dynamic key in JSON file (used by backbone.js models for example...)

You won't be able to do that directly; the left side of the ":" in an object literal must be a constant. What you can do is this:

window.MyModel = Backbone.Model.extend({    myDynamicVar : "country",    urlRoot: "api/myModel",    defaults: function(fieldName) {      var value = {        "id": null,        "country":  {            "route":"test/test/test",            "class":".usa2",            "txt":"USA 2"        },          "region":  "California",        "year":  "",        "description":  "",        "picture":  ""      };      value[fieldName] = "USA";      return value;    }(this.myDynamicVar)});

edit — ah, I just noticed that "myDynamicVar" itself is part of the outer object. There's no way to refer to it with this in such a way, so if that's what you're trying to do the code would be different:

window.MyModel = Backbone.Model.extend(function() {  var value = {    myDynamicVar : "country",    urlRoot: "api/myModel",    defaults: {        "id": null,        "country":  {            "route":"test/test/test",            "class":".usa2",            "txt":"USA 2"        },          "region":  "California",        "year":  "",        "description":  "",        "picture":  ""      }   };   value.defaults.[value.myDynamicVar] = "USA";   return value;}());

If you want myDynamicVar itself to be dynamic, then you could pass it in as a parameter:

window.MyModel = Backbone.Model.extend(function(whichProperty) {  var value = {    myDynamicVar : "country",    urlRoot: "api/myModel",    defaults: {        "id": null,        "country":  {            "route":"test/test/test",            "class":".usa2",            "txt":"USA 2"        },          "region":  "California",        "year":  "",        "description":  "",        "picture":  ""      }   };   value.defaults.[value[whichProperty]] = "USA";   return value;}("myDynamicVar"));

You cannot use a variable in defining the object key. You can try something like this.

window.MyModel = Backbone.Model.extend({    myDynamicVar : "country",    urlRoot: "api/myModel",    defaults: {        "id": null,        "country":  {            "route":"test/test/test",            "class":".usa2",            "txt":"USA 2"        },          "region":  "California",        "year":  "",        "description":  "",        "picture":  ""      }});window.MyModel.defaults[this.myDynamicVar] = "USA";