How do I create a dynamic key to be added to a JavaScript object variable

Question

I m trying something like this  but this example does not work   jsObj        for  var i   1  i  lt   10  i          jsObj  key    i     example     1      What can I do to make a dynamic key like this

User · Answer

Associative Arrays in JavaScript don t really work the same as they do in other languages   for each statements are complicated  because they enumerate inherited prototype properties    You could declare properties on an object associative array as Pointy mentioned  but really for this sort of thing you should use an array with the push method   jsArr         for  var i   1  i  lt   10  i           jsArr push  example     1         Just don t forget that indexed arrays are zero-based so the first element will be jsArr 0   not jsArr 1

User · Answer

Square brackets  jsObj  key    i     example    1   In JavaScript  all arrays are objects  but not all objects are arrays  The primary difference  and one that s pretty hard to mimic with straight JavaScript and plain objects  is that array instances maintain the length property so that it reflects one plus the numeric value of the property whose name is numeric and whose value  when converted to a number  is the largest of all such properties  That sounds really weird  but it just means that given an array instance  the properties with names like  quot 0 quot    quot 5 quot    quot 207 quot   and so on  are all treated specially in that their existence determines the value of length  And  on top of that  the value of length can be set to remove such properties  Setting the length of an array to 0 effectively removes all properties whose names look like whole numbers  OK  so that s what makes an array special  All of that  however  has nothing at all to do with how the JavaScript     operator works  That operator is an object property access mechanism which works on any object  It s important to note in that regard that numeric array property names are not special as far as simple property access goes   They re just strings that happen to look like numbers  but JavaScript object property names can be any sort of string you like  Thus  the way the     operator works in a for loop iterating through an array  for  var i   0  i  lt  myArray length    i      var value   myArray i      property access             is really no different from the way     works when accessing a property whose name is some computed string  var value   jsObj  quot key quot    i    The     operator there is doing precisely the same thing in both instances  The fact that in one case the object involved happens to be an array is unimportant  in other words  When setting property values using      the story is the same except for the special behavior around maintaining the length property  If you set a property with a numeric key on an array instance  myArray 200    5   then  assuming that  quot 200 quot  is the biggest numeric property name  the length property will be updated to 201 as a side-effect of the property assignment  If the same thing is done to a plain object  however  myObj 200    5   there s no such side-effect   The property called  quot 200 quot  of both the array and the object will be set to the value 5 in otherwise the exact same way  One might think that because that length behavior is kind-of handy  you might as well make all objects instances of the Array constructor instead of plain objects   There s nothing directly wrong about that  though it can be confusing  especially for people familiar with some other languages  for some properties to be included in the length but not others    However  if you re working with JSON serialization  a fairly common thing   understand that array instances are serialized to JSON in a way that only involves the numerically-named properties  Other properties added to the array will never appear in the serialized JSON form  So for example  var obj       obj 0     quot hello world quot   obj  quot something quot     5000   var objJSON   JSON stringify obj    the value of  quot objJSON quot  will be a string containing just   quot hello world quot    the  quot something quot  property will be lost  ES2015  If you re able to use ES6 JavaScript features  you can use Computed Property Names to handle this very easily  var key    DYNAMIC KEY       obj              key    ES6           console log obj       gt     DYNAMIC KEY    ES6

[javascript] How do I create a dynamic key to be added to a JavaScript object variable

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