How to efficiently check if variable is Array or Object in NodeJS V8

Question

Is there any way to efficiently check if the variable is Object or Array  in NodeJS  amp  V8    I m writing a Model for MongoDB and NodeJS  and to traverse the object tree I need to know if the object is simple  Number  String       or composite  Hash  Array    It seems that V8 has fast built-in Array isArray  but how to check if object is an Object  I mean complex object like hash    or instance of class  not something like new String     Usually it may be done as this   Object prototype toString call object       object Object     or this   object     Object object    But it seems that this operations aren t cheap  maybe there s some more efficient  It s ok if it s not universal and doesn t works on all engines  I need it only to work on V8

User · Answer

If its just about detecting whether or not you're dealing with an Object, I could think of

Object.getPrototypeOf( obj ) === Object.prototype

However, this would probably fail for non-object primitive values. Actually there is nothing wrong with invoking .toString() to retreive the [[cclass]] property. You can even create a nice syntax like

var type = Function.prototype.call.bind( Object.prototype.toString );

and then use it like

if( type( obj ) === '[object Object]' ) { }

It might not be the fastest operation but I don't think the performance leak there is too big.

User · Answer

I know it s been a while  but I thought i would update the answer since there are new  faster and simpler  ways to solve this problem  Since ECMAscript 5 1 yo can use the isArray   method avaiable in the Array class   Yo can see it s documentation in MDN here   I think you shouldn t have a compatibility problem nowadays  but just in case  if you add this to your code you should be always safe that Array isArray   is polyfilled   if   Array isArray      Array isArray   function arg        return Object prototype toString call arg        object Array

User · Answer

If you know that a parameter will definitely be either an array or an object  it may be easier to check for an array compared to checking for an object with something like this   function myIsArray  arr        return  arr constructor     Array

User · Answer

I use typeof to determine if the variable I m looking at is an object  If it is then I use instanceof to determine what kind it is  var type   typeof elem  if  type     number            do stuff   else if  type     string            do stuff   else if  type     object        either array or object     if  elem instanceof Buffer           other stuff

User · Answer

I ve used this function to solve   function isArray myArray        return myArray constructor toString   indexOf  Array    gt  -1

User · Answer

The Best Way I Can Use My Project Use hasOwnProperty in Tricky Way    var arr        or  arr   new Array    var obj        or  arr   new Object     arr constructor prototype hasOwnProperty  push     true  This is an Array   obj constructor prototype hasOwnProperty  push      false  This is an Object

User · Answer

looking at jQuery they in there jQuery isArray      they do       isArray   Array isArray    function  obj         return jQuery type obj       array       this leads us to  jQuery type       type   function  obj         return obj    null           String  obj             class2type  toString call obj        object       and again we have to look in  class2type  class2type           Populate the class2type map jQuery each  Boolean Number String Function Array Date RegExp Object  split       function i  name        class2type    object     name           name toLowerCase          and in native JS   var a  t    Boolean Number String Function Array Date RegExp Object  split       for  a in t         class2type    object     t a            t a  toLowerCase        this ends up with   var isArray   Array isArray    function  obj         return toString call obj        object Array

User · Answer

Just found a quick and simple solution to discover type of a variable   ES6  export const isType    type  val    gt  val constructor name toLowerCase       type toLowerCase      ES5   function isType type  val      return val constructor name toLowerCase       type toLowerCase        Examples   isType  array       true isType  array       false isType  string       true isType  string   1  false isType  number       false isType  number   1  true isType  boolean   1  false isType  boolean   true  true   EDIT  Improvment to prevent  undefined  and  null  values   ES6  export const isType    type  val    gt     val constructor  amp  amp  val constructor name toLowerCase       type toLowerCase       ES5   function isType type  val      return    val constructor  amp  amp  val constructor name toLowerCase       type toLowerCase

User · Answer

All objects are instances of at least one class     Object     in ECMAScript   You can only differentiate between instances of built-in classes and normal objects using Object toString   They all have the same level of complexity  for instance  whether they are created using    or the new operator   Object prototype toString call object  is your best bet to differentiate between normal objects and instances of other built-in classes  as object     Object object  doesn t work here   However  I can t see a reason why you would need to do what you re doing  so perhaps if you share the use case I can offer a little more help

User · Answer

Hi I know this topic is old but there is a much better way to differentiate an Array in Node js from any other Object have a look at the docs   var util   require  util     util isArray         true util isArray         false  var obj       typeof obj      Object     true

User · Answer

For simply checking against Object or Array without additional function call  speed    isArray    isArray   function a        return    a   amp  amp   a constructor     Array      console log isArray                false console log isArray     null       false console log isArray     true       false console log isArray        1       false console log isArray     str        false console log isArray                false console log isArray new Date       false console log isArray                true   isObject    isObject   function a        return    a   amp  amp   a constructor     Object      console log isObject                false console log isObject     null       false console log isObject     true       false console log isObject        1       false console log isObject     str        false console log isObject                false console log isObject new Date       false console log isObject                true

User · Answer

underscore js is using the following  toString   Object prototype toString     isArray   nativeIsArray    function obj        return toString call obj       object Array            isObject   function obj        return obj     Object obj           isFunction   function obj        return toString call obj       object Function

[javascript] How to efficiently check if variable is Array or Object (in NodeJS & V8)?

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