Location hack: Difference between revisions

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The '''location hack''' is an ugly but useful way to interact with the content scope of the page being [[user script]]ed. It does this by indirectly [http://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Reference:Global_Functions:eval evaling] strings within that scope.
The location hack is an older technique for crossing the barrier between the user script scope and the content window scope, when it exists.
The previous version of this page may be read [http://wiki.greasespot.net/index.php?title=Location_hack&oldid=7202 via history].


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In Firefox version 39.0.3 Mozilla patched a known [https://www.mozilla.org/en-US/security/advisories/mfsa2015-78/ security vulnerability].
As a side effect, this broke the location hack.


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Thankfully in modern versions of Firefox and Greasemonkey, the location hack is no longer necessary.
 
Read about how to execute code in the content scope, like previously possible with the location hack, at [[Content Script Injection]].
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== Percent encoding issue ==
 
Sometimes percent-encoding the percent symbol is required. For example,
 
location.href = ("javascript:(" + function() {
  var n = 44;
  if(!('''n%22''')) alert('n is a multiple of 22');
} + ")()");
 
The above code will cause error because %22 is interpreted as double quotation mark. The workaround is:
 
location.href = "javascript:(" + encodeURI(
  function() {
  var n = 44;
  if(!(n%22)) alert('n is a multiple of 22');
  }) + ")()";
 
See also [http://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Reference:Global_Functions:encodeURI encodeURI()].
 
== Returning values ==
 
The location hack is really handy for passing values to the content scope, or to call functions defined there.
It is not, however, capable of directly reading a variable or value returned from a function.
Furthermore, it is run asynchronously, much like <code>setTimeout()</code>, so you cannot immediately rely on side effects.
(If you use the location hack to, for example, store a value in the DOM and then attempt to read it, it will only be available at some other later point in time.)
For reading javascript values from the content scope inside the sandbox, see [[Reading Content Globals]] for a reliable example.
 
== See Also ==
 
* [[Reading Content Globals]]
* [[Content Script Injection]]


[[Category:Coding Tips:Interacting With The Page]]
[[Category:Coding Tips:Interacting With The Page]]

Latest revision as of 15:54, 23 September 2015

The location hack is an older technique for crossing the barrier between the user script scope and the content window scope, when it exists. The previous version of this page may be read via history.

In Firefox version 39.0.3 Mozilla patched a known security vulnerability. As a side effect, this broke the location hack.

Thankfully in modern versions of Firefox and Greasemonkey, the location hack is no longer necessary. Read about how to execute code in the content scope, like previously possible with the location hack, at Content Script Injection.

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