Proj4js is a JavaScript library to transform point coordinates from one coordinate system to another, including datum transformations. Originally a port of PROJ (then known as PROJ.4) and GCTCP C (Archive) it is a part of the MetaCRS group of projects.

Depending on your preferences

npm install proj4
bower install proj4
component install proj4js/proj4js

or just manually grab the file proj4.js from the latest release's dist/ folder.

If you do not want to download anything, Proj4js is also hosted on cdnjs for direct use in your browser applications.

The basic signature is:

proj4(fromProjection[, toProjection, coordinates])

Projections can be proj or wkt strings.

Coordinates may an object of the form {x:x,y:y} or an array of the form [x,y].

When all 3 arguments are given, the result is that the coordinates are transformed from projection1 to projection 2. And returned in the same format that they were given in.

var firstProjection = 'PROJCS["NAD83 / Massachusetts Mainland",GEOGCS["NAD83",DATUM["North_American_Datum_1983",SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],AUTHORITY["EPSG","6269"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4269"]],UNIT["metre",1,AUTHORITY["EPSG","9001"]],PROJECTION["Lambert_Conformal_Conic_2SP"],PARAMETER["standard_parallel_1",42.68333333333333],PARAMETER["standard_parallel_2",41.71666666666667],PARAMETER["latitude_of_origin",41],PARAMETER["central_meridian",-71.5],PARAMETER["false_easting",200000],PARAMETER["false_northing",750000],AUTHORITY["EPSG","26986"],AXIS["X",EAST],AXIS["Y",NORTH]]';
var secondProjection = "+proj=gnom +lat_0=90 +lon_0=0 +x_0=6300000 +y_0=6300000 +ellps=WGS84 +datum=WGS84 +units=m +no_defs";
//I'm not going to redefine those two in latter examples.
proj4(firstProjection,secondProjection,[2,5]);
// [-2690666.2977344505, 3662659.885459918]

If only 1 projection is given then it is assumed that it is being projected from WGS84 (fromProjection is WGS84).

proj4(firstProjection,[-71,41]);
// [242075.00535055372, 750123.32090043]

If no coordinates are given an object with two methods is returned, its methods are forward which projects from the first projection to the second and inverse which projects from the second to the first.

proj4(firstProjection,secondProjection).forward([2,5]);
// [-2690666.2977344505, 3662659.885459918]
proj4(secondProjection,firstProjection).inverse([2,5]);
// [-2690666.2977344505, 3662659.885459918]

And as above if only one projection is given, it's assumed to be coming from wgs84:

proj4(firstProjection).forward([-71,41]);
// [242075.00535055372, 750123.32090043]
proj4(firstProjection).inverse([242075.00535055372, 750123.32090043]);
//[-71, 40.99999999999986]
//the floating points to answer your question

If you prefer to define a projection as a string and reference it that way, you may use the proj4.defs method which can be called 2 ways, with a name and projection:

proj4.defs('WGS84', "+title=WGS 84 (long/lat) +proj=longlat +ellps=WGS84 +datum=WGS84 +units=degrees");

or with an array

proj4.defs([
  [
    'EPSG:4326',
    '+title=WGS 84 (long/lat) +proj=longlat +ellps=WGS84 +datum=WGS84 +units=degrees'],
  [
    'EPSG:4269',
    '+title=NAD83 (long/lat) +proj=longlat +a=6378137.0 +b=6356752.31414036 +ellps=GRS80 +datum=NAD83 +units=degrees'
  ]
]);

you can then do

proj4('EPSG:4326');

instead of writing out the whole proj definition, by default proj4 has the following projections predefined:

• 'EPSG:4326', which has the following alias
  • 'WGS84'
• 'EPSG:4269'
• 'EPSG:3857', which has the following aliases
  • 'EPSG:3785'
  • 'GOOGLE'
  • 'EPSG:900913'
  • 'EPSG:102113'

Defined projections can also be accessed through the proj4.defs function (proj4.defs('EPSG:4326')).

proj4.defs can also be used to define a named alias:

proj4.defs('urn:x-ogc:def:crs:EPSG:4326', proj4.defs('EPSG:4326'));

By default, proj4 uses [x,y] axis order for projected (cartesian) coordinate systems and [x=longitude,y=latitude] for geographic coordinates. To enforce the axis order of the provided proj or wkt string, use the

proj4(fromProjection, toProjection).forward(coordinate, enforceAxis);
proj4(fromProjection, toProjection).inverse(coordinate, enforceAxis);

signatures with enforceAxis set to true:

proj4('+proj=longlat +ellps=WGS84 +datum=WGS84 +units=degrees +axis=neu', firstProjection).forward([41, -71], true);
// [242075.00535055372, 750123.32090043]
proj4('+proj=longlat +ellps=WGS84 +datum=WGS84 +units=degrees +axis=neu', firstProjection).inverse([242075.00535055372, 750123.32090043], true);
//[40.99999999999986, -71]
//the floating points to answer your question

To use +nadgrids= in a proj definition, first read your NTv2 .gsb file (e.g. from https://github.com/OSGeo/proj-datumgrid) into an ArrayBuffer, then pass it to proj4.nadgrid. E.g:

const buffer = fs.readFileSync('ntv2.gsb').buffer
proj4.nadgrid('key', buffer);

then use the given key in your definition, e.g. +nadgrids=@key,null. See Grid Based Datum Adjustments.

TypeScript implementation was added to the DefinitelyTyped repository.

$ npm install --save @types/proj4

To set up build tools make sure you have node and grunt-cli installed and then run npm install.

To do the complete build and browser tests run:

node_modules/.bin/grunt

To run node tests run:

npm test

To run node tests with coverage run:

npm test --coverage

To create a build with only default projections (latlon and Mercator) run:

node_modules/.bin/grunt build

To create a build with only custom projections include a comma separated list of projections codes (the file name in 'lib/projections' without the '.js') after a colon, e.g.:

node_modules/.bin/grunt build:tmerc
#includes transverse Mercator
node_modules/.bin/grunt build:lcc
#includes lambert conformal conic
node_modules/.bin/grunt build:omerc,moll
#includes oblique Mercator and Mollweide