mock.js

/**
 * Mocks a function with a custom mocked handler for the duration
 * of the execution of a context function
 * @param {Object} parent the parent scope of the function being mocked
 * @param {String} functionName the name of the function being mocked
 * @param {Function} mockedFunction replacement function to use as the mocked function
 * @param {Function} context code to execute while function is mocked
 */
var mock = function(parent, functionName, mockedFunction, context) {
    var nativeFunction = parent[functionName];
    try {
        parent[functionName] = mockedFunction;
        context();
    } finally {
        parent[functionName] = nativeFunction;
    }   
};

/**
 * Usage/Examples
 */

// Given a function and call like:

var sayHello = function(name){
    return 'Hello ' + name;
};

var message = sayHello("John");
console.log(message);

// You can mock it:

mock(window, 'sayHello', 
    // a function to use as the mock
    function(name) {
        return "Goodbye " + name;
    }, 
    // wrap the code to execute while mocked
    function(){
        var message = sayHello("John");
        // assert message = "Goodbye John"
    });
    
// And so, for $.ajax, you can both assert expected values from your code 
// were passed to $.ajax as well as supply your code with fake data.

mock($,'ajax',
    function(opts){
        // assert opts.url = 'response.html'
        // assert opts.data had expected values 
        // then call the success property, passing fake data to calling code
        opts.success('some fake data');
    },
    function(){
        // the code being tested in the context of a mocked $.ajax
        $.ajax({
            url: 'response.html',
            success: function(data) {
                console.log('success:');
                console.log(data);
            }
        });            
    });