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decent programming advice

written by ben cherry

Spying Constructors in JavaScript

When writing unit-tests for code, a common technique is spying, where you set expectations on a method’s invocation, run some code, and verify that the method was invoked as expected. This is pretty straightforward. Here’s a simple example using JsMockito:

function foo(a) { return a; }
foo = spy(foo);
verify(foo)(1); // verified!
verify(foo)(2); // never run

Here, we’re spying on the foo method, and checking that it was invoked at least once with the parameter 1, and once with the parameter 2. As it turns out, this spy method does not work well with JavaScript constructors, in JsMockito, Jasmine, or many other testing frameworks. The basic problem is that the prototype is not transferred appropriately, so code like this will fail:

function Foo(a) {
	this.a = a;
Foo.prototype = {
	bar: function () {

var f = new Foo(1);; // 1

Foo = spy(Foo);
var g = new Foo(1);; // error

verify(Foo)(1); // not reached

It turns out it’s really easy to write a constructor-safe spying function, and it doesn’t even take very many lines of code.

function spy(F) {
	function G() {
		var args =;
		F.apply(this, args);

	G.prototype = F.prototype;
	G.calls = [];

	return G;

This spy function works just like the one in JsMockito, but it doesn’t fail with constructors. For completeness, here’s an implementation of a simple verify function:

function verify(F) {
	return function () {
		var args =,
			count = 0,

		for (i = 0; i < F.calls.length; i += 1) {
			call = F.calls[i];
			matched = true;
			for (j = 0; j < args.length; j += 1) {
				if (args[j] !== call[j]) {
					matched = false;
			if (matched) {
				count += 1;

		return count > 0;

It would be easy to extend this verify implementation to allow more types of verify like .once() or .never(), working off the count variable.

And that’s it! Here’s an example of code that will work with this spy implementation:

function Foo(name, id) { = name; = id;

Foo.prototype = {
	log: function () {
		console.log("Foo %o:%o",,;

var f = new Foo("test", 1);

Foo = spy(Foo);

var f2 = new Foo("spied", 2);

console.log("verify Foo(\"spied\", 2): %o", verify(Foo)("spied", 2));
console.log("verify Foo(\"something\", 2): %o", verify(Foo)("something", 2));

var baz = {
	spam: function (a) {
		console.log("calling baz.spam(%o), this.other=%o", a, this.other);
	other: 10

baz.spam = spy(baz.spam);

console.log("verify baz.spam(1)", verify(baz.spam)(1));
console.log("verify baz.spam(2)", verify(baz.spam)(2));

The other neat thing is that, so long as you’re not trapping stale references to the original constructor function before it got spied, JavaScript’s instanceof operator should work just fine:

function F() {}
F = spy(F)
new F() instanceof F; // true

You can find the complete code (and a bit more) for this excercise at I hope this was informative. I think I’ll probably end up either contributing a patch to JsMockito with this, or building my own bare-bones set of mocking/spying functions for use with QUnit.

___P.S. It's been some time since I've updated, but I'm hoping this will be the first of many new, interesting JavaScript posts inspired by the work I'm doing at Twitter with @bs, @hoverbird, @ded, and @dsa.___

filed under javascript and testing