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.
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.
.overload(
Date, // consturctor function
function foo(d) { return d.getDay(); }
)
.overload(
'string', // predefined datatype
function bar(s) { return new Date(s).getDay(); }
)
.overload(
'number', 'number', 'number',
function quz(year, month, date) { return new Date(year, month - 1, date).getDay(); }
)
.overload(
4, // length of arguments
function four() { console.log('too many arguments'); }
)
;
getDay(new Date);
// foo(d) invoked
getDay('2000-1-1');
// bar(s) invoked
getDay(2000, 1, 1);
// quz(year, month, date) invoked
getDay(1, 2, 3, 4);
// four() invoked
According to overload2 , there are different ways to define a datatype.
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
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 Data Type | Remark |
|---|---|
| overload2.ANY | Anything. |
| overload2.BOOLEAN | It must be true or false, anything else including instance of Boolean is unmatched. |
| overload2.CHAR | A string whose length equals 1, e.g. "a" |
| overload2.NUMBER | A number, but NOT instance of Number. |
| overload2.SCALAR | A number, string or boolean, but NOT instance of Number`, StringorBoolean``. |
| overload2.STRING | A string, but NOT instance of String. |
ATTENTION: Datatype aliases are CaseSensitive strings.
| Alias | Corresponding Datetype |
|---|---|
| * | overload2.ANY |
| any | overload2.ANY |
| boolean | overload2.BOOLEAN |
| char | overload2.CHAR |
| number | overload2.NUMBER |
| scalar | overload2.SCALAR |
| string | overload2.STRING |
overload2 offers some factory methods to create frequently used datatypes, e.g. enum.
Beyond the basic use, overload2 is also suitable with more complex and large-scale programs. See the class hierarchy shown below:
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 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.
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.
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. |
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().
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().
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().
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> )
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().
Advanced Usage
Use overload2 in complex situations.
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.
FAQs
Elegant solution for function overloading in JavaScript
We found that overload2 demonstrated a not healthy version release cadence and project activity because the last version was released a year ago. It has 2 open source maintainers collaborating on the project.
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