Very simple mock library

gistfile1.js

/**
 * Note: Has hard-coded dependency on a CommonJS Unit Test 1.0 assertion module,
 * but doesn't  have to.
 */
function Mock(intf) {
    var expectations, failures, expect = {};

    for (var p in intf) {
        if (intf.hasOwnProperty(p) && typeof intf[p] === "function") {
            (function ($p) {
                this[$p] = function () {
                    var args = [].slice.call(arguments),
                        expectation = expectations.shift();
                    
                    try {
                        assert.ok(expectation,
                            "Unexpected function call");
                        assert.equal(expectation.name, $p,
                            "Unmatched function call");
                        for (var i = 0; i < args.length; ++i) {
                            assert.deepEqual(expectation.args[i], args[i],
                                "Unmatched argument at index " + i);
                        }
                        return expectation.action && expectation.action();
                    } catch (e) {
                        failures.push(e.message ? e.message : e);
                    }
                };
                
                expect[$p] = function () {
                    var expectation = {
                        name: $p,
                        args: [].slice.call(arguments)
                    };
                    
                    expectations.push(expectation);
                    
                    return {
                        thenReturn: function (ret) {
                            expectation.action = function () {
                                return ret;
                            };
                        },
                        thenDo: function (fn) {
                            expectation.action = fn;
                            return this;
                        }
                    };
                };
            }).call(this, p);
        }
    }
    
    this.reset = function () {
        expectations = [];
        failures = [];
        return this;
    };

    this.expect = function () {
        return expect;
    };

    this.verify = function () {
        if (failures.length) {
            assert.ok(false, failures.join());
        }
        return this;
    };
    
    this.reset();
}

Usage:

var mock = new Mock({
    doThis: function () {},
    doThat: function () {}
});

mock.expect().doThis(42);
mock.expect().doThat("abc", "def").thenReturn("abcdef");

// Do something that uses the Mock instance

mock.verify();