How can I make a yup number accept nullable values?

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You have to pass true to nullable –

nrOfApples: yup.number().min(0).max(999).nullable(true);

From: https://github.com/jquense/yup/issues/500

Working example: https://runkit.com/xdumaine/5f2816c75a0ba5001aa312b2

Note that if you add required().nullable(true) the required overrides the nullable and null will not validate.

Update:

You can use a transform to convert the NaN value into null. I updated the runkit with this:

const contactSchema = yup.object({
  name: yup.string()
    .required(),
  nrOfApples: yup
    .number()
    .min(0)
    .max(999)
    .nullable(true)
    // checking self-equality works for NaN, transforming it to null
    .transform((_, val) => val === Number(val) ? val : null) 
})

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