How do I transform ErrorBuilder output?

ErrorBuilder.addTransform('toObject', function(isResponse) {

    var builder = {};

    for (var i = 0, length = this.items.length; i < length; i++) {
        var err = this.items[i];
        // err.name
        // err.error
        // err.path
        // err.index
        builder[err.name] = err.error;
    }

    // HTTP status (if the schema is used in the routing)
    this.status = 400;
    
    // Returns a new transformed object
    return isResponse ? JSON.stringify(builder) : builder;
});

// Sets the new transformation as a default output for all ErrorBuilders
ErrorBuilder.setDefaultTransform('toObject');

var err = new ErrorBuilder();
err.push('email', 'Email is not valid');
err.push('age', 'You are too young');
err.push('something'); // error message will be read from a resource filfe
console.log(err.output());
// { email: 'Email is not valid', age: 'You are too young' }

ErrorBuilder.addTransform('csv', function() {
   // ...
});

ErrorBuilder.addTransform('xml', function() {
   // ...
});

var err = new ErrorBuilder();
err.push('email', 'Email is not valid');
err.push('age', 'You are too young');
err.setTransform('xml');

// OR

var err = new ErrorBuilder();
err.push('email', 'Email is not valid');
err.push('age', 'You are too young');
err.setTransform('csv');