This is a JavaScript library that provides methods for coordinate transformations between map projections and longitude/latitude, including datum transformations, in a web client.
To use the Proj4js, you first create a source and destination Proj4js.Proj objects, passing in a projection code (e.g. EPSG:4326). You can then use the Proj4js.transform() method, passing in map XY as a point object and the source and destination projection objects, and it returns the point coordinate in the destination projection.
For more information on Proj4js and to report bugs, please visit the Proj4js Trac and Wiki at OSGeo.