javascript optional first argument in function

Question

I have a typical javascript function with some parameters  my function   function content  options    action     if I call the function as such    my function  options    the argument  options  is passed as  content   how do i go around this so I can either have both arguments passed or just one   thank you

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my function   function hash       use hash options and hash content         and then call   my function    options  options     my function    options  options  content  content

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Call like this  my function      options

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You can pass all your optional arguments in an object as the first argument  The second argument is your callback  Now you can accept as many arguments as you want in your first argument object  and make it optional like so   function my func op  cb        var options    typeof arguments 0       function    arguments 0                callback    typeof arguments 0       function    arguments 1    arguments 0        console log options       console log callback       If you call it without passing the options argument  it will default to an empty object   my func function           gt  options       gt  callback  function        If you call it with the options argument you get all your params   my func  param1   param1   param2   param2    function           gt  options   param1   param1   param2   param2     gt  callback  function        This could obviously be tweaked to work with more arguments than two  but it get s more confusing  If you can just use an object as your first argument then you can pass an unlimited amount of arguments using that object  If you absolutely need more optional arguments  e g  my func arg1  arg2  arg3       arg10  fn    then I would suggest using a library like ArgueJS  I have not personally used it  but it looks promising

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You will get the un passed argument value as undefined   But in your case you have to pass at least null value in the first argument   Or you have to change the method definition like  my function   function options  content    action

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There is a nice read on Default parameters in ES6 on the MDN website here   In ES6 you can now do the following   secondDefaultValue    indirectSecondDefaultValue    function MyObject  param1    firstDefaultValue   param2   secondDefaultValue        this first   param1      this second   param2      You can use this also as follows   var object   new MyObject  undefined  options      Which will set default value  firstDefaultValue  for first param1 and your options for second param2   Here a demonstration in a fiddle

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With ES6   function test a  b   3       console log a       b      test 1           Output  1 3 test 1  2        Output  1 2

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There are 2 ways to do this   1   function something options        var timeToLive   options timeToLive    200     default to 200 if not set             2   function something timeToDie     timeToLive          var timeToLive   arguments 1     200     default to 200 if not set            In 1   options is a JS object with what ever attributes are required  This is easier to maintain and extend   In 2   the function signature is clear to read and understand that a second argument can be provided  I ve seen this style used in Mozilla code and documentation

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Like this   my function  null  options     for options only my function  content     for content only my function  content  options     for both

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You have to decide as which parameter you want to treat a single argument  You cannot treat it as both  content and options   I see two possibilities    Either change the order of your arguments  i e  function options  content  Check whether options is defined   function content  options        if typeof options      undefined             options   content          content   null              action     But then you have to document properly  what happens if you only pass one argument to the function  as this is not immediately clear by looking at the signature

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You could also put a check in the action like  options    options   content   options that sets options to the first argument if no second was passed in  and then you just set content to null  or ignore it  however you want to check that

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Just to kick a long-dead horse  because I ve had to implement an optional argument in the middle of two or more required arguments  Use the arguments array and use the last one as the required non-optional argument   my function       var options   arguments argument length - 1     var content   arguments length  gt  1   arguments 0    null

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Or you also can differentiate by what type of content you got  Options used to be an object the content is used to be a string  so you could say   if   typeof content      object        options   content    content   null      Or if you are confused with renaming  you can use the arguments array which can be more straightforward   if   arguments length     1       options   arguments 0     content   null

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I ve created a simple library for handling optional arguments with JavaScript functions  see https   github com ovatto argShim  The library is developed with Node js in mind but should be easily ported to work with e g  browsers   Example   var argShim   require  argShim    var defaultOptions       myOption  123     var my function   argShim        optional  String         optional  Object  default defaultOptions       function content  options        console log  content    content   options    options          my function    my function  str    my function  myOption 42    my function  str    myOption 42      Output   content  undefined options    myOption  123   content  str options    myOption  123   content  undefined options    myOption  42   content  str options    myOption  42     The main target for the library are module interfaces where you need to be able to handle different invocations of exported module functions

[javascript] javascript: optional first argument in function

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