@jc-lab/jose/lib/jwt/shared_validations.js

JSON Web Almost Everything - JWA, JWS, JWE, JWK, JWT, JWKS for Node.js with minimal dependencies

const isNotString = val => typeof val !== 'string' || val.length === 0

module.exports.isNotString = isNotString
module.exports.isString = function isString (Err, value, label, required = false) {
  if (required && value === undefined) {
    throw new Err(`${label} is missing`)
  }

  if (value !== undefined && isNotString(value)) {
    throw new Err(`${label} must be a string`)
  }
}