Set type for function parameters

Question

Is there a way to let a javascript function know that a certain parameter is of a certain type   Being able to do something like this would be perfect   function myFunction Date myDate  String myString          do stuff     Thank you   Update  Being that the answer is a resounding  no   if I want myDate to be treated as a date  in order to call date functions on it   I have to cast it as a date inside the function or set a new variable of type Date to it

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I ve been thinking about this too  From a C background  you can simulate function return code types  as well as  parameter types  using something like the following   function top function         var rc      console log  1st call        rc   Number test function  number   1   string    my string         console log  typeof rc      typeof rc       rc      rc       console log  2nd call        rc   Number test function  number    a    string    my string         console log  typeof rc      typeof rc       rc      rc     function test function parm type 1  parm val 1  parm type 2  parm val 2        if  typeof parm val 1     parm type 1  console log  Parm 1 not correct type        if  typeof parm val 2     parm type 2  console log  Parm 2 not correct type        return parm val 1      The Number before the calling function returns a Number type regardless of the type of the actual value returned  as seen in the 2nd call where typeof rc   number but the value is NaN  the console log for the above is   1st call typeof rc  number   rc  1 2nd call Parm 1 not correct type typeof rc  number   rc  NaN

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You can implement a system that handles the type checks automatically  using a wrapper in your function      With this approach  you can build a complete declarative type check system that will manage for you the type checks     If you are interested in taking a more in depth look at this concept  check the Functyped library   The following implementation illustrates the main idea  in a simplistic  but operative way      x000D   x000D     x000D     checkType     Test the type of the value  If succeds return true   x000D     if fails  throw an Error x000D      x000D  function checkType value type  i   x000D       perform the appropiate test to the passed  x000D       value according to the provided type x000D    switch type   x000D      case Boolean    x000D        if typeof value      boolean   return true  x000D        break  x000D      case String    x000D        if typeof value      string   return true  x000D        break  x000D      case Number    x000D        if typeof value      number   return true  x000D        break  x000D      default   x000D        throw new Error  TypeError   Unknown type provided in argument   i 1     x000D      x000D       test didn t succeed   throw error x000D    throw new Error  TypeError   Expecting a   type name  in argument   i 1     x000D    x000D   x000D   x000D     x000D     typedFunction     Constructor that returns a wrapper x000D     to handle each function call  performing automatic  x000D     arguments type checking x000D      x000D  function typedFunction  parameterTypes  func    x000D       types definitions and function parameters  x000D       count must match x000D    if parameterTypes length     func length  throw new Error  Function has   func length  arguments  but type definition has   parameterTypes length     x000D       return the wrapper    x000D    return function    args   x000D         provided arguments count must match types x000D         definitions count x000D      if parameterTypes length     args length  throw new Error  Function expects   func length  arguments  instead   args length  found     x000D         iterate each argument value  and perform a x000D         type check against it  using the type definitions x000D         provided in the construction stage x000D      for let i 0  i lt args length i    checkType  args i   parameterTypes i    i  x000D         if no error has been thrown  type check succeed x000D         execute function  x000D      return func    args   x000D      x000D    x000D   x000D     Play time   x000D     Declare a function that expects 2 Numbers x000D  let myFunc   typedFunction    Number  Number      a b   gt   x000D    return a b  x000D      x000D   x000D     call the function  with an invalid second argument x000D  myFunc 123   456   x000D     ERROR  Uncaught Error  TypeError   Expecting a Number in argument 2 x000D   x000D   x000D

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It can easilly be done with ArgueJS   function myFunction        arguments       myDate  Date  myString  String         do stuff

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TypeScript is one of the best solution for now     TypeScript extends JavaScript by adding types to the language    https   www typescriptlang org

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No  JavaScript is not a statically typed language  Sometimes you may need to manually check types of parameters in your function body

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Edit  Seven years later  this answer still gets occasional upvotes   It s fine if you are looking for runtime checking  but I would now recommend compile-time type checking using Typescript  or possibly Flow  See https   stackoverflow com a 31420719 610585 above for more   Original answer   It s not built into the language  but you can do it yourself quite easily   Vibhu s answer is what I would consider the typical way of type checking in Javascript   If you want something more generalized  try something like this   just an example to get you started   typedFunction   function paramsList  f         optionally  ensure that typedFunction is being called properly  -- here s a start      if    paramsList instanceof Array   throw Error  invalid argument  paramsList must be an array           the type-checked function     return function            for var i 0 p arg p paramsList i  arg arguments i  i lt paramsList length  i                 if  typeof p      string                    if  typeof arg     p  throw new Error  expected type     p      got     typeof arg                             else     function                 if    arg instanceof p   throw new Error  expected type     String p  replace   s                 got     typeof arg                                     type checking passed  call the function itself         return f apply this  arguments              usage  var ds   typedFunction  Date   string    function d  s       console log d toDateString    s substr 0         ds  notadate    test      Error  expected type function Date    got string ds      Error  expected type function Date    got undefined ds new Date    42     Error  expected type string  got number ds new Date     success      Fri Jun 14 2013 success

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Check out the new Flow library from Facebook   a static type checker  designed to find type errors in JavaScript programs   Definition          flow    function foo x  string  y  number   string     return x length   y    foo  Hello   42     Type checking     gt  flow hello js 3 10 21  number This type is incompatible with   hello js 2 37 42  string   And here is how to run it

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Not in javascript it self but using Google Closure Compiler s advanced mode you can do that           param  Date  myDate The date     param  string  myString The string     function myFunction myDate  myString          do stuff     See http   code google com closure compiler docs js-for-compiler html

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Maybe a helper function like this  But if you see yourself using such syntax regularly  you should probably switch to Typescript   function check caller args     types        if  types every  type  index    gt            if typeof type      string               return typeof caller args index      type         return caller args index  instanceof type          throw Error  Illegal argument given       function abc name  id  bla       check arguments   string    number   MyClass        code

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While you can t inform JavaScript the language about types  you can inform your IDE about them  so you get much more useful autocompletion   Here are two ways to do that    Use JSDoc  a system for documenting JavaScript code in comments  In particular  you ll need the  param directive           param  Date  myDate - The date     param  string  myString - The string     function myFunction myDate  myString                 You can also use JSDoc to define custom types and specify those in  param directives  but note that JSDoc won t do any type checking  it s only a documentation tool  To check types defined in JSDoc  look into TypeScript  which can parse JSDoc tags  Use type hinting by specifying the type right before the parameter in a    comment        This is a pretty widespread technique  used by ReactJS for instance  Very handy for parameters of callbacks passed to 3rd party libraries    TypeScript  For actual type checking  the closest solution is to use TypeScript  a  mostly  superset of JavaScript  Here s TypeScript in 5 minutes

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No  instead you would need to do something like this depending on your needs   function myFunction myDate  myString      if arguments length  gt  1  amp  amp  typeof Date parse myDate       number   amp  amp  typeof myString      string           Code here

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Explanation I m not sure if my answer is direct answer to original question  but as I suppose a lot of people come here to just find a way to tell their IDEs to understand types  I ll share what I found  If you want to tell VSCode to understand your types  do as follows  Please pay attention that js runtime and NodeJS does not care about these types at all  Solution 1- Create a file with  d ts ending  e g  index d ts  You can create this file in another folder  for example  types index d ts 2- Suppose we want to have a function called view  Add these lines to index d ts         Use express res render function to render view file inside layout file         param  string  view The path of the view file  relative to view root dir      param  object  options The options to send to view file for ejs to use when rendering      returns  Express Response render        view view  string  options   object   Express Response render   3- Create a jsconfig json file in you project s root   It seems that just creating this file is enough for VSCode to search for your types   A bit more Now suppose we want to add this type to another library types   As my own situation   We can use some ts keywords  And as long as VSCode understands ts we have no problem with it   For example if you want to add this view function to response from expressjs  change index d ts file as follows  export declare global     namespace Express       interface Response                      Use express res render function to render view file inside layout file                     param  string  view The path of the view file  relative to view root dir            param  object  options The options to send to view file for ejs to use when rendering            returns  Express Response render                    view view  string  options   object   Express Response render               Result

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Use typeof or instanceof   const assert   require  assert     function myFunction Date myDate  String myString        assert  typeof myString       string     Error message about incorrect arg type        assert  myDate instanceof Date           Error message about incorrect arg type

[javascript] Set type for function parameters?

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