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Flexible data validation using a duck type interface. For JavaScript and Node.js.

As JavaScript is a loosely typed language, any variable can contain any type of data, and any type of data can be passed as arguments any function. When dealing with data inputs coming from external sources, there is a need to validate the type and contents of the data. Ducktype offers an easy way to validate both basic data types as well as complex structured data types in a flexible way.

var ducktype = require('ducktype');
var person = ducktype({
  name: String,
  age: Number
person.test({name: 'John', age: 34}); // true 
person.test({name: 'Mary'});          // false 



npm install ducktype


bower install ducktype



var ducktype = require('ducktype');


    <script src="ducktype.js" type="text/javascript"></script> 
    <script type="text/javascript">
        // use ducktype... 


Basic types

// use built-in types 
ducktype.number.test(2.3);      // true 
ducktype.number.test('hi');     // false 
ducktype.number.test(true);     // false Date()); // true;        // false 
ducktype.string.test('hello');  // true 
// create a ducktype 
var type = ducktype(Number);
type.test(2.3);                 // true 
type.test('hi');                // false 
type.test(true);                // false 
// create a ducktype with options 
var nullableString = ducktype(String, {nullable: true});
nullableString.test('string');  // true 
nullableString.test(null);      // true 
nullableString.test(2.3);       // false 

Combined types

// combination of types 
var combi = ducktype(Number, String);
combi.test(2.3);   // true 
combi.test('hi');  // true 
combi.test(true);  // false 

Structured objects

// structured object 
var person = ducktype({
  name: String,
  age: Number,
  address: {
    city: String,
    street: String,
    country: String
  email: ducktype(String, {optional: true})
  name: 'John',
  age: 32,
  address: {
    city: 'Sunnyvale, CA 95125',
    street: '701 First Ave.',
    country: 'United States'
}); // true 
  name: 'Mary',
  age: 26
}); // false 

Structured Arrays

// structured arrays 
var numberArray = ducktype([Number]);
numberArray.test([1, 2, 3]);        // true 
numberArray.test([1, 'string', 3]); // false 
// structured object and array 
var family = ducktype({
  name: String,
  age: ducktype(Number, {optional: true}),
  children: [
      name: String,
      age: ducktype(Number, {optional: true})
  name: 'John',
  children: [
      'name': 'Mary',
      'age': 6
      'name': 'Grant'
}); // true 
  name: 'John',
  children: [
      'firstName': 'Mary',
      'age': 6
      'firstName': 'Grant'
}); // false 

Function arguments

var type = ducktype([Number, Number]);
function add (a, b) {
  return a + b;
var sum = add(2, 3);        // ok 
var sum = add(2, 'string'); // will throw a TypeError 

Alternatively, a ducktype wrapper can be created which validates the function arguments against the ducktype:

var add = ducktype([Number, Number]).wrap(function add (a, b) {
  return a + b;
var sum = add(2, 3);        // ok 
var sum = add(2, 'string'); // will throw a TypeError 



A ducktype can be constructed as:

ducktype(type, options)
ducktype(type1, type2, ...)
ducktype(type1, type2, ..., options)


  • type can be:

    • A basic type. Choose from Array, Boolean, Date, Function, Number, Object, RegExp, String, null, undefined.
    • Another ducktype.
    • An object. All properties of the object will be checked. Each property can be a basic type, ducktype, object, or array.
    • An array. An array can have zero, one or multiple elements which can be a basic type, ducktype, object, or array. Providing an array with zero elements will just return a ducktype(Array). Providing an array with one element will return a ducktype which will test each of tested arrays elements against the given type, for example ducktype([Number]).test(1, 2, 3). Providing an array with multiple elements will validate the length of the tested array, and validate each of the array elements one to one against the provided types. This can be used to test the number and type of function arguments. Example: ducktype([Number, String]).test(2, 'str').
  • options is an object with properties:

    • A string name (optional)
    • A boolean optional (optional)
    • A boolean nullable (optional)

A created ducktype has functions:

  • test(object). A function which returns true when provided object matches the ducktype, and false otherwise.
  • validate(object). A function which will throw a TypeError when the provided object does not match the ducktype.
  • wrap(fn). Creates a wrapper function around the provided function, which validates the function arguments against the ducktype. Only applicable for ducktypes containing an array, as the ducktype is tested against an array with the function arguments.

Built-in types

Ducktype comes with a set of built-in types:

  • ducktype.array
  • ducktype.boolean
  • ducktype.function
  • ducktype.number
  • ducktype.object
  • ducktype.regexp
  • ducktype.string
  • ducktype.null
  • ducktype.undefined

The built-in types can be used as:

ducktype.number.test(2.3); // true 
ducktype.string.test(2.3); // false 


To execute tests for the library, run:

npm test


  • Implement common data types like url, phone number, email, postcode, etc.
  • Implement extra options for specific types:
    • Number: integer, min, max, finite, odd, even, positive, negative, nan, ...
    • String: lowercase, uppercase, alpha, alphanumeric, empty, ...
    • Array: length, length.min, length.max, ...
  • Implement a parser accepting a string describing a type in annotations.
  • Implement support to define your own tests for custom types.
  • Implement non-strict type checking: when an object can be converted to the desired type, it is ok. For example a string containing a numeric value can be considered a valid Number, or a string containing an ISO date can be considered a valid Date.


Copyright (C) 2013 Jos de Jong

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.