How to check if object property exists with a variable holding the property name

Question

I am checking for the existence of an object property with a variable holding the property name in question    var myObj  myObj prop    exists   var myProp    p   r   o   p    if myObj myProp       alert  yes  i have that property         This is undefined because it s looking for myObj myProp but I want it to check for myObj prop

User · Answer

You can use hasOwnProperty   as well as in operator

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For own property     var loan     amount  150    if Object prototype hasOwnProperty call loan   amount             will execute     Note  using Object prototype hasOwnProperty is better than loan hasOwnProperty      in case a custom hasOwnProperty is defined in the prototype chain  which is not the case here   like   var foo           hasOwnProperty  function             return false                 bar   Here be dragons              https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Object hasOwnProperty  To include inherited properties in the finding use the in operator   but you must place an object at the right side of  in   primitive values will throw error  e g    length  in  home  will throw error  but  length  in new String  home   won t   const yoshi     skulk  true    const hattori     sneak  true    const kuma     creep  true    if   skulk  in yoshi       console log  Yoshi can skulk     if     sneak  in yoshi        console log  Yoshi cannot sneak     if     creep  in yoshi        console log  Yoshi cannot creep     Object setPrototypeOf yoshi  hattori    if   sneak  in yoshi      console log  Yoshi can now sneak    if     creep  in hattori       console log  Hattori cannot creep     Object setPrototypeOf hattori  kuma    if   creep  in hattori      console log  Hattori can now creep    if   creep  in yoshi      console log  Yoshi can also creep         https   developer mozilla org en-US docs Web JavaScript Reference Operators in  Note  One may be tempted to use typeof and     property accessor as the following code which doesn t work always      var loan     amount  150     loan installment   undefined   if  installment  in loan     correct          will execute    if typeof loan  installment        undefined      incorrect          will not execute

User · Answer

Several ways to check if an object property exists  const dog     name   quot Spot quot     if  dog name  console log  quot Yay 1 quot       Prints  if  dog sex  console log  quot Yay 2 quot       Doesn t print    if   quot name quot  in dog  console log  quot Yay 3 quot       Prints  if   quot sex quot  in dog  console log  quot Yay 4 quot       Doesn t print   if  dog hasOwnProperty  quot name quot    console log  quot Yay 5 quot       Prints  if  dog hasOwnProperty  quot sex quot    console log  quot Yay 6 quot       Doesn t print  but prints undefined

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Thank you for everyone s assistance and pushing to get rid of the eval statement   Variables needed to be in brackets  not dot notation  This works and is clean  proper code    Each of these are variables  appChoice  underI  underObstr   if typeof tData tonicdata appChoice  underI  underObstr       undefined          enter code here

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You can use hasOwnProperty  but based on the reference you need quotes when using this method   if  myObj hasOwnProperty  myProp             do something     https   developer mozilla org en-US docs Web JavaScript Reference Global Objects Object hasOwnProperty  Another way is to use in operator  but you need quotes here as well   if   myProp  in myObj           do something     https   developer mozilla org en-US docs Web JavaScript Reference Operators in

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var myProp    prop   if myObj hasOwnProperty myProp        alert  yes  i have that property        Or  var myProp    prop   if myProp in myObj       alert  yes  i have that property        Or  if  prop  in myObj       alert  yes  i have that property        Note that hasOwnProperty doesn t check for inherited properties  whereas in does   For example  constructor  in myObj is true  but myObj hasOwnProperty  constructor   is not

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A much more secure way to check if property exists on the object is to use empty object or object prototype to call hasOwnProperty    var foo       hasOwnProperty  function         return false         bar   Here be dragons      foo hasOwnProperty  bar       always returns false     Use another Object s hasOwnProperty and call it with  this  set to foo      hasOwnProperty call foo   bar       true     It s also possible to use the hasOwnProperty property from the Object    prototype for this purpose Object prototype hasOwnProperty call foo   bar       true   Reference from MDN Web Docs - Object prototype hasOwnProperty

[javascript] How to check if object property exists with a variable holding the property name?

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