0.3.2 • Public • Published


Elegant solution for function overloading in JavaScript.

When you are tired with writing tasteless code to do with arguments, overload2 will MAKE THINGS EASY.

On programming with strongly-typed languages such as C++ and Java, function overloading is frequently employed to make API more convenient to be used. As a weakly-typed language, JavaScript does not support function overloading. At the same time, fortunately, functions in JavaScript may be passed with any arguments that is why overload2 is feasible.

Table of contents


Get Started

Install overload2 firstly.

# Install overload2 and save as a dependency of current package. 
npm install overload2 --save

Open node and run next code:

const overload2 = require('overload2');
// Create a function with overloaded implementations.
var getDay = overload2()
    // Create an overloaded implementation with method overload().
    // The last argument is the implementation function,
    // and the previous is/are used to qualify the datatypes or number of real arguments.
        Date, // consturctor function
        function foo(d) { return d.getDay(); }
        'string', // predefined datatype
        function bar(s) { return new Date(s).getDay(); }
        'number', 'number', 'number',
        function quz(year, month, date) { return new Date(year, month - 1, date).getDay(); }
        2, // length of arguments
        function md(month, date) {
            var d = new Date;
            return d.setMonth(month - 1), d.setDate(date), d.getDay();
        '*', Date, '*',
        function select(some, d, others) {
            return d.getDay();
getDay(new Date);
// foo(d) invoked
// bar(s) invoked
getDay(2000, 1, 1);
// quz(year, month, date) invoked
getDay(12, 1);
// md() invoked
getDay('foo', 'bar', new Date, 'quz');
// select() invoked


According to overload2 , there are different ways to define a datatype.

Constructor Function

overload2 can match any instance with its constructor function, e.g. [0,1] is matched with Array. See another example:

var getDay = overlaod2()
    .overload(Date, function foo(d) { return d.getDay(); })
    .overload(String, function bar(s) { return new Date(s).getDay(); })
getDay(new Date);  // foo() invoked
getDay(new String('2000-1-1'));  // bar() invoked

Customized Datatype

You may create customized datatypes by new overload2.Type(fn), e.g.

// Create a Type object.
var MonthName = new overload2.Type(function(value) {
    var names = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'];
    return names.indexOf(value) >= 0;
// Customized type may be used on overloading.
var getDay = overlaod2()
    .overload(Date, function foo(d) { return d.getDay(); })
    .overload('number', MonthName, 'number', function() {
        return new Date(year, month - 1, date).getDay();
getDay(2000, 'Jan', 1);

Predefined Datatype

Predefined Data Type Remark
overload2.Type.ANY Anything.
overload2.Type.BOOLEAN It must be true or false, anything else including instance of Boolean is unmatched.
overload2.Type.CHAR A string whose length equals 1, e.g. "a"
overload2.Type.NUMBER A number, but NOT instance of Number.
overload2.Type.PLAIN_OBJECT An object whose constructor is Object instead of anything else.
overload2.Type.SCALAR A number, string or boolean, but NOT instance of Number, String or Boolean.
overload2.Type.STRING A string, but NOT instance of String.


  1. Predefined datatypes named Type.BOOLEAN, Type.NUMBER and Type.STRING refers to primitive values with types of boolean, number and string, not their Object-wrapped forms. If Object-wrapped form required, just directly use constructor functions Boolean, Number and String as datatype, see Datatypes: Constructor Function.

  2. Before version 0.1.0, predefined datatypes are appended directly to the overload2 module. To avoid ambiguity in future, predefined datatypes will be appended to overload2.Type. Although the old ones reserved, it's strongly suggested not to use overload2.<PREDEFINED_TYPE_NAME> any longer.

Datatype Alias

ATTENTION: Datatype aliases are CaseSensitive strings.

Alias Corresponding Datetype
? overload2.Type.ANY
any overload2.Type.ANY
boolean overload2.Type.BOOLEAN
char overload2.Type.CHAR
number overload2.Type.NUMBER
object overload2.Type.PLAIN_OBJECT
scalar overload2.Type.SCALAR
string overload2.Type.STRING

Create Datatype With Factory Method

overload2 offers some factory methods to create frequently used datatypes, e.g. enum.

  • overload2.Type.enum(item [, ...])
    Return an enumeration type.

  • overload2.Type.and(type1, type2 [, ...])
    Create a compound type.

  • overload2.Type.or(type1, type2 [, ...])
    Create a compound type.

  • overload2.Type.not(type)
    Create a new type which is complementary to the origin type.

Mutable Parameter

By appending size decorator, we can define mutable parameters. E.g.

var add = overload2()
    .overload('number *', function(numbers) {
        var ret = 0;
        numbers.forEach(function(number) { ret += number; })
        return ret;
    .overload('boolean {2,3}', function(bools) {
        var ret = false;
        for (var i = 0; !ret && i < bools.length; i++) {
            ret = bools[i];
        return ret;
    .overload([ Date, '+' ], function(dates) {
        var date = dates[0];
        for (var i = 1; i < dates.length; i++) {
            if (date < dates[i]) {
                date = dates[i];
        return date;

Size decorators look like repetition in regular expression. Here are some examples for overload param with size decorators:

[ Date, '{2,}']  // takes at least 2 arguments which are instances of Date
'number *'  // takes any number (including zero) of arguments of type number
'*'         // takes any number (including zero) of arguments of any type
'+'         // takes at least one argument
'?'         // takes one argument
'{2}'       // takes 2 arguments
'{2,4}'     // takes 2 to 4 arguments
'{2,}'      // takes at least 2 arguments
'{,4{}'     // takes no more than 4 arguments
// The braces may be omitted, so the following are also valid.

Move Forward

Beyond the basic use, overload2 is also suitable with more complex and large-scale programs. See the class hierarchy shown below:
overload2 hierarchy

The usage of classes in overload2 is explained in the next table:

Class Remark
overload2.OverloadedFunction wrapper of overloaded function, not a function instance itself
overload2.Overload to define overloading implementation
overload2.ParamList to define a parameter list
overload2.Param to define a parameter
overload2.Type wrapper of class (consturctor function), or to customise some datatype

Instances of Type, Param, ParamList and Overload are able to be created independently and be re-used in creating instances of superior class(es).

Here is an example for advanced mode.


overload2(), Create An Overloaded Function

overload2 itself is a function, when invoked, it will return an overloded function instance.

  • <fn> overload2()
    Create a new overloaded function. The function has no implementations before .overload() called.

  • <fn> <fn>.overload( [ <datatype>, ... ] function <implementation> )
    Append an overloading implementation to existing overloaded function.

  • <fn> <fn>.default( function <implementation> )
    Set default implementation function for existing overloaded function.

class overload2.Type

To define a datatype in context of overload2, there are different ways including overload2.Type. And all other datatypes will be converted to instances of overload2.Type before being used.

  • new overload2.Type( function | RegExp <matcher> )
    Here matcher may be a function or RegExp object.

  • private boolean <type>.match( <value> )
    Return true if value matches the datatype, otherwise return false.

class overload2.Param

A Param is made up of a Type and some decorators. Available decorators are:

Decorator Remark
null If argument equals null, it matches the parameter.
undefined If argument equals undefined (the place should be occupied), it matches the parameter.
absent The argument may be absent (optional). See example code for more details.
  • new overload2.Param( string "<alias> <decorator> ..." )
    The alias should be one of alias listed in table Datatype Alias.

  • new overload2.Param( Type | function | string <datatype>, string <decorator(s)> [ , string <decorator(s)> ] )
    Here datatype may be instance of Type, or construtor function, or datatype alias.

  • private boolean <param>.satisfy( <value> )
    To judge if the argument value satisfy the parameter.

  • Param overload2.Param.parse( ? )
    Arguments suitable for new Param() are also suitable for the Param.parse().

class overload2.ParamList

  • new overload2.ParamList( [ Param | Array | String <param> [ , ... ] ] )
    Here param may be an instance of Param, or a string or an array which may used as argument(s) for new Param().

  • private boolean <paramList>.satisfy( Array | Arguments <args> )
    To check arguments with parameters, return true if matched or false if not.

  • ParamList overload2.ParamList.parse( ? )
    Arguments suitable for new ParamList() are also suitable for the ParamList.parse().

class overload2.Overload

  • new overload2.Overload( number , function <implementation> )
    Create an Overload instance by restricting the number of arguments.

  • new overload2.Overload( ParamList, function <implementation> )
    Create an Overload instance bound to specified ParamList.

  • new overload2.Overload( <param> [ , ... ] , function <implementation> )
    Create an Overload instance with optional definitions of Param.

  • new overload2.Overload(function <implementation> )
    Create an Overload instance which will be invoked while arguments length equals 0.

  • Overload overload2.Overload.parse( ? )
    Arguments suitable for new Overload() are also suitable for the Overload.parse().

class overload2.OverloadedFunction

  • new overload2.OverloadedFunction()
    The instance of OverloadedFunction is a wrapper, not a function itself.

  • <wrapper>.exec( ... )
    Run the overloaded function.

  • <wrapper>.apply( <scope>, Array | Arguments <args> )
    Run the overloaded function under specified scope, passing arguments as an array or Arguments instance.

  • <wrapper>.call( <scope> [ , <arg> [ , ... ] ] )
    Run the overloaded function under specified scope, passing arguments one by one.

  • <wrapper>.overload( Overload <overloadInstance> [ , Boolean ] )
    Append an overloading implementation.

  • <wrapper>.overload( ? )
    Append an overloading implementation, arguments suitable for new Overload() are also suitable for the <wrapper>.overload().


  • Basic Usage
    To create overloaded function in simple way.

  • Unit Test
    Another way to understand overload2 is via reading unit-test code. To run the unit test on the local installed module, please:

    # Change to the installing directory of overload2. 
    cd node_modules/overload2
    # To install devDependencies. 
    npm intall
    # Run test. 
    npm test
  • Overloaded Constructor Function
    Overloaded function created by overload2 may also be used as class constructor.

  • Run Overloaded Function Under Specified Scope
    A function created by overload2 may also be invoked by .apply(), .call(), as normal functions do. And, it may also be bound to specified scope with .bind().

  • Mutable Parameters
    Explain how to define mutable parameters.

  • Optional Parameters
    Explain show how to indicate a param which may be absent (that means it is optional), and how to set the default value.

  • Parameter Decorators
    Introduce available decorators used to define a more complex parameter.

  • Advanced Usage
    Use overload2 in complex situations.

Why overload2

There have been dozens of packages devoted to function overloading in JavaScript, and some of them are really not bad, e.g.

So, is overload2 redundant? I donnot know. Each of previous is unsatisfactory more or less, of course overload2 is not perfect either. Maybe future ECMAScript specification will support function overloading. However, until then, I will depend on overload2 while coding in JavaScript.

Honorable Dependents

Welcome to be the first dependent of overload2!



Why postfixed the package name with number 2? Since name "overload" has been occupied, inspired by well-known package "pm2" and "through2", I thought "overload2" is not bad. The most important reason why I choose "overload2" was because 2 /tu:/ is pronounced like tool /tu:l/.


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