Finding Variable Type in JavaScript

Question

In Java  you can use instanceOf or getClass   on a variable to find out its type   How do I find out a variable s type in JavaScript which isn t strongly-typed   For example  how do I know if the bar is a Boolean or a Number  or a String   function foo bar           what do I do here

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In Javascript you can do that by using the typeof function  function foo bar     alert typeof bar

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To be a little more ECMAScript-5 1-precise than the other answers  some might say pedantic    In JavaScript  variables  and properties  don t have types  values do  Further  there are only 6 types of values  Undefined  Null  Boolean  String  Number  and Object   Technically  there are also 7  specification types   but you can t store values of those types as properties of objects or values of variables--they are only used within the spec itself  to define how the language works  The values you can explicitly manipulate are of only the 6 types I listed    The spec uses the notation  Type x   when it wants to talk about  the type of x   This is only a notation used within the spec  it is not a feature of the language   As other answers make clear  in practice you may well want to know more than the type of a value--particularly when the type is Object  Regardless  and for completeness  here is a simple JavaScript implementation of Type x  as it is used in the spec   function Type x         if  x     null            return  Null              switch  typeof x        case  undefined   return  Undefined       case  boolean     return  Boolean       case  number      return  Number       case  string      return  String       default           return  Object

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Using type      Numbers typeof 37                     number   typeof 3 14                   number   typeof Math LN2               number   typeof Infinity               number   typeof NaN                    number      Despite being  Not-A-Number  typeof Number 1               number      but never use this form      Strings typeof                        string   typeof  bla                   string   typeof  typeof 1              string      typeof always return a string typeof String  abc            string      but never use this form      Booleans typeof true                   boolean   typeof false                  boolean   typeof Boolean true           boolean      but never use this form      Undefined typeof undefined              undefined   typeof blabla                 undefined      an undefined variable     Objects typeof  a 1                   object   typeof  1  2  4               object      use Array isArray or Object prototype toString call to differentiate regular objects from arrays typeof new Date               object   typeof new Boolean true       object      this is confusing  Don t use  typeof new Number 1           object      this is confusing  Don t use  typeof new String  abc        object       this is confusing  Don t use      Functions typeof function               function   typeof Math sin               function

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I find it frustrating that typeof is so limited  Here   s an improved version   var realtypeof   function  obj        switch  typeof obj                object prototypes         case  object               if  obj instanceof Array                  return   object Array                if  obj instanceof Date                  return   object Date                if  obj instanceof RegExp                  return   object regexp                if  obj instanceof String                  return   object String                if  obj instanceof Number                  return   object Number                 return  object              object literals         default              return typeof obj                 sample test   realtypeof           string  realtypeof  new String            object String   Object prototype toString call  foo bar       object String

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For builtin JS types you can use   function getTypeName val        return    toString call val  slice 8  -1       Here we use  toString  method from  Object  class which works different than the same method of another types   Examples      Primitives getTypeName 42              Number  getTypeName  hi             String  getTypeName true            Boolean  getTypeName Symbol  s       Symbol  getTypeName null            Null  getTypeName undefined       Undefined      Non-primitives getTypeName                     Object  getTypeName                     Array  getTypeName new Date            Date  getTypeName function            Function  getTypeName  a                  RegExp  getTypeName new Error           Error    If you need a class name you can use   instance constructor name   Examples        constructor name           Object     constructor name             Array   new Date  constructor name     Date   function MyClass      let my   new MyClass    my constructor name             MyClass    But this feature was added in ES2015

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In JavaScript everything is an object  console log type of         Object console log type of         Object   To get Real type   use this  console log Object prototype toString call           object Object  console log Object prototype toString call           object Array    Hope this helps

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Use typeof    gt  typeof  foo   string   gt  typeof true  boolean   gt  typeof 42  number    So you can do   if typeof bar      number          whatever     Be careful though if you define these primitives with their object wrappers  which you should never do  use literals where ever possible     gt  typeof new Boolean false   object   gt  typeof new String  foo    object   gt  typeof new Number 42   object    The type of an array is still object  Here you really need the instanceof operator   Update   Another interesting way is to examine the output of Object prototype toString    gt  Object prototype toString call  1 2 3     object Array    gt  Object prototype toString call  foo bar     object String    gt  Object prototype toString call 45    object Number    gt  Object prototype toString call false    object Boolean    gt  Object prototype toString call new String  foo bar      object String    gt  Object prototype toString call null    object Null    gt  Object prototype toString call  123     object RegExp    gt  Object prototype toString call undefined    object Undefined     With that you would not have to distinguish between primitive values and objects

User · Answer

typeof is only good for returning the  primitive  types such as number  boolean  object  string and symbols  You can also use instanceof to test if an object is of a specific type   function MyObj prop      this prop   prop     var obj   new MyObj 10    console log obj instanceof MyObj  amp  amp  obj instanceof Object      outputs true

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Here is the Complete solution      You can also use it as a Helper class in your Projects     x000D   x000D   use strict   x000D      x000D      description Util file x000D      author Tarandeep Singh x000D      created 2016-08-09 x000D      x000D   x000D  window Sys       x000D   x000D  Sys     x000D    isEmptyObject  function val    x000D      return this isObject val   amp  amp  Object keys val  length  x000D       x000D        This Returns Object Type    x000D    getType  function val    x000D      return Object prototype toString call val   x000D       x000D        This Checks and Return if Object is Defined    x000D    isDefined  function val    x000D      return val     void 0    typeof val      undefined   x000D       x000D        Run a Map on an Array     x000D    map  function arr  fn    x000D      var res       x000D        i   0  x000D      for    i  lt  arr length    i    x000D        res push fn arr i   i    x000D        x000D      arr   null  x000D      return res  x000D       x000D        Checks and Return if the prop is Objects own Property    x000D    hasOwnProp  function obj  val    x000D      return Object prototype hasOwnProperty call obj  val   x000D       x000D        Extend properties from extending Object to initial Object    x000D    extend  function newObj  oldObj    x000D      if  this isDefined newObj   amp  amp  this isDefined oldObj     x000D        for  var prop in oldObj    x000D          if  this hasOwnProp oldObj  prop     x000D            newObj prop    oldObj prop   x000D            x000D          x000D        return newObj  x000D        else   x000D        return newObj    oldObj        x000D        x000D      x000D     x000D   x000D     This Method will create Multiple functions in the Sys object that can be used to test type of x000D    Arguments    Function    String    Number    Date    RegExp    Object    Array    Undefined   x000D   forEach  x000D    function name    x000D      Sys  is    name    function obj    x000D        return toString call obj       object     name        x000D         x000D      x000D     x000D   lt h1 gt Use the Helper JavaScript Methods   lt  h1 gt  x000D   lt code gt use  if Sys isDefined jQuery  console log  O Yeah           lt  code gt  x000D   x000D   x000D    For Exportable CommonJs Module or RequireJS Module       use strict         Helper Utils               description Util file    From Vault     author Tarandeep Singh     created 2016-08-09      var Sys        Sys         isEmptyObject  function val           return this isObject val   amp  amp  Object keys val  length                 This Returns Object Type        getType  function val           return Object prototype toString call val                  This Checks and Return if Object is Defined        isDefined  function val           return val     void 0    typeof val      undefined                  Run a Map on an Array         map  function arr fn           var res       i 0          for    i lt arr length    i               res push fn arr i   i                      arr   null          return res                 Checks and Return if the prop is Objects own Property        hasOwnProp  function obj  val           return Object prototype hasOwnProperty call obj  val                  Extend properties from extending Object to initial Object        extend  function newObj  oldObj           if this isDefined newObj   amp  amp  this isDefined oldObj                for var prop in oldObj                   if this hasOwnProp oldObj  prop                        newObj prop    oldObj prop                                               return newObj           else               return newObj    oldObj                                   This isn t Required but just makes WebStorm color Code Better  D       Sys isObject       Sys isArguments       Sys isFunction       Sys isString       Sys isArray       Sys isUndefined       Sys isDate       Sys isNumber       Sys isRegExp                This Method will create Multiple functions in the Sys object that can be used to test type of        Arguments    Function    String    Number    Date    RegExp    Object    Array    Undefined        forEach          function name                Sys  is    name    function obj                    return toString call obj       object     name                                          module exports   Sys       Currently in Use on a public git repo     Github Project   Now you can import this Sys code in a Sys js file   then you can use this Sys object functions to find out the type of JavaScript Objects   you can also check is Object is Defined or type is Function or the Object is Empty    etc     Sys isObject Sys isArguments Sys isFunction Sys isString Sys isArray Sys isUndefined Sys isDate Sys isNumber Sys isRegExp   For Example  var m   function      Sys isObject      Sys isFunction m   Sys isString m    console log Sys isDefined jQuery

[javascript] Finding Variable Type in JavaScript

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