mimock

Mimock (mini mock) is a small simple mocking library with a low learning curve.

1. Use guide.
   1. Import.
   2. Creating a complete new object type.
   3. Object instrumentation.
   4. Object method wrapping.
      1. Method stubbing wrappers.
      2. Method pass through wrappers.
   5. Layered wrappers.
   6. Wrapping a function.
   7. Wrapping a module.
      1. Modules exporting a function.
      2. Modules exporting an object.
   8. Module wrapping behaviours.

Use guide

Test scenarios are illustrated with mocha/karma it and jasmine/chai style expect assertions.

Import

Import the mockset constructor as follows:

let mimock  = require('mimock');
let mockset = mimock.mockset;

The mockset constructor creates objects which are essentially a set of mocks. When mocking behaviour is introduced, it is done via the mock set object, and this makes it easy to undo.

Creating a complete new object type

let mocks = new mockset();

let play_object = mockset.object_type({
    con: function (params) {
        this.foo_val = params.foo;
        this.bar_val = params.bar;
    },
    methods: {
        foo: function (new_val) {
            let orig_val = this.foo_val;
            if ('0' in arguments)
                this.foo_val = new_val;
            return orig_val;
        },
        bar: function (new_val) {
            let orig_val = this.bar_val;
            if ('0' in arguments)
                this.bar_val = new_val;
            return orig_val;
        },
    },
});

Now, objects of this type may be created as usual:

let ball = new play_object({
    colour: 'blue',
    foo:    'round',
    bar:    'bouncy',
    });

let ball_foo     = ball.foo();
let old_ball_bar = ball.bar('fun');
let ball_bar     = ball.bar();

Constructor and methods are optional.

original state just by calling restore() on it.

Object instrumentation

You can instrument an object method like this (o can be used instead of object and m instead of method):

let mocks = new mockset();
let some_object = get_some_object();
let some_method = mocks.object(some_object).method('some_method');

That changes the some_method call on some_object so that calls to the target method are 'observed' from then on.

The number of times the method has been called (after observation start) is then available by calling some_method.call_count(). The calls themselves can be accessed by calling some_method.calls(), which returns an array of objects (sorted be 'called'), each object having the following keys:

Key	Description
called	A Date object; when the call was made
returned	A Date object; when the call returned
args	An array comprising the arguments the caller passed
retval	The return value, included unless it threw an exception
exception	An exception value, if it threw an exception

A call counting test can be done using a wrap, like this test that checks a method on an object is called twice:

it('is called twice', function () {
    let mocks = new mockset();
    let call_count = 0;
    mocks.object(some_object).method('some_method').wrap(function () {
        call_count++;
    });
    do_the_thing();
    expect(call_count).toBe(2);
    mocks.restore();
});

Instrumention allows you to do it like this however:

it('is called twice', function () {
    let mocks = new mockset();
    let some_method = mocks.object(some_object).method('some_method');
    do_the_thing();
    expect(some_method.call_count()).toBe(2);
    mocks.restore();
});

Instrumentation can be cancelled either by calling some_method.restore(), or mocks.restore() will cancel all wrappers, and anything else, done with mocks.

Object method wrapping

A wrapper is a function that is called instead of an objects usual method. The wrapper can effectively stub the call by returning some value of its choosing to the caller, or invoke the original method (with or without modified arguments, and optionally modifying the return value), or it could throw an exception. Because the wrapper is invoked instead of the original method it can really do anything it wants.

The objects original method is referred to below as the target method.

Method stubbing wrappers

This is the simplest possible type of wrapper; here this target method is replaced with this function that always just returns 30 (w can be used instead of wrap):

let mocks = new mockset();

let some_method = mocks.object(some_object).method('some_method');
let wrap = some_method.wrap(function () {
    return 30;
});

mocks.restore();

Method pass through wrappers

The wrapper is passed an argument (helper in the examples below), providing access to various things, including the original/target function. A helper method called continue causes the target function to be called (with whatever arguments were passed by the caller). The return value is whatever is returned by the original/target. A completely benign wrapper could be applied like this therefore:

let mocks = new mockset();

let some_method = mocks.object(some_object).method('some_method');
some_method.wrap(function (helper) {
    return helper.continue();
});

mocks.restore();

The arguments, conspicuously missing above, are accessible as an array, helper.args, and modifying this array causes different arguments to be passed on to the target method when continue is called.

So, to cause the target method to be invoked with the first argument multipled by ten, you could do this:

let mocks = new mockset();

let some_method = mocks.object(some_object).method('some_method');
some_method.wrap(function (helper) {
    helper.args[0] *= 10;
    return helper.continue();
});

mocks.restore();

Or you could throw an exception if the second argument is equal "dog":

let mocks = new mockset();

let some_method = mocks.object(some_object).method('some_method');
some_method.wrap(function (helper) {
    if (helper.args[0] === 'dog')
        throw new Error('dogs not allowed');
    return helper.continue();
});

mocks.restore();

Modifying the return value is probably fairly obvious, here the return value is multipled by five:

let mocks = new mockset();

let some_method = mocks.object(some_object).method('some_method');
let wrap = some_method.wrap(function (helper) {
    return helper.continue() * 5;
});

mocks.restore();

The wrapper can be cancelled. Calling mocks.restore() will cancel everything created with the mocks object, some_method.unwrap(function) or wrap.restore() will remove just that wrapper function, leaving any other wraps and instrumentation operable, or some_method.restore() will return that method to original service (all wrappers removed and no instrumentation).

Layered wrappers

What if you wrap a method already wrapped?

The answer is, it's wrapped again. The new wrapper is called first (think of it a present you're wrapping in layers of wrapping paper, the latest layer goes on top) then the previously set wrapper, then the real function (assuming both wrappers call helper.continue() of course). If the first (to be called) wrapper modifies the arguments, the second gets the modifications and sees nothing of the originals. If the first returns (as a stub), or throws an exception, the second wrapper will never see the light of day.

Wrapping a function

A function, not on an object, can't be usefully changed as such, if something already has a reference to it there's not much you can do, but it can be substituted:

let mocks = new mockset();

let orig_fun = get_callback();
let some_fun_mock = mocks.fun(orig_fun)

Calling replacement() will provide a replacement function which passes control to the original after instrumentation:

let new_fun = some_fun_mock.replacement();
let fun_wrap = some_fun_mock.wrap(function (helper) {
    return helper.continue();
});
new_fun();
expect(some_fun_mock.call_count()).toBe(1);

mocks.restore();

The call history (when, args, return value, etc) is also available by calling calls().

Instrumentation and all wraps are cancelled by calling some_fun_mock.restore(), that one wrap can be removed with some_fun_mock.unwrap(function) or fun_wrap.restore(), or everything done with mocks can be cancelled with mocks.restore().

Wrapping a module

If you want to wrap a function on an object that another function creates that's a bit of a problem, because you never get your hands on the object to instrument or wrapper it. You can solve this (probably) by wrapping the whole module/library (these examples use real existing modules testob (a test object module) and polylock (a multiple concurrent resource lock module)):

let mocks = new mockset();

let testob_lib = mocks.library('testob');

Once you've done this, first the library is instrumented, so you can check how many times it has been required:

let testob = require('testob');
expect(testob_lib.require_count()).toBe(1);

mocks.restore();

Bits of the module can now be wrapped. Modules can be anything of course, some export a function, and others export an object, often with functions in/on it, though it is perfectly possible for a module to export a number or string too. You must tell mimock which elements of a module's exports you want to mock.

Modules exporting a function

If the module exports a function (such as polylock or PouchDB) do this (note l can be used instead of library):

let mocks = new mockset();

let polylock_lib = mocks.library('polylock');
let root_export = polylock_lib.export();

The 'root' export, polylock's constructor, is now instrumented, and you can wrap it just like functions or object methods:

let polylock = require('polylock');

let locks = new polylock();
expect(root_export.call_count()).toBe(1);
root_export.wrap(function (helper) {
    return helper.continue();
});

Now if you want to wrap methods on the objects constructed you can (here the test_locks methods on all new objects is wrapped):

let polylock = require('polylock');

let test_locks_method;
root_export.wrap(function (helper) {
    let new_obj = helper.continue();
	test_locks_method = mocks.o(new_obj).m('test_locks');
    test_locks_method.w(function (helper) {
        throw new Error('gotcha');
	});
	return new_obj
});

let locks = new polylock();
expect(typeof test_locks_method).not.toBe('undefined');
expect(test_locks_method.call_count()).toBe(0);
try {
    let retval = locks.test_locks({});
    console.log('Return value: '+retval);
} catch (err) {
    console.log('Exception: '+err);
}
expect(test_locks_method.call_count()).toBe(1);

mocks.restore();

Modules exporting an object

If the module exports an object (such as testob) do this:

let mocks = new mockset();

let testob_lib = mocks.library('testob');
let basic_export = testob_lib.export('basic');

The 'basic' export, references the 'basic' element from the testob module exports. It is now (as of the above) instrumented, and you can wrap it:

let testob = require('testob');
let testob_basic = testob.basic;

let basic_object = new testob_basic();
expect(basic_export.call_count()).toBe(1);
basic_export.wrap(function (helper) {
    return helper.continue();
});

Wrapping methods on the objects constructed is the same as above, for modules exporting a function.

Module wrapping behaviours

Any module required before a call to (new mockset()).library(name) is entirely free from any Mimock influence.

Any module required after a call to (new mockset()).library(name) will be affected.

Where a module exports a function, the function is replaced (and instrumented) even if you don't ask for this. The reason this is done is to ensure consistent behaviour as compared with modules that expport an object... generally the principals of engineering here are that the order things are done in should have as little impact as possible (i.e. A then B yields the same result as B then A), and that performing an operation on X should have the same (context allowing) impact as the same operation on Y...

...thus where a module exports an object with functions on/in, those functions can be changed later, substituted within the object, but where a module exports a function, this can't be done... so the function is wrapped regardless so that later a call to lib.e(undefined) will work as well as lib.e('name_of_function').