toString () {
        var value = this.valueOf();

        // returns String representation of null as it's a valid JSON type
        // refer: https://github.com/postmanlabs/postman-app-support/issues/8493
        if (value === null) {
            return NULL;
        }

        // returns empty string if the value is undefined or does not implement
        // the toString method
        return (!_.isNil(value) && _.isFunction(value.toString)) ? value.toString() : E;
    }
Returns the stringified value of the variable.

@returns {String}