Custom error objects for Mongoose schema paths

Question

Take the following Mongoose schema.

const userSchema = mongoose.Schema({
    username: {
        type: String,
        required: true,
        unique: true,
        minLength: 3
    },
    name: String,
    passwordHash: String
})

const User = mongoose.model('User', userSchema)

By Mongoose official documentation You can make a custom error message for each schema path as following.

 minLength: [3, 'username must be at least 3-character long']

In Node.js tests, you can easily test wrong cases as follows:

test('a user with too short username cannot be added into database', async () => {
    try {
        const user = new User({
            username: 'x'
        })
    
        await user.save()
    } catch (error) {
        expect(error.username.message).toBe('username must be at least 3-character long')
    }
})

For more readability though, I have a large validation error object, where each error is basically an object containing a message property and a code property:

const ERRORS = {
    USER: {
        USERNAME: {
            USERNAME_TOO_SHORT: {
                code: 'USERNAME_TOO_SHORT',
                message: 'Username is too short.'
            }
        }
    }
}

This way I'd only have to compare error codes in the tests not error messages:

expect(error.username.code).toBe(ERRORS.USER.USERNAME.USERNAME_TOO_SHORT.code)

The problem is mongoose does not allow "error objects" for each path:

 minLength: [3, ERRORS.USER.USERNAME.USERNAME_TOO_SHORT]

This results in [object Object] as the message, which is not useful for testing.

I'm looking for a way to test mongoose validation errors based on an error code not message. As mentioned above mongoose won't allow error objects for a schema path, it only expects strings.

As mentioned in Staging Ground, you can provide just the error code and then have a mapping of codes to full error messages (e.g. as a flat dictionary instead of nested like your current setup). This is typically how error code schemes work, like from linters, for example. — agilgur5
– agilgur5, Commented Nov 1 at 16:45

Shehan Lakshitha · Accepted Answer · 2025-11-05 14:37:24Z

0

Mongoose’s built-in validation system only supports strings as error messages for path validators. When you pass an object (like your ERRORS.USER.USERNAME.USERNAME_TOO_SHORT), Mongoose calls .toString() on it, which is why you see [object Object].

But you can define a custom validator function that throws a mongoose.Error.ValidatorError with both a message and a code.

https://mongoosejs.com/docs/validation.html

answered Nov 5 at 14:37

Shehan Lakshitha

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1 Comment

Ahmad Nov 6 at 18:56

Can you explain how it's done specifically? From @agilgur5 's comment though, all I had to do was passing error codes for each path and then mapping them into error messages if necessary.

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