Set default value of javascript object attributes

Question

Is there a way to set the default attribute of a javascript object such that   var emptyObj          do some magic emptyObj nonExistingAttribute      gt  defaultValue   IE can be disregarded  Chrome Frame has relieved me of that headache

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Object withDefault    defaultValue o       gt      return new Proxy o        get   o  k    gt   k in o    o k    defaultValue           o   Object withDefault 42   o x      gt  42  o x   10 o x      gt  10 o xx     gt  42

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Simplest of all Solutions   dict     first   1           second   2           third   3    Now   dict  last       Excluded    will return  Excluded   which is the default value

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I saw an article yesterday that mentions an Object   noSuchMethod   property  JavascriptTips I ve not had a chance to play around with it  so I don t know about browser support  but maybe you could use that in some way

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I m surprised nobody has mentioned ternary operator yet   var emptyObj    a  123   b  234   c 0   var defaultValue    defaultValue   var attr    someNonExistAttribute   emptyObj hasOwnProperty attr    emptyObj attr    defaultValue     gt   defaultValue    attr    c        gt   c  emptyObj hasOwnProperty attr    emptyObj attr    defaultValue       gt  0   In this way  even if the value of  c  is 0  it will still get the correct value

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The way I achieve this is with the object assign function  const defaultProperties      foo    bar    bar    foo     const overwriteProperties      foo    foo     const newObj   Object assign     defaultProperties  overwriteProperties   console log defaultProperties         foo    bar    bar    foo   console log overwriteProperties          foo    foo     console log newObj          foo    foo    bar    foo

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Use destructuring  new in ES6   There is great documentation by Mozila as well as a fantastic blog post that explains the syntax better than I can   To Answer Your Question  var emptyObj       const   nonExistingAttribute   defaultValue     emptyObj  console log nonExistingAttribute      defaultValue   Going Further  Can I rename this variable  Sure   const   nonExistingAttribute  coolerName   15    emptyObj  console log coolerName      15   What about nested data  Bring it on   var nestedData         name   Awesome Programmer       languages                          name   javascript               proficiency  4                       country   Canada       var  name  realName  languages    name  languageName      nestedData    console log realName      Awesome Programmer console log languageName      javascript

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var obj       a  2    b  4     console log obj    -- gt   a  2  b  4   function applyDefaults obj      obj a     10    obj b     10    obj c     10        do some magic applyDefaults obj    console log obj    -- gt   a  2  b  4  c  10   This works because undefined     quot 1111111 quot  -- gt   quot 1111111 quot   quot 0000000 quot      quot 1111111 quot  -- gt   quot 0000000 quot   as null  undefined  NaN  0   quot  quot   Empty String   false itself  are all considered to be equivalent to false  falsy   Anything else is true  truthy   Note that this is not uniformly supported across browsers and nodejs versions  confirm for yourself   So two troublesome cases are the empty String  quot  quot  and 0  zero   If it is important not to override those  you might need to rewrite this as  if  typeof obj d     quot undefined quot   obj d    quot default quot   This will be better supported across browsers also  Alternatively you could write this as  obj d      quot default quot   This is the nullish assignment which applies only to values that are null or undefined  nullish  - of which the empty string is not part  However  this has again a diminished cross-browser support  See also on the official Mozilla Website - Assigning a default value to a variable

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This seems to me the most simple and readable way of doing so   let options    name  James   const default options    name  John   surname  Doe    options   Object assign     default options  options    Object assign   reference

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my code is   function s       s             top  s top    100        default value or s top         left  s left    300      default value or s left           alert s top

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This is actually possible to do with Object create  It will not work for  non defined  properties  But for the ones that has been given a default value   var defaults         a   test1       b   test2       Then when you create your properties object you do it with Object create  properties   Object create defaults     Now you will have two object where the first object is empty  but the prototype points to the defaults object  To test   console log  Unchanged   properties   properties a    updated   console log  Updated   properties   console log  Defaults   Object getPrototypeOf properties

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Since I asked the question several years ago things have progressed nicely    Proxies are part of ES6  The following example works in Chrome  Firefox  Safari and Edge   var handler       get  function target  name        return target hasOwnProperty name    target name    42          var p   new Proxy     handler    p answerToTheUltimateQuestionOfLife      gt  42     Read more in Mozilla s documentation on Proxies

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I think the simplest approach is using Object assign   If you have this Class   class MyHelper       constructor options            this options   Object assign               name   John               surname   Doe               birthDate   1980-08-08             options             You can use it like this   let helper   new MyHelper   name   Mark      console log helper options surname      this will output  Doe    Documentation  with polyfill   https   developer mozilla org it docs Web JavaScript Reference Global Objects Object assign

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Or you can try this  dict       somekey    somevalue      val   dict  anotherkey       anotherval

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One approach would be to take a defaults object and merge it with the target object   The target object would override values in the defaults object   jQuery has the  extend   method that does this   jQuery is not needed however as there are vanilla JS implementations such as can be found here   http   gomakethings com vanilla-javascript-version-of-jquery-extend

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I came here looking for a solution because the header matched my problem description but it isn t what i was looking for but i got a solution to my problem I wanted to have a default value for an attribute which would be dynamic something like date    let Blog     title    String  image    String  body     String  created   type  Date  default  Date now      The above code was the solution for which i finally settled

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This sure sounds like the typical use of protoype-based objects      define a new type of object var foo   function             define a default attribute and value that all objects of this type will have foo prototype attribute1    defaultValue1         create a new object of my type var emptyObj   new foo    console log emptyObj attribute1            outputs defaultValue1

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There isn t a way to set this in Javascript - returning undefined for non-existent properties is a part of the core Javascript spec  See the discussion for this similar question  As I suggested there  one approach  though I can t really recommend it  would be to define a global getProperty function   function getProperty o  prop        if  o prop      undefined  return o prop       else return  my default      var o         foo  1     getProperty o   foo       1 getProperty o   bar        my default    But this would lead to a bunch of non-standard code that would be difficult for others to read  and it might have unintended consequences in areas where you d expect or want an undefined value  Better to just check as you go    var someVar   o someVar     my default

[javascript] Set default value of javascript object attributes

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