Empty string is not a string in Joi validation

We’re using Joi validation to ensure that we’re saving the data of the correct types and with the correct fields into our database.

I had created a new field which was a string, so I added this to the validation:

  body: Joi.object({
   // some other stuff
    infoForSchool: Joi.string(),

However, when I submitted the form with the info for school field empty, I got an error saying that infoForSchool is not allowed to be empty. If the field was left empty in the UI it would submit an empty string, which is a string! I didn't consider this field to be empty. I mean, an empty string is also a string, right? Well, turns out that Joi doesn't allow empty strings when you put Joi.string() - even though I didn't specifically make it a required field and all validations are optional by default. I hadn't specifically put Joi.string().required(). Strange. Well, turns out that string is the only primitive in Joi that doesn't allow an empty value. There a reason for that: a request that includes q= for example will be submitted as an empty string. And in most cases this is not an expected result. That's why you explicitly have to allow empty strings like this: Joi.string().allow(‘’). Good to know.

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