Back in Jan, I was trying to understand different ways of modifying the url of a page without actually having to refresh it. A typical case is search pages. If you pull your results in through Ajax -as you modify the search parameters- you also want the url to be modified/constructed to reflect your refined search.
Previously modifying the url of a page by way of js would force a refresh:
There are a whole bunch of posts up now on the subject, but this particular one caught my eye as it was one of the early ones and really did a great job explaining the concept with a nice tutorial.
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Other nice resources: