How to Path an Image File in a JavaScript File in CakePHP

If you write any path in the JavaScript file under webroot/js/,
It is the path seen from the accessed URL of the HTML file from which the file was loaded.

• In the Example 1js file, "img/a.png"
http://foo.example.com/user/img/a.png,
  when accessed by http://foo.example.com/user/ http://foo.example.com/user/index/img/a.png when accessed by http://foo.example.com/user/index/

• In the Example 2js file, "/img/a.png"
http://foo.example.com/img/a.png or http://foo.example.com/user/index/ whichever you access, http://foo.example.com/user/
  However, if you have apps under a specific subdirectory with features such as Alias, (like http://foo.example.com/appbase/user/) will also be http://foo.example.com/img/a.png.

Example 1 "img/a.png"
in the js file http://foo.example.com/user/img/a.png,
when accessed by http://foo.example.com/user/ http://foo.example.com/user/index/img/a.png when accessed by http://foo.example.com/user/index/

Example 2 In the js file "/img/a.png"
http://foo.example.com/img/a.png or http://foo.example.com/user/index/ whichever you access, http://foo.example.com/user/
However, if you have a application under a specific subdirectory with features such as Alias, it will also be http://foo.example.com/img/a.png.

Perhaps the questioner is operating under a subdirectory.
In that case, you need to supplement the underlying directory, such as /appbase/img/a.png, or pass $this->request->base to call processing from a script in the View template.