How to overload functions in javascript?

Question

Classical (non-js) approach to overloading:

function myFunc(){
 //code
}

function myFunc(overloaded){
 //other code
}

Javascript wont let more than one function be defined with the same name. As such, things like this show up:

function myFunc(options){
 if(options["overloaded"]){
  //code
 }
}

Is there a better workaround for function overloading in javascript other than passing an object with the overloads in it?

Passing in overloads can quickly cause a function to become too verbose because each possible overload would then need a conditional statement. Using functions to accomplish the //code inside of those conditional statements can cause tricky situations with scopes.

Answer 1

There are multiple aspects to argument overloading in Javascript:

1. Variable arguments - You can pass different sets of arguments (in both type and quantity) and the function will behave in a way that matches the arguments passed to it.

2. Default arguments - You can define a default value for an argument if it is not passed.

3. Named arguments - Argument order becomes irrelevant and you just name which arguments you want to pass to the function.

Below is a section on each of these categories of argument handling.

Variable Arguments

Because javascript has no type checking on arguments or required qty of arguments, you can just have one implementation of myFunc() that can adapt to what arguments were passed to it by checking the type, presence or quantity of arguments.

jQuery does this all the time. You can make some of the arguments optional or you can branch in your function depending upon what arguments are passed to it.

In implementing these types of overloads, you have several different techniques you can use:

1. You can check for the presence of any given argument by checking to see if the declared argument name value is undefined.
2. You can check the total quantity or arguments with arguments.length.
3. You can check the type of any given argument.
4. For variable numbers of arguments, you can use the arguments pseudo-array to access any given argument with arguments[i].

Here are some examples:

Let's look at jQuery's obj.data() method. It supports four different forms of usage:

obj.data("key");
obj.data("key", value);
obj.data();
obj.data(object);

Each one triggers a different behavior and, without using this dynamic form of overloading, would require four separate functions.

Here's how one can discern between all these options in English and then I'll combine them all in code:

// get the data element associated with a particular key value
obj.data("key");

If the first argument passed to .data() is a string and the second argument is undefined, then the caller must be using this form.

---

// set the value associated with a particular key
obj.data("key", value);

If the second argument is not undefined, then set the value of a particular key.

---

// get all keys/values
obj.data();

If no arguments are passed, then return all keys/values in a returned object.

---

// set all keys/values from the passed in object
obj.data(object);

If the type of the first argument is a plain object, then set all keys/values from that object.

---

Here's how you could combine all of those in one set of javascript logic:

 // method declaration for .data()
 data: function(key, value) {
     if (arguments.length === 0) {
         // .data()
         // no args passed, return all keys/values in an object
     } else if (typeof key === "string") {
         // first arg is a string, look at type of second arg
         if (typeof value !== "undefined") {
             // .data("key", value)
             // set the value for a particular key
         } else {
             // .data("key")
             // retrieve a value for a key
         }
     } else if (typeof key === "object") {
         // .data(object)
         // set all key/value pairs from this object
     } else {
         // unsupported arguments passed
     }
 },

---

The key to this technique is to make sure that all forms of arguments you want to accept are uniquely identifiable and there is never any confusion about which form the caller is using. This generally requires ordering the arguments appropriately and making sure that there is enough uniqueness in the type and position of the arguments that you can always tell which form is being used.

For example, if you have a function that takes three string arguments:

obj.query("firstArg", "secondArg", "thirdArg");

You can easily make the third argument optional and you can easily detect that condition, but you cannot make only the second argument optional because you can't tell which of these the caller means to be passing because there is no way to identify if the second argument is meant to be the second argument or the second argument was omitted so what's in the second argument's spot is actually the third argument:

obj.query("firstArg", "secondArg");
obj.query("firstArg", "thirdArg");

Since all three arguments are the same type, you can't tell the difference between different arguments so you don't know what the caller intended. With this calling style, only the third argument can be optional. If you wanted to omit the second argument, it would have to be passed as null (or some other detectable value) instead and your code would detect that:

obj.query("firstArg", null, "thirdArg");

---

Here's a jQuery example of optional arguments. both arguments are optional and take on default values if not passed:

clone: function( dataAndEvents, deepDataAndEvents ) {
    dataAndEvents = dataAndEvents == null ? false : dataAndEvents;
    deepDataAndEvents = deepDataAndEvents == null ? dataAndEvents : deepDataAndEvents;

    return this.map( function () {
        return jQuery.clone( this, dataAndEvents, deepDataAndEvents );
    });
},

Here's a jQuery example where the argument can be missing or any one of three different types which gives you four different overloads:

html: function( value ) {
    if ( value === undefined ) {
        return this[0] && this[0].nodeType === 1 ?
            this[0].innerHTML.replace(rinlinejQuery, "") :
            null;

    // See if we can take a shortcut and just use innerHTML
    } else if ( typeof value === "string" && !rnoInnerhtml.test( value ) &&
        (jQuery.support.leadingWhitespace || !rleadingWhitespace.test( value )) &&
        !wrapMap[ (rtagName.exec( value ) || ["", ""])[1].toLowerCase() ] ) {

        value = value.replace(rxhtmlTag, "<$1></$2>");

        try {
            for ( var i = 0, l = this.length; i < l; i++ ) {
                // Remove element nodes and prevent memory leaks
                if ( this[i].nodeType === 1 ) {
                    jQuery.cleanData( this[i].getElementsByTagName("*") );
                    this[i].innerHTML = value;
                }
            }

        // If using innerHTML throws an exception, use the fallback method
        } catch(e) {
            this.empty().append( value );
        }

    } else if ( jQuery.isFunction( value ) ) {
        this.each(function(i){
            var self = jQuery( this );

            self.html( value.call(this, i, self.html()) );
        });

    } else {
        this.empty().append( value );
    }

    return this;
},

---

Named Arguments

Other languages (like Python) allow one to pass named arguments as a means of passing only some arguments and making the arguments independent of the order they are passed in. Javascript does not directly support the feature of named arguments. A design pattern that is commonly used in its place is to pass a map of properties/values. This can be done by passing an object with properties and values or in ES6 and above, you could actually pass a Map object itself.

Here's a simple ES5 example:

jQuery's $.ajax() accepts a form of usage where you just pass it a single parameter which is a regular Javascript object with properties and values. Which properties you pass it determine which arguments/options are being passed to the ajax call. Some may be required, many are optional. Since they are properties on an object, there is no specific order. In fact, there are more than 30 different properties that can be passed on that object, only one (the url) is required.

Here's an example:

$.ajax({url: "http://www.example.com/somepath", data: myArgs, dataType: "json"}).then(function(result) {
    // process result here
});

Inside of the $.ajax() implementation, it can then just interrogate which properties were passed on the incoming object and use those as named arguments. This can be done either with for (prop in obj) or by getting all the properties into an array with Object.keys(obj) and then iterating that array.

This technique is used very commonly in Javascript when there are large numbers of arguments and/or many arguments are optional. Note: this puts an onus on the implementating function to make sure that a minimal valid set of arguments is present and to give the caller some debug feedback what is missing if insufficient arguments are passed (probably by throwing an exception with a helpful error message).

In an ES6 environment, it is possible to use destructuring to create default properties/values for the above passed object. This is discussed in more detail in this reference article.

Here's one example from that article:

function selectEntries({ start=0, end=-1, step=1 } = {}) {
    ···
};

Then, you can call this like any of these:

selectEntries({start: 5});
selectEntries({start: 5, end: 10});
selectEntries({start: 5, end: 10, step: 2});
selectEntries({step: 3});
selectEntries();

The arguments you do not list in the function call will pick up their default values from the function declaration.

This creates default properties and values for the start, end and step properties on an object passed to the selectEntries() function.

Default values for function arguments

In ES6, Javascript adds built-in language support for default values for arguments.

For example:

function multiply(a, b = 1) {
  return a*b;
}

multiply(5); // 5

Further description of the ways this can be used here on MDN.

Answer 2

Overloading a function in JavaScript can be done in many ways. All of them involve a single master function that either performs all the processes, or delegates to sub-functions/processes.

One of the most common simple techniques involves a simple switch:

function foo(a, b) {
    switch (arguments.length) {
    case 0:
        //do basic code
        break;
    case 1:
        //do code with `a`
        break;
    case 2:
    default:
        //do code with `a` & `b`
        break;
    }
}

A more elegant technique would be to use an array (or object if you're not making overloads for every argument count):

fooArr = [
    function () {
    },
    function (a) {
    },
    function (a,b) {
    }
];
function foo(a, b) {
    return fooArr[arguments.length](a, b);
}

That previous example isn't very elegant, anyone could modify fooArr, and it would fail if someone passes in more than 2 arguments to foo, so a better form would be to use a module pattern and a few checks:

var foo = (function () {
    var fns;
    fns = [
        function () {
        },
        function (a) {
        },
        function (a, b) {
        }
    ];
    function foo(a, b) {
        var fnIndex;
        fnIndex = arguments.length;
        if (fnIndex > foo.length) {
            fnIndex = foo.length;
        }
        return fns[fnIndex].call(this, a, b);
    }
    return foo;
}());

Of course your overloads might want to use a dynamic number of parameters, so you could use an object for the fns collection.

var foo = (function () {
    var fns;
    fns = {};
    fns[0] = function () {
    };
    fns[1] = function (a) {
    };
    fns[2] = function (a, b) {
    };
    fns.params = function (a, b /*, params */) {
    };
    function foo(a, b) {
        var fnIndex;
        fnIndex = arguments.length;
        if (fnIndex > foo.length) {
            fnIndex = 'params';
        }
        return fns[fnIndex].apply(this, Array.prototype.slice.call(arguments));
    }
    return foo;
}());

---

My personal preference tends to be the switch, although it does bulk up the master function. A common example of where I'd use this technique would be a accessor/mutator method:

function Foo() {} //constructor
Foo.prototype = {
    bar: function (val) {
        switch (arguments.length) {
        case 0:
            return this._bar;
        case 1:
            this._bar = val;
            return this;
        }
    }
}

Answer 3

You cannot do method overloading in strict sense. Not like the way it is supported in java or c#.

The issue is that JavaScript does NOT natively support method overloading. So, if it sees/parses two or more functions with a same names it’ll just consider the last defined function and overwrite the previous ones.

One of the way I think is suitable for most of the case is follows -

Lets say you have method

function foo(x)
{
} 

Instead of overloading method which is not possible in javascript you can define a new method

fooNew(x,y,z)
{
}

and then modify the 1st function as follows -

function foo(x)
{
  if(arguments.length==2)
  {
     return fooNew(arguments[0],  arguments[1]);
  }
} 

If you have many such overloaded method consider using switch than just if-else statements.

(more details) PS: Above link goes to my personal blog that has additional details on this.

Answer 4

I am using a bit different overloading approach based on arguments number. However i believe John Fawcett's approach is also good. Here the example, code based on John Resig's (jQuery's Author) explanations.

// o = existing object, n = function name, f = function.
    function overload(o, n, f){
        var old = o[n];
        o[n] = function(){
            if(f.length == arguments.length){
                return f.apply(this, arguments);
            }
            else if(typeof o == 'function'){
                return old.apply(this, arguments);
            }
        };
    }

usability:

var obj = {};
overload(obj, 'function_name', function(){ /* what we will do if no args passed? */});
overload(obj, 'function_name', function(first){ /* what we will do if 1 arg passed? */});
overload(obj, 'function_name', function(first, second){ /* what we will do if 2 args passed? */});
overload(obj, 'function_name', function(first,second,third){ /* what we will do if 3 args passed? */});
//... etc :)

Answer 5

I tried to develop an elegant solution to this problem described here. And you can find the demo here. The usage looks like this:

var out = def({
    'int': function(a) {
        alert('Here is int '+a);
    },

    'float': function(a) {
        alert('Here is float '+a);
    },

    'string': function(a) {
        alert('Here is string '+a);
    },

    'int,string': function(a, b) {
        alert('Here is an int '+a+' and a string '+b);
    },
    'default': function(obj) {
        alert('Here is some other value '+ obj);
    }

});

out('ten');
out(1);
out(2, 'robot');
out(2.5);
out(true);

The methods used to achieve this:

var def = function(functions, parent) {
 return function() {
    var types = [];
    var args = [];
    eachArg(arguments, function(i, elem) {
        args.push(elem);
        types.push(whatis(elem));
    });
    if(functions.hasOwnProperty(types.join())) {
        return functions[types.join()].apply(parent, args);
    } else {
        if (typeof functions === 'function')
            return functions.apply(parent, args);
        if (functions.hasOwnProperty('default'))
            return functions['default'].apply(parent, args);        
    }
  };
};

var eachArg = function(args, fn) {
 var i = 0;
 while (args.hasOwnProperty(i)) {
    if(fn !== undefined)
        fn(i, args[i]);
    i++;
 }
 return i-1;
};

var whatis = function(val) {

 if(val === undefined)
    return 'undefined';
 if(val === null)
    return 'null';

 var type = typeof val;

 if(type === 'object') {
    if(val.hasOwnProperty('length') && val.hasOwnProperty('push'))
        return 'array';
    if(val.hasOwnProperty('getDate') && val.hasOwnProperty('toLocaleTimeString'))
        return 'date';
    if(val.hasOwnProperty('toExponential'))
        type = 'number';
    if(val.hasOwnProperty('substring') && val.hasOwnProperty('length'))
        return 'string';
 }

 if(type === 'number') {
    if(val.toString().indexOf('.') > 0)
        return 'float';
    else
        return 'int';
 }

 return type;
};

Answer 6

In javascript you can implement the function just once and invoke the function without the parameters myFunc() You then check to see if options is 'undefined'

function myFunc(options){
 if(typeof options != 'undefined'){
  //code
 }
}

Answer 7

https://github.com/jrf0110/leFunc

var getItems = leFunc({
  "string": function(id){
    // Do something
  },
  "string,object": function(id, options){
    // Do something else
  },
  "string,object,function": function(id, options, callback){
    // Do something different
    callback();
  },
  "object,string,function": function(options, message, callback){
    // Do something ca-raaaaazzzy
    callback();
  }
});

getItems("123abc"); // Calls the first function - "string"
getItems("123abc", {poop: true}); // Calls the second function - "string,object"
getItems("123abc", {butt: true}, function(){}); // Calls the third function - "string,object,function"
getItems({butt: true}, "What what?" function(){}); // Calls the fourth function - "object,string,function"

Answer 8

No Problem with Overloading in JS , The pb how to maintain a clean code when overloading function ?

You can use a forward to have clean code, based on two things:

1. Number of arguments (when calling the function).

2. Type of arguments (when calling the function)

     function myFunc(){
         return window['myFunc_'+arguments.length+Array.from(arguments).map((arg)=>typeof arg).join('_')](...arguments);
      }
   
       /** one argument & this argument is string */
     function myFunc_1_string(){
   
     }
      //------------
      /** one argument & this argument is object */
     function myFunc_1_object(){
   
     }
     //----------
     /** two arguments & those arguments are both string */
     function myFunc_2_string_string(){
   
     }
      //--------
     /** Three arguments & those arguments are : id(number),name(string), callback(function) */
     function myFunc_3_number_string_function(){
               let args=arguments;
                 new Person(args[0],args[1]).onReady(args[3]);
     }
   
      //--- And so on ....   
   

Answer 9

How about using a proxy (ES6 Feature)?
I didn't find anywhere mentioning this method of doing it. It might be impractical but it's an interesting way nonetheless.
It's similar to Lua's metatables, where you can "overload" the call operator with the __call metamethod in order to achieve overloading.
In JS, it can be done with the apply method in a Proxy handler. You can check the arguments' existence, types, etc. inside the said method, without having to do it in the actual function.

MDN: proxy apply method

function overloads() {}

overloads.overload1 = (a, b) => {
  return a + b;
}
overloads.overload2 = (a, b, c) => {
  return a + b + c;
}

const overloadedFn = new Proxy(overloads, { // the first arg needs to be an Call-able object
  apply(target, thisArg, args) {
    if (args[2]) {
      return target.overload2(...args);
    }
    return target.overload1(...args);
  }
})

console.log(overloadedFn(1, 2, 3)); // 6
console.log(overloadedFn(1, 2)); // 3

Answer 10

Check this out:

http://www.codeproject.com/Articles/688869/Overloading-JavaScript-Functions

Basically in your class, you number your functions that you want to be overloaded and then with one function call you add function overloading, fast and easy.

Answer 11

Since JavaScript doesn't have function overload options object can be used instead. If there are one or two required arguments, it's better to keep them separate from the options object. Here is an example on how to use options object and populated values to default value in case if value was not passed in options object.

function optionsObjectTest(x, y, opts) {
    opts = opts || {}; // default to an empty options object

    var stringValue = opts.stringValue || "string default value";
    var boolValue = !!opts.boolValue; // coerces value to boolean with a double negation pattern
    var numericValue = opts.numericValue === undefined ? 123 : opts.numericValue;

    return "{x:" + x + ", y:" + y + ", stringValue:'" + stringValue + "', boolValue:" + boolValue + ", numericValue:" + numericValue + "}";

}

here is an example on how to use options object

Answer 12

For this you need to create a function that adds the function to an object, then it will execute depending on the amount of arguments you send to the function:

<script > 
//Main function to add the methods
function addMethod(object, name, fn) {
  var old = object[name];
  object[name] = function(){
    if (fn.length == arguments.length)
      return fn.apply(this, arguments)
    else if (typeof old == 'function')
      return old.apply(this, arguments);
  };
}


  var ninjas = {
   values: ["Dean Edwards", "Sam Stephenson", "Alex Russell"]
};

//Here we declare the first function with no arguments passed
  addMethod(ninjas, "find", function(){
    return this.values;
});

//Second function with one argument
  addMethod(ninjas, "find", function(name){
    var ret = [];
    for (var i = 0; i < this.values.length; i++)
      if (this.values[i].indexOf(name) == 0)
        ret.push(this.values[i]);
    return ret;
  });

//Third function with two arguments
  addMethod(ninjas, "find", function(first, last){
    var ret = [];
    for (var i = 0; i < this.values.length; i++)
      if (this.values[i] == (first + " " + last))
        ret.push(this.values[i]);
    return ret;
  });


//Now you can do:
ninjas.find();
ninjas.find("Sam");
ninjas.find("Dean", "Edwards")
</script>

Answer 13

How about using spread operator as a parameter? The same block can be called with Multiple parameters. All the parameters are added into an array and inside the method you can loop in based on the length.

    function mName(...opt){
        console.log(opt); 
    }
    mName(1,2,3,4); //[1,2,3,4]
    mName(1,2,3); //[1,2,3]

Answer 14

I like to add sub functions within a parent function to achieve the ability to differentiate between argument groups for the same functionality.

var doSomething = function() {
    var foo;
    var bar;
};

doSomething.withArgSet1 = function(arg0, arg1) {
    var obj = new doSomething();
    // do something the first way
    return obj;
};

doSomething.withArgSet2 = function(arg2, arg3) {
    var obj = new doSomething();
    // do something the second way
    return obj;
};

Answer 15

What you are trying to achieve is best done using the function's local arguments variable.

function foo() {
    if (arguments.length === 0) {
        //do something
    }
    if (arguments.length === 1) {
        //do something else
    }
}

foo(); //do something
foo('one'); //do something else

You can find a better explanation of how this works here.

Answer 16

(() => {
  //array that store functions
    var Funcs = []
     /**
     * @param {function} f overload function
     * @param {string} fname overload function name
   * @param {parameters} vtypes function parameters type descriptor (number,string,object....etc
     */
    overloadFunction = function(f, fname, ...vtypes) {
        var k,l, n = false;
        if (!Funcs.hasOwnProperty(fname)) Funcs[fname] = [];
        Funcs[fname].push([f, vtypes?vtypes: 0 ]);
        window[fname] = function() {
            for (k = 0; k < Funcs[fname].length; k++)
                if (arguments.length == Funcs[fname][k][0].length) {
                    n=true;
                    if (Funcs[fname][k][1]!=0)
                    for(i=0;i<arguments.length;i++)
                    {
                        if(typeof arguments[i]!=Funcs[fname][k][1][i])
                        {
                            n=false;
                        }
                    }
                    if(n) return Funcs[fname][k][0].apply(this, arguments);
                }
        }
    }
})();

//First sum function definition with parameter type descriptors
overloadFunction(function(a,b){return a+b},"sum","number","number")
//Second sum function definition with parameter with parameter type descriptors
overloadFunction(function(a,b){return a+" "+b},"sum","string","string")
//Third sum function definition (not need parameter type descriptors,because no other functions with the same number of parameters
overloadFunction(function(a,b,c){return a+b+c},"sum")

//call first function
console.log(sum(4,2));//return 6
//call second function
console.log(sum("4","2"));//return "4 2"
//call third function
console.log(sum(3,2,5));//return 10
//ETC...

Answer 17

One nontrivial concern with overloaded functions is that sometimes the parameter positions vary.

For example, here is a function in Clojure where from moves the 2nd position to the 3rd, depending on the arity. This is not an uncommon practice.

(into)(into to)(into to from)(into to xform from)

So when you take a function and try to define the parameters outright it communicates one case well, but not the others:

function into(to, xform, from){ //not necessarily!
  if (arguments.length === 2) { 
     from = xform; //an unclear, avoidable practice
  }
}

Far clearer with destructuring:

function into(...args){
   switch(args.length){
     case 0: {
       //return ??
     }
     case 1: {
       const [to] = args;
       //return ??
     }
     case 2: {
       const [to, from] = args;
       //return ??
     }
     case 3: {
       const [to, xform, from] = args;
       //return ??
     }
   }
}

And sometimes related but different implementations return different types.

(map f)(map f coll)(map f c1 c2)(map f c1 c2 c3)(map f c1 c2 c3 & colls)

The artiy 1 map returns a transducer while all other arities return a lazy sequence. Also, not uncommon.

An overloaded function is actually just separate functions which all reside at the same named address and whose arity decides how to route the function call. Sharing the name is better than trying to provide an alternate name for the odd one out.

(mapt f)
(map f coll)(map f c1 c2)(map f c1 c2 c3)(map f c1 c2 c3 & colls)

Related is related. So let the overloads share a name! And why should this sort of thing not be allowed in JavaScript.

What I actually do is define the arities as separate functions and unite them with a higher-order function, like so:

function overload(...fs) {
  const fallback = fs[fs.length - 1];
  return function (...args) {
    const f = fs[args.length] || (args.length >= fs.length ? fallback : null);
    return f.apply(this, args);
  };
}

const into = overload(null, into1, into2, into3); //null since no arity 0