Example of maven project using npm for managing js dependencies, lessc for CSS preprocessing and r.js for js modules assembly.

build.js

#!/usr/bin/env node

var exec = require('child_process').exec,
    mkdirp = require('mkdirp'),
	projectPath = __dirname + '/../..',
	targetPath = __dirname + '/../../target/assets';

// Task execution helpers
// ----------------------

function Task(msg, fn) {
	if (typeof msg === 'function') {
		this.fn = msg;
	} else {
		this.msg = msg;
		this.fn = fn;
	}
};

Task.prototype.run = function () {
	var msg = this.msg,
		fn = this.fn,
		next = this.next;
	if (msg) {
		console.log('Running ' + msg + '...');
	}
	fn(function(err, out) {
		if (err) {
			console.error('ERROR: ' + err.toString());
			if (out) {
				console.info(out);
			}
			process.exit(1);
			return;
		}
		if (msg) {
			console.log('Done\n');
		}
		if (next) {
			next.run();
		}
	});
};

Task.prototype.than = function (msg, fn) {
	this.next = new Task(msg, fn);
	return this.next;
};

Task.run = function(msg, fn) {
	var task = new Task(msg, fn);
	process.nextTick(function() {
		task.run();
	});
	return task;
};


// Build task definitions
// ----------------------

Task.run(function (cb) {
	mkdirp(targetPath, cb);
}).than('Less compilation', function (cb) {
	exec('node ' + [
		projectPath + '/node_modules/.bin/lessc',
		'-x', 
		'-O2',
		'--include-paths=' + __dirname + '/../main/css',
	    projectPath + '/src/main/css/style.less',
		targetPath + '/css/style.css'
	].join(' '), cb);
}).than('Require.js dependency resolution and minification', function (cb) {
	exec('node ' + [
		projectPath + '/lib/r.js',
        '-o',
        projectPath + '/src/main/config/r-js.json',
        'baseUrl=' + projectPath + '/src/main/js',
		'dir=' + targetPath + '/js'
	].join(' '), cb);
});

package.json

{
    "name": "...",
    "descrition": "...",
    "version": "1.0.0",
    "private": true,
    "devDependencies": {
        "express": "3.2.x",
        "less": "1.3.x",
        "mocha": "1.11.x",
        "socket.io": "0.9.x",
        "mkdirp": "0.3.x"
    },
    "config": {
    },
    "scripts": {
        "test": "mocha test",
        "prepublish": "node build.js"
    }
}

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <build>
        <plugins>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>1.1.1</version>
                <executions>

                    <!-- Install all npm dependencies and call prepublish script in maven generate-resources pahase -->
                    <execution>
                        <id>npm-install</id>
                        <phase>generate-resources</phase>
                        <goals>
                            <goal>exec</goal>
                        </goals>
                        <configuration>
                            <executable>npm</executable>
                            <arguments>
                                <argument>install</argument>
                            </arguments>
                        </configuration>
                    </execution>

                    <!-- Run npm tests in maven test phase -->
                    <execution>
                        <id>npm-test</id>
                        <phase>test</phase>
                        <goals>
                            <goal>exec</goal>
                        </goals>
                        <configuration>
                            <executable>npm</executable>
                            <arguments>
                                <argument>test</argument>
                            </arguments>
                        </configuration>
                    </execution>
                    
                </executions>
            </plugin>
        </plugins>
    </build>
</project>