform);
= RegExp escape string =
Escapes regexp meta characters in a string.
function escapeRegexp(s) {
  return s.replace(/([.*+?^${}()|[\]\/\\])/g, '\\$1');
Example usage:
var re = new RegExp("^" + escapeRegexp("fo*bar") + "$");
"fo*bar".match(re);  // Matches
"foobar".match(re);  // Doesn't match
= !important Style =
Appends !important to each rule then adds the CSS to the page letting you override the default formatting.
  function addStyle(css) {
    GM_addStyle(css.replace(/;/g,' !important;'));
Example usage:
  addStyle('a {text-decoration:none;}');
= make an array persistent in globalStorage =
[http://developer.mozilla.org/en/docs/DOM:Storage DOM:Storage] is available in Firefox 2.0 and up.
    Makes an given Array persistent in the globalStorage Object. It will not work
    with arrays that get additional mutator functions after it was made persistent.
    @param {String} name - name of the property of this that is the Array
    @param {String} domain - domain parameter of globalStorage
    @param {Array{String, String, ...}} add_mutator - additional non standard (Javascript 1.7) functions
  function makeArrayPersistent(_name, _domain, _add_mutator){
    //workaround for scripts that work on pages stored on file://
    var domain = _domain || ".localdomain";
    //if the array is not defined yet we define it and fill it
    //with values stored in globalStorage
      var evalStr = String(globalStorage[domain][_name]);
      this[_name] = eval(evalStr);
    //if it is defined allready we store it in globalStorage
      globalStorage[domain][_name] = uneval(this[_name]);
    //Watch will intercept asignments to this[_name] and store the new array in
    //globalStorage. The original array in globalStorage is discarded.
    this.watch(_name, function(_prop, _oldVal, _newVal){
      globalStorage[domain][_name] = uneval(_newVal);
      return _newVal;
    //see a few lines below
    ["push", "pop", "reverse", "shift", "sort", "splice", "unshift"].forEach(function(_f){
    //you can supply additional functions that will be wrapped
    //member functions that alter the array itself are wrapped. The wrapper will
    //call the mutator function and store the altered array in globalStorage
    function makeMutatorFunctionGlobal(_f){
      this[_name][_f] = function(){
        var f = this[_name][_f];
        return function(){
          var retVal;
          retVal = f.apply(this, arguments);
          globalStorage[domain].trolls = uneval(this);
          return retVal;
== Example Usage ==
We create an array in window.
  var a = [1,4,3,2];
The following would not work:
  function baz(){
    var b;
    makeArrayPersistent("b", "somewhere.org");
We cant keep track of variables that that are not kept track of by Javascript itself.
  makeArrayPersistent("a", "somewhere.org");
We have to supply the variables name due to watch. this["a"] equals this.a in our case. If you want to store an object in a different scope you can use makeArrayPersistent.call(some.object, "a", "somewhere.org"); . The domain has to match the domain of the site where your script is injected too. Read up on globalObject for details.
Now a equals [1,4,3,2,5]. It is stored in globalObject that way.
And now it's [1,2,3,4,5], stored again.
  delete this.a;
Now the array does not exist in this anymore. But it is kept in globalObject. In the next line a will be defined in this and filled from globalObject.
  makeArrayPersistent("a", "dexhome.homelinux.org");
And this.a equals [1,2,3,4,5] again.
If the user of your script got the same page open in two tabs the two scripts will start to fight over globalStorage and you will lose data. You have to alter makeMutatorFunctionGlobal with some meaningfull logic to counter this. The idea is to retrieve the array from globalStorage, combine it with the local copy and run the wrapped mutator function. Afterward it's stored in globalStorage again.

Latest revision as of 14:09, 4 February 2010