JSON-tryparse parses JSON (using JSON.parse) and returns promise to handle success / error of parsing

jsontryParse.js

  if (!JSON.tryParse)
    {

        JSON.tryParse = function (str) {
           
            var successState=1, errorState = 2, result, errorResult,
                promise = {
                then: function(successCallback, errorCallback)
                {
                    if (this.state === successState)
                    {
                        if (successCallback)
                            successCallback(this.result);
                    }
                    else if (errorCallback)
                    {
                        errorCallback(this.errorResult);
                    }
                    return this;
                },
                done: function(callback)
                {
                    callback();
                    return this;
                }
            }
            try {
                promise.result = JSON.parse(str);
                promise.state = successState;
            }
            catch (err) {
                promise.state = errorState;
                promise.errorResult = err;
            }

            return promise;
        };
    }