an implementation of require() that runs in a browser and downloads code from a server

require.js

// require.js 
// a browser-side implementation of require()
// (c) 2010 Jan Bölsche <[email protected]>
// See my blog post at http://blog.lagomorph.de/2010/11/require-requirements
// This code is licenced under the terms of the GPL (any version you prefer)

// requires JQuery.

var moduleExports = {};  // hash of hashes, stores exported objects of all loaded modules

// loadModule() loads a module from the web server and resolve all
// requirements (by loading more code from the server)
// The exported objects of all loaded modules can be found
// in moduleExports.
// This function returns immediately and has no return value. 
// If the operation succeeds, onLoad is called with the name of the
// module that was just loaded as the first argument and its exports
// as the second argument. onError is called with the name of the module
// in case the file could not be downloaded.
     
// CAVEATS
// TODO: catch and report errors that occur while evaluating a module's code.
// FIXME: modules may be loaded and evaluated more then once.

function loadModule(moduleName, onLoad, onError) {

    // evaluateModule() runs downloaded code and
    // potentially triggers the download of its dependencies.
    // It only succeeds if all dependencies are already loaded.
    // Otherwise it will call loadModule which in turn will
    // call back evaluateModule() once the new code is loaded.
    function evaluateModule(moduleName, code, onSuccess, onError) {
        exports = {};  // will be filled by the module to be evaluated
    
        // The following is the implementation of require()
        // that is visible to downloaded modules
        // while they are being evaluated.   
        
        // If the path specified refers to a module
        // that was already loaded, the method succeeds.
        // Otherwise it triggers the download and fails with 
        // an exception.
        // Once the download is finished, the callback function
        // retries to evaluate the module that originally called
        // require().
        var require = function(path) {
            if (path.substring(0, 2) == "./") {
                path = path.substring(2);
            } 
            if (path in moduleExports) {
                console.log("required module " + path + " is already loaded.");
                return moduleExports[path];
            }
            console.log("required module " + path + " is not loaded yet.");

            loadModule(path, function() {
                    evaluateModule(moduleName, code, onSuccess, onError);
                }, onError
            );
            throw "required module " + path + " not yet loaded (now laoding)";
        }

        // the following line might throw an
        // excpetion due to an unresolved required module
        // in that case we will be called again after
        // that module has been loaded, so we just return.
        eval(code);
    
        // otherwise we get here, store the exports
        // exposed by the new module for future use
        // and report success.
        moduleExports[moduleName] = exports;
        onSuccess();
        return exports;
    }   
    
    if (moduleName in moduleExports) {
        if (onSuccess != undefined) {
            onSuccess(moduleName, moduleExports[moduleName]);
        }
        return;
    }

    console.log("trying to load module " + moduleName + " ...");
    jQuery.ajax({
        dataType: 'text',
        url: '/' + moduleName.toLowerCase() + '.js',
        'success': function(data, textStatus, request) {
            evaluateModule(moduleName, data, 
                function() {
                    if (onLoad != undefined) {
                        onLoad(moduleName, moduleExports[moduleName]);
                    }
                },
                function() {
                    if (onError != undefined) {
                        onError(moduleName);
                    }
                }  
            );
        },
        'error': function(request, textStatus, errorThrown) {
            console.log("failed to download module " + moduleName);
            if (onError != undefined) {
                onError(moduleName);
            }
        }
    });
}