How to determine whether an object has a given property in JavaScript

Question

How can I determine whether an object x has a defined property y  regardless of the value of x y   I m currently using  if  typeof x y       undefined     but that seems a bit clunky  Is there a better way

User · Answer

includes Object keys x  includes  y     The Array prototype includes   method determines whether an array includes a certain value among its entries  returning true or false as appropriate  and Object keys   returns an array of strings that represent all the enumerable properties of the given object   hasOwnProperty   and the ES6     -optional-chaining like  if  x  y  are very good 2020  options as well

User · Answer

If you want to know if the object physically contains the property  gnarf s answer using hasOwnProperty will do the work    If you re want to know if the property exists anywhere  either on the object itself or up in the prototype chain  you can use the in operator   if   prop  in obj                 Eg    var obj         toString  in obj    true     inherited from Object prototype obj hasOwnProperty  toString      false     doesn t contains it physically

User · Answer

Object has property   If you are testing for properties that are on the object itself  not a part of its prototype chain  you can use  hasOwnProperty     if  x hasOwnProperty  y                       Object or its prototype has a property   You can use the in operator to test for properties that are inherited as well   if   y  in x

User · Answer

Why not simply    if  typeof myObject myProperty     undefined   alert  myProperty is not defined       Or if you expect a specific type   if  typeof myObject myProperty     string   alert  myProperty has wrong type or does not exist

User · Answer

One feature of my original code  if   typeof x y      undefined          that might be useful in some situations is that it is safe to use whether x exists or not  With either of the methods in gnarf s answer  one should first test for x if there is any doubt if it exists   So perhaps all three methods have a place in one s bag of tricks

User · Answer

Since question was regarding clunkiness of property checking  and one regular usecase for that being validation of function argument options objects  thought I d mention a library-free short way of testing existence of multiple properties  Disclaimer  It does require ECMAScript 5  but IMO anyone still using IE8 deserves a broken web    function f opts      if    req1   req2   every opts hasOwnProperty  opts           throw new Error  IllegalArgumentException          alert  ok      f  req1  123        error f  req1  123  req2  456        ok

User · Answer

const data      quot b quot  1  quot c quot  100    quot a quot  1  quot b quot  1  quot c quot  150    quot a quot  1  quot b quot  2  quot c quot  100    quot a quot  2  quot b quot  1  quot c quot  13    const result   data reduce  r  e     gt      r  a       e  a     e  a     0      r  d       e  b     e  b     0    r  c       e  c     e  c     0     return r      a  0   d  0   c  0    console log result   result    a  4  d  5  c  363

User · Answer

You can trim that up a bit like this   if   x y     undefined

User · Answer

Underscore js or Lodash  if    has x   y

User · Answer

ES6   There is a new feature on ES6  that you can check it like below  if  x  y   Actually  the interpretor checks the existence of x and then call the y and because of putting inside if parentheses the coercion happens and x  y converted to boolean

[javascript] How to determine whether an object has a given property in JavaScript

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