Mask letters without first and last 3 characters

mask.js

const maskString = data => {
  const firstThreeChar = /(^.{3})/g
  const lastThreeChar = /(.{3}$)/g
  const maskedString = data.replace(/./g, String.fromCharCode(32, 183, 32)).slice(6)

  return data.match(firstThreeChar) + maskedString + data.match(lastThreeChar)
}