Is there a way to provide named parameters in a function call in JavaScript

Question

I find the named parameters feature in C  quite useful in some cases   calculateBMI 70  height  175     What can I use if I want this in JavaScript      What I don   t want is this   myFunction   param1  70  param2  175      function myFunction params        Check if params is an object      Check if the parameters I need are non-null      Blah blah     That approach I   ve already used  Is there another way   I   m okay using any library to do this

User · Answer

Coming from Python this bugged me  I wrote a simple wrapper Proxy for node that will accept both positional and keyword objects    https   github com vinces1979 node-def blob master README md

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Contrary to what is commonly believed  named parameters can be implemented in standard  old-school JavaScript  for boolean parameters only  by means of a simple  neat coding convention  as shown below   function f p1 true  p2 false               f    p1   false     p2   true      call f p1 false  p2 true    Caveats    Ordering of arguments must be preserved - but the pattern is still useful  since it makes it obvious which actual argument is meant for which formal parameter without having to grep for the function signature or use an IDE  This only works for booleans  However  I m sure a similar pattern could be developed for other types using JavaScript s unique type coercion semantics

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No - the object approach is JavaScript s answer to this  There is no problem with this provided your function expects an object rather than separate params

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There is another way   If you re passing an object by reference  that object s properties will appear in the function s local scope  I know this works for Safari  haven t checked other browsers  and I don t know if this feature has a name  but the below example illustrates its use   Although in practice I don t think that this offers any functional value beyond the technique you re already using  it s a little cleaner semantically   And it still requires passing a object reference or an object literal     function sum   a a  b b         console log a     b       if a  undefined  a 0      if b  undefined  b 0      return  a b         will work  returns 9 and 3 respectively  console log sum  a 4 b 5     console log sum  a 3         will not work  returns 0  console log sum 4 5    console log sum 4

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Calling function f with named parameters passed as the object   o    height  1  width  5         is basically calling its composition f    g o   where I am using the spread syntax and g is a  binding  map connecting the object values with their parameter positions    The binding map is precisely the missing ingredient  that can be represented by the array of its keys      map  height  to the first and  width  to the second param binding     height    width       take binding and arg object and return aray of args withNamed    bnd  o    gt  bnd map param   gt  o param       call f with named args via binding f    withNamed binding   hight  1  width  5      Note the three decoupled ingredients  the function  the object with named arguments and the binding  This decoupling allows for a lot of flexibility to use this construct  where the binding can be arbitrarily customized in function s definition and arbitrarily extended at the function call time    For instance  you may want to abbreviate height and width as h and w inside your function s definition  to make it shorter and cleaner  while you still want to call it with full names for clarity      use short params f    h  w    gt          modify f to be called with named args ff   o   gt  f    withNamed   height    width    o       now call with real more descriptive names ff  height  1  width  5     This flexibility is also more useful for functional programming  where functions can be arbitrarily transformed with their original param names getting lost

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Trying Node-6 4 0   process versions v8    5 0 71 60   and Node Chakracore-v7 0 0-pre8 and then Chrome-52  V8 5 2 361 49   I ve noticed that named parameters are almost implemented  but that order has still precedence  I can t find what the ECMA standard says    gt function f a 1  b 2   console log  a   a    b   b      a b        gt  f   a 1   b 2   3  gt  f a 5  a 5   b 2   7  gt  f a 7  b 10  a 7   b 10   17   But order is required   Is it the standard behaviour    gt  f b 10  a 10   b 2   12

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Another way would be to use attributes of a suitable object  e g  like so   function plus a b    return a b      Plus     a  function x    return   b  function y    return plus x y                 b  function y    return   a  function x    return plus x y         sum   Plus a 3  b 5     Of course for this made up example it is somewhat meaningless  But in cases where the function looks like  do something some connection handle  some context parameter  some value    it might be more useful  It also could be combined with  parameterfy  idea to create such an object out of an existing function in a generic way  That is for each parameter it would create a member that can evaluate to a partial evaluated version of the function   This idea is of course related to Sch  nfinkeling aka Currying

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If you want to make it clear what each of the parameters are  rather than just calling   someFunction 70  115     why not do the following  var width   70  height   115  someFunction width  height     sure  it s an extra line of code  but it wins on readability

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ES2015 and later  In ES2015  parameter destructuring can be used to simulate named parameters  It would require the caller to pass an object  but you can avoid all of the checks inside the function if you also use default parameters   myFunction   param1   70  param2   175     function myFunction  param1  param2               function body          Or with defaults   function myFunc     name    Default user     age    N A                  function body          ES5  There is a way to come close to what you want  but it is based on the output of Function prototype toString  ES5   which is implementation dependent to some degree  so it might not be cross-browser compatible   The idea is to parse the parameter names from the string representation of the function so that you can associate the properties of an object with the corresponding parameter   A function call could then look like  func a  b   someArg       someOtherArg           where a and b are positional arguments and the last argument is an object with named arguments   For example   var parameterfy    function         var pattern    function                        return function func               fails horribly for parameterless functions            var args   func toString   match pattern  1  split    s              return function                 var named params   arguments arguments length - 1               if  typeof named params      object                     var params      slice call arguments  0  -1                   if  params length  lt  args length                        for  var i   params length  l   args length  i  lt  l  i                              params push named params args i                                               return func apply this  params                                               return func apply null  arguments                             Which you would use as   var foo   parameterfy function a  b  c        console log  a is     a      b is     b      c is     c             foo 1  2  3      a is 1    b is 2    c is 3 foo 1   b 2  c 3       a is 1    b is 2    c is 3 foo 1   c 3       a is 1    b is undefined    c is 3 foo  a  1  c 3       a is 1    b is undefined    c is 3    DEMO  There are some drawbacks to this approach  you have been warned      If the last argument is an object  it is treated as a  named argument objects  You will always get as many arguments as you defined in the function  but some of them might have the value undefined  that s different from having no value at all   That means you cannot use arguments length to test how many arguments have been passed      Instead of having a function creating the wrapper  you could also have a function which accepts a function and various values as arguments  such as  call func  a  b   posArg            or even extend Function prototype so that you could do   foo execute a  b   posArg

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Lot s of people say to just use the  quot Pass an object quot  trick so that you have named parameters         My Function        param  Object  arg1 Named arguments     function myFunc arg1       myFunc   param1   70  param2   175     And that works great  except      when it comes to most IDEs out there  a lot of us developers rely on type   argument hints within our IDE  I personally use PHP Storm  Along with other JetBrains IDEs like PyCharm for python and AppCode for Objective C  And the biggest problem with using the  quot Pass an object quot  trick is that when you are calling the function  the IDE gives you a single type hint and that s it    How are we supposed to know what parameters and types should go into the arg1 object   So    the  quot Pass an object quot  trick doesn t work for me    It actually causes more headaches with having to look at each function s docblock before I know what parameters the function expects     Sure  it s great for when you are maintaining existing code  but it s horrible for writing new code  Well  this is the technique I use     Now  there may be some issues with it  and some developers may tell me I m doing it wrong  and I have an open mind when it comes to these things    I am always willing to look at better ways of accomplishing a task    So  if there is an issue with this technique  then comments are welcome         My Function        param  string  arg1 Argument 1     param  string  arg2 Argument 2     function myFunc arg1  arg2       var arg1  arg2  myFunc arg1  Param1   arg2  Param2     This way  I have the best of both worlds    new code is easy to write as my IDE gives me all the proper argument hints    And  while maintaining code later on  I can see at a glance  not only the value passed to the function  but also the name of the argument  The only overhead I see is declaring your argument names as local variables to keep from polluting the global namespace  Sure  it s a bit of extra typing  but trivial compared to the time it takes to look up docblocks while writing new code or maintaining existing code

[javascript] Is there a way to provide named parameters in a function call in JavaScript?

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