Handling optional parameters in javascript

Question

I have a static javascript function that can take 1  2 or 3 parameters   function getData id  parameters  callback    parameters  associative array  and callback  function  are optional   I know I can always test if a given parameter is undefined  but how would I know if what was passed was the parameter or the callback   What s the best way of doing this     Examples of what could be passed in   1   getData  offers      2   var array   new Array    array  type    lalal   getData  offers  array     3   var foo   function         getData  offers  foo     4   getData  offers  array foo

User · Accepted Answer

You can know how many arguments were passed to your function and you can check if your second argument is a function or not:

function getData (id, parameters, callback) {
  if (arguments.length == 2) { // if only two arguments were supplied
    if (Object.prototype.toString.call(parameters) == "[object Function]") {
      callback = parameters; 
    }
  }
  //...
}

You can also use the arguments object in this way:

function getData (/*id, parameters, callback*/) {
  var id = arguments[0], parameters, callback;

  if (arguments.length == 2) { // only two arguments supplied
    if (Object.prototype.toString.call(arguments[1]) == "[object Function]") {
      callback = arguments[1]; // if is a function, set as 'callback'
    } else {
      parameters = arguments[1]; // if not a function, set as 'parameters'
    }
  } else if (arguments.length == 3) { // three arguments supplied
      parameters = arguments[1];
      callback = arguments[2];
  }
  //...
}

If you are interested, give a look to this article by John Resig, about a technique to simulate method overloading on JavaScript.

User · Answer

You can use the arguments object property inside the function

User · Answer

Can you override the function   Will this not work   function doSomething id    function doSomething id parameters    function doSomething id parameters callback

User · Answer

So use the typeof operator to determine if the second parameter is an Array or function   This can give some suggestions  http   www planetpdf com developer article asp ContentID testing for object types in ja  I am not certain if this is work or homework  so I don t want to give you the answer at the moment  but the typeof will help you determine it

User · Answer

If your problem is only with function overloading  you need to check if  parameters  parameter is  parameters  and not  callback    i would recommend you don t bother about argument type and use this approach  The idea is simple - use literal objects to combine your parameters     function getData id  opt       var data   voodooMagic id  opt parameters       if  opt callback  undefined        opt callback call data       return data              getData 5   parameters   1 2 3   callback       function   for  i 0 i lt  1 i-- alert  FAIL

User · Answer

I know this is a pretty old question  but I dealt with this recently  Let me know what you think of this solution   I created a utility that lets me strongly type arguments and let them be optional  You basically wrap your function in a proxy  If you skip an argument  it s undefined  It may get quirky if you have multiple optional arguments with the same type right next to each other   There are options to pass functions instead of types to do custom argument checks  as well as specifying default values for each parameter    This is what the implementation looks like   function displayOverlay   message  timeout  callback        return arrangeArgs arguments  String  Number  Function       function message  timeout  callback                 your code                     For clarity  here is what is going on   function displayOverlay   message  timeout  callback          arrangeArgs is the proxy   return arrangeArgs               first pass in the original arguments            arguments                then pass in the type for each argument            String   Number   Function                lastly  pass in your function and the proxy will do the rest             function message  timeout  callback                    debug output of each argument to verify it s working              console log  message   message   timeout   timeout   callback   callback                        your code                                       You can view the arrangeArgs proxy code in my GitHub repository here   https   github com joelvh Sysmo js blob master sysmo js  Here is the utility function with some comments copied from the repository              Overview               Strongly type a function s arguments to allow for any arguments to be optional         Other resources     http   ejohn org blog javascript-method-overloading              Example implementation                 all args are optional    will display overlay with default settings    var displayOverlay   function          return Sysmo optionalArgs arguments                 String   Number  false  0   Function                 function message  timeout  callback                    var overlay   new Overlay message                   overlay timeout   timeout                  overlay display  onDisplayed  callback                                       Example function call                 the window alert   function is the callback  message and timeout are not defined     displayOverlay alert            displays the overlay after 500 miliseconds  then alerts    message is not defined     displayOverlay 500  alert               Setup               arguments   the original arguments to the function defined in your javascript API     config   describe the argument type     - Class - specify the type  e g  String  Number  Function  Array       -  Class function  boolean  default  - pass an array where the first value is a class or a function                                               The  boolean  indicates if the first value should be treated as a function                                             The  default  is an optional default value to use instead of undefined          arrangeArgs  function     arguments  config1    config2    callback           config format   String  false        Number  false  0    Function  false  function          config doesn t need a default value      config can also be classes instead of an array if not required and no default value     var configs   Sysmo makeArray arguments         values   Sysmo makeArray configs shift           callback   configs pop          args             done   function               add the proper number of arguments before adding remaining values         if   args length              args   Array configs length                       fire callback with args and remaining values concatenated         return callback apply null  args concat values                  if there are not values to process  just fire callback   if   values length        return done             loop through configs to create more easily readable objects   for  var i   0  i  lt  configs length  i           var config   configs i          make sure there s a value     if  values length             type or validator function       var fn   config 0     config              if config 1  is true  use fn as validator               otherwise create a validator from a closure to preserve fn for later use           validate    config 1     fn   function value                return value constructor     fn                        see if arg value matches config       if  validate values 0              args push values shift             continue                       add a default value if there is no value in the original args       or if the type didn t match     args push config 2           return done

User · Answer

Er - that would imply that you are invoking your function with arguments which aren t in the proper order    which I would not recommend   I would recommend instead feeding an object to your function like so   function getData  props         props   props            props params   props params            props id   props id    1      props callback   props callback    function          alert  props callback       getData        id  3      callback  function    alert  hi             Benefits     you don t have to account for argument order you don t have to do type checking it s easier to define default values because no type checking is necessary less headaches  imagine if you added a fourth argument  you d have to update your type checking every single time  and what if the fourth or consecutive are also functions    Drawbacks     time to refactor code   If you have no choice  you could use a function to detect whether an object is indeed a function   see last example     Note  This is the proper way to detect a function   function isFunction obj        return Object prototype toString call obj        object Function       isFunction  function

User · Answer

This I guess may be self explanatory example   function clickOn elem   bubble  cancelable          var bubble        arguments length  gt  1     arguments 1    true      var cancelable    arguments length    3    arguments 2    true       var cle   document createEvent  MouseEvent        cle initEvent  click   bubble  cancelable       elem dispatchEvent cle

User · Answer

I recommend you to use ArgueJS   You can just type your function this way   function getData      arguments       id  String  parameters   Object   callback   Function          and now access your arguments by arguments id                arguments parameters and arguments callback     I considered by your examples that you want your id parameter to be a string  right  Now  getData is requiring a String id and is accepting the optionals Object parameters and Function callback  All the use cases you posted will work as expected

User · Answer

You should check type of received parameters  Maybe you should use arguments array since second parameter can sometimes be  parameters  and sometimes  callback  and naming it parameters might be misleading

User · Answer

Are you saying you can have calls like these  getData id  parameters   getData id  callback    In this case you can t obviously rely on position and you have to rely on analysing the type  getType   and then if necessary getTypeName    Check if the parameter in question is an array or a function

User · Answer

I think you want to use typeof   here   function f id  parameters  callback      console log typeof parameters      typeof callback       f  hi     a   boo    f     prints  object function  f  hi   f    a   boo       prints  function object

[javascript] Handling optional parameters in javascript

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