require.js

// ==================================================================
// browser code in development
 
// locked into on of these options
//
//  1) using a special XHR synchronous script loader and    
//      a) seeing poor error messages with strange line numbering
//         not corresponding with source code line numbering or
//      b) using a JavaScript interpreter written in JavaScript
//         to execute the code retrieved by the XHR. Line numbers
//         would then correspond to source code line numbers.
//         (Seems like an extreme solution.)
//
//  2) edit-compile-load-test cycle using the code below. Also
//     strange numbering in error messages that does not match
//     source files. Tools could make compile automatic but that
//     means tools are required and that is not the case in 
//     the current browser scripting world.
 
 
// ==================================================================
// compiled for production in browser
 
//
// library.js
//
var require = (function() {
 
    // memoized export objects
    var exportsObjects = {}
 
    // don't want outsider redefining "require" and don't want
    // to use arguments.callee so name the function here.
    var require = function(name) {
        if (exportsObjects.hasOwnProperty(name)) {
            return exportsObjects[name];
        }
        var exports = {};
        // memoize before executing module for cyclic dependencies
        exportsObjects[name] = exports;
        modules[name](require, exports);
        return exports;
    };
 
    return require;
})();
 
var run = function(name) {
    require(name); // doesn't return exports
};
 
var modules = {};
 
//
// compiledModules.js
//
modules["math"] = function(require, exports) {
    exports.add = function() {
        var sum = 0, i = 0, args = arguments, l = args.length;
        while (i < l) {
            sum += args[i++];
        }
        return sum;
    };
};
 
modules["increment"] = function(require, exports) {
    var add = require('math').add;
    exports.increment = function(val) {
        add(val, 1);
    };
};
 
modules["program"] = function(require, exports) {
    var inc = require('increment').increment;
    var a = 1;
    inc(a); // 2
};
 
//
// html in document head
//
<script src="library.js" type="text/javascript"></script>
<script src="compiledModules.js" type="text/javascript"></script>
<script type="text/javascript">
    // You might not use use the window.onload property
    // but rather addEventListener/attachEvent.
    window.onload = function() {
        run("program");
    };
</script>