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.