Helper for JavaScript duck typing checks
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A JavaScript duck typing checker module

Mission statement

When operating on an object in JavaScript, one often wants to check for the object's capabilities instead of its origin. This means checking for it having a couple of specified properties, instead of doing something like obj instanceof Foo - sometimes there isn't even a Foo constructor to check for.

Of course this could be taken really seriously, leading to writing veritable unit tests for the poor object that wants to quack for us. A middle ground is checking for existence and type of the properties that make an object able to quack like the duck of your choice.

This module is meant to make it easy to set up such a quacking standard, and check objects for conformance.

Using ActsAs

Obviously, you need to include the file ActsAs.js.

Then, create a signature for the characteristics you are after. A signaure is an object with a defined characteristic for each property you are interested in validating.

// an entity with name and age, is insane and can perform lambada
var insaneLambadaEnabled = {
  name: "type-string"
  , age: "type-number"
  , lambada: "type-function"
  , insane: "value-true"

Check for conformance with this signature by calling Acts.As(signature, obj):

function foo(obj) {
  if (!Acts.As(insaneLambadaEnabled, obj)) return; // failing early. no lambada for you, obj!
  console.log("I hereby announce our next contestant, an insane lambada dancer named ""!");

// A dedicated lambadaist constructor
function Lambadaist(name, age, insane) { = name;
  this.age = age;
  this.insane = !!insane;
Lambadaist.prototype.lambada = function () {
  return console.log(": NAAA NA-NA-NA-NAAA...");

// Middle-aged and insane object named Ljörgen has lambada chops
foo({name:"Ljörgen", age: 47, insane: true, lambada: function () {console.log(": Naaa na-na-na-naaa... (ducky)");}});
// A card carrying Lambdaist obj has lambada chops..
var lambadaist = new Lambadaist("Kalenderhielm", 56, true);
foo(lambadaist);  // ok
// .. after being on medication for a while, no longer applicable for foo()
lambadaist.insane = false;
foo(lambadaist); // throws

The result is true, or false. In the case of false, you might be interested in what went wrong. Failure info (first point of failure), is available via

Here's how to use conformance failure info for debugging or whatever:

function bar(obj) {
  if (!Acts.As(insaneLambadaEnabled, obj)) {throw Error(;}

// will throw with msg "prop 'insane' was not value-true, but: string (TRUE)"
bar({name:"Ljörgen", age: 47, insane: "TRUE", lambada: function () {console.log(": Naaa na-na-na-naaa... (ducky)");}});

One way to resue a signature is of course to store it in a variable or a property. An alternate route is to create a tester function, like this:

var isInsaneLambadaEnabled = Acts.As.BuildTest({
  name: "type-string"
  , age: "type-number"
  , lambada: "type-function"
  , insane: "value-true"

and the usage is like:

for (var idx=0, len=arr.length, obj; idx<len; ++idx) {
  obj = arr[idx];
  if (!isInsaneLambadaEnabled(obj)) {
    console.error("obj %s is not fit for insane lambada: %s", idx,;

The mismatch info message is available as a method on the test function as well as via the "static"


Out of the box, a signature explicitly supports the following property characteristics:


"type-boolean"      // any boolean will do


"value-true"        // property must be === true
"value-false"       // property must be === false
"value-null"        // property must be === null
"value-undefined"   // property must not be defined

Constrained values



Foo     // property must be instanceof Foo

The constructor validating thing is demonstrated in the logGrunt.signature below. We also see how one could go about validating function parameters in general:

function logGrunt(ape, what) {
  if (!Acts.As(logGrunt.signature, {"gruntee": ape, "what": what})) {throw Error(;}
logGrunt.signature = {
  "gruntee": Ape
  , "what": "non-blank-string"
function Ape(name) { = name;
Ape.prototype.grunt = function (what) {return " grunts: '"+what+"'";};

logGrunt(new Ape("Ljörgen"), "fool.."); // ok
logGrunt("Ljörgen", "fool.."); // throws

Custom validations are coming up when I decided on a nice API for it...


As shown above, a nice place for signatures can be on a function that is meant to operate on just a certain breed of objects.

Happy ducktyping!