[SuperZen]

<script>
  const obj = {
    id: 1,
    firstName: 'Вася',
    birthday: new Date()
  }

  const toLocale = date => date.toLocaleDateString('ru', {year: 'numeric', month: '2-digit', day: '2-digit'})

  const fields = obj => {
    return Object.keys(obj).map(key => {
      switch (typeof obj[key]) {
        case 'string':
          return `${key} = '${obj[key]}'`
        case 'number':
          return `${key} = ${obj[key]}`
        default: 
          return `${key} = '${toLocale(obj[key])}'`
      }
    }).join(', ')
  }

  const str = `UPDATE users SET ${fields(obj)} WHERE id = ${obj.id}`;

  console.log(str)
</script>

p.s. после сборки доработать напильником %)