// 通过 ID
const form = document.getElementById('myForm')

// 通过 name
const form2 = document.forms['loginForm']

// 通过索引
const form3 = document.forms[0]

// 使用 querySelector
const form4 = document.querySelector('form.login')

const form = document.getElementById('myForm')

// 通过 name 属性
const username = form.elements['username']
const password = form.elements['password']

// 通过索引
const firstInput = form.elements[0]

// 所有表单元素
console.log(form.elements.length)

// 简写方式
const email = form.email // 等同于 form.elements['email']

const input = document.getElementById('username')

// 获取/设置值
console.log(input.value)
input.value = '新值'

// 获取属性
input.type // 'text'
input.name // 'username'
input.placeholder // '请输入用户名'
input.maxLength // 20
input.disabled // false
input.readOnly // false

// 设置属性
input.disabled = true
input.readOnly = true

// 焦点控制
input.focus()
input.blur()

// 选中文本
input.select()
input.setSelectionRange(0, 5) // 选中前5个字符

// 复选框
const checkbox = document.getElementById('agree')
console.log(checkbox.checked) // true/false
checkbox.checked = true

// 获取所有选��的复选框
const checked = document.querySelectorAll('input[name="hobbies"]:checked')
const values = [...checked].map((cb) => cb.value)

// 单选框
const radios = document.querySelectorAll('input[name="gender"]')
let selectedValue
radios.forEach((radio) => {
  if (radio.checked) {
    selectedValue = radio.value
  }
})

// 或使用 :checked
const selected = document.querySelector('input[name="gender"]:checked')
console.log(selected?.value)

const select = document.getElementById('city')

// 获取选中值
console.log(select.value)

// 获取选中的 option
console.log(select.selectedIndex)
console.log(select.options[select.selectedIndex])
console.log(select.selectedOptions) // 多选时

// 设置选中
select.value = 'beijing'
select.selectedIndex = 2

// 获取所有选项
for (const option of select.options) {
  console.log(option.value, option.text)
}

// 动态添加选项
const newOption = new Option('上海', 'shanghai')
select.add(newOption)
// 或
select.innerHTML += '<option value="shenzhen">深圳</option>'

// 多选下拉框
const multiSelect = document.getElementById('skills')
const selectedValues = [...multiSelect.selectedOptions].map((o) => o.value)

const textarea = document.getElementById('content')

// 获取/设置内容
console.log(textarea.value)
textarea.value = '新内容'

// 行数和列数
console.log(textarea.rows)
console.log(textarea.cols)

// 自动调整高度
textarea.addEventListener('input', function () {
  this.style.height = 'auto'
  this.style.height = this.scrollHeight + 'px'
})

const form = document.getElementById('myForm')
const input = document.getElementById('username')

// 表单提交
form.addEventListener('submit', (e) => {
  e.preventDefault() // 阻止默认提交
  console.log('表单提交')
})

// 表单重置
form.addEventListener('reset', (e) => {
  console.log('表单重置')
})

// 输入事件（实时触发）
input.addEventListener('input', (e) => {
  console.log('输入:', e.target.value)
})

// 改变事件（失焦后触发）
input.addEventListener('change', (e) => {
  console.log('改变:', e.target.value)
})

// 获得焦点
input.addEventListener('focus', () => {
  console.log('获得焦点')
})

// 失去焦点
input.addEventListener('blur', () => {
  console.log('失去焦点')
})

// 按键事件
input.addEventListener('keydown', (e) => {
  if (e.key === 'Enter') {
    console.log('按下回车')
  }
})

const form = document.getElementById('myForm')

// 统一处理所有输入
form.addEventListener('input', (e) => {
  const { name, value } = e.target
  console.log(`${name}: ${value}`)

  // 实时验证
  validateField(e.target)
})

// 统一处理失焦验证
form.addEventListener('focusout', (e) => {
  if (e.target.matches('input, textarea, select')) {
    validateField(e.target)
  }
})

<form id="myForm">
  <!-- 必填 -->
  <input type="text" name="username" required />

  <!-- 邮箱格式 -->
  <input type="email" name="email" required />

  <!-- 最小/最大长度 -->
  <input type="text" name="password" minlength="6" maxlength="20" />

  <!-- 数字范围 -->
  <input type="number" name="age" min="0" max="150" />

  <!-- 正则模式 -->
  <input type="text" name="phone" pattern="1[3-9]\d{9}" />

  <button type="submit">提交</button>
</form>

const form = document.getElementById('myForm')

form.addEventListener('submit', (e) => {
  // 检查表单是否有效
  if (!form.checkValidity()) {
    e.preventDefault()
    // 显示验证错误
    form.reportValidity()
  }
})

// 检查单个字段
const input = form.username
console.log(input.validity.valid) // 是否有效
console.log(input.validity.valueMissing) // 是否缺少值
console.log(input.validity.typeMismatch) // 类型不匹配
console.log(input.validity.patternMismatch) // 模式不匹配
console.log(input.validity.tooShort) // 太短
console.log(input.validity.tooLong) // 太长
console.log(input.validity.rangeUnderflow) // 小于最小值
console.log(input.validity.rangeOverflow) // 大于最大值

// 自定义错误消息
input.setCustomValidity('用户名已存在')
input.setCustomValidity('') // 清除自定义错误

const validators = {
  required(value) {
    return value.trim() !== '' || '此字段必填'
  },

  email(value) {
    const regex = /^[\w.-]+@[\w.-]+\.\w+$/
    return regex.test(value) || '请输入有效的邮箱'
  },

  phone(value) {
    const regex = /^1[3-9]\d{9}$/
    return regex.test(value) || '请输入有效的手机号'
  },

  minLength(min) {
    return (value) => value.length >= min || `至少需要 ${min} 个字符`
  },

  maxLength(max) {
    return (value) => value.length <= max || `最多 ${max} 个字符`
  },

  pattern(regex, message) {
    return (value) => regex.test(value) || message
  },

  match(fieldName) {
    return (value, form) => {
      const other = form.elements[fieldName].value
      return value === other || '两次输入不一致'
    }
  },
}

// 验证函数
function validateField(field, form) {
  const rules = field.dataset.validate?.split('|') || []
  const value = field.value

  for (const rule of rules) {
    let validator
    let params

    if (rule.includes(':')) {
      const [name, param] = rule.split(':')
      validator = validators[name]
      params = param
    } else {
      validator = validators[rule]
    }

    if (typeof validator === 'function') {
      const fn = params ? validator(params) : validator
      const result = fn(value, form)

      if (result !== true) {
        return result // 返回错误消息
      }
    }
  }

  return true
}

// 使用
// <input name="email" data-validate="required|email">
// <input name="password" data-validate="required|minLength:6">

class FormValidator {
  constructor(form) {
    this.form = form
    this.errors = {}
    this.setupListeners()
  }

  setupListeners() {
    this.form.addEventListener('submit', (e) => {
      if (!this.validate()) {
        e.preventDefault()
      }
    })

    this.form.addEventListener(
      'blur',
      (e) => {
        if (e.target.matches('input, textarea, select')) {
          this.validateField(e.target)
          this.showFieldError(e.target)
        }
      },
      true
    )
  }

  validate() {
    this.errors = {}

    for (const field of this.form.elements) {
      if (field.name) {
        this.validateField(field)
      }
    }

    this.showErrors()
    return Object.keys(this.errors).length === 0
  }

  validateField(field) {
    const { name, value } = field
    const rules = this.getRules(field)

    delete this.errors[name]

    for (const { validator, message } of rules) {
      if (!validator(value)) {
        this.errors[name] = message
        break
      }
    }
  }

  getRules(field) {
    const rules = []

    if (field.required) {
      rules.push({
        validator: (v) => v.trim() !== '',
        message: '此字段必填',
      })
    }

    if (field.type === 'email') {
      rules.push({
        validator: (v) => /^[\w.-]+@[\w.-]+\.\w+$/.test(v),
        message: '请输入有效的邮箱',
      })
    }

    if (field.minLength > 0) {
      rules.push({
        validator: (v) => v.length >= field.minLength,
        message: `至少 ${field.minLength} 个字符`,
      })
    }

    return rules
  }

  showErrors() {
    // 清除所有错误
    this.form.querySelectorAll('.error-message').forEach((el) => el.remove())
    this.form.querySelectorAll('.is-invalid').forEach((el) => {
      el.classList.remove('is-invalid')
    })

    // 显示新错误
    for (const [name, message] of Object.entries(this.errors)) {
      const field = this.form.elements[name]
      this.showFieldError(field, message)
    }
  }

  showFieldError(field, message = this.errors[field.name]) {
    field.classList.toggle('is-invalid', !!message)

    const existing = field.parentNode.querySelector('.error-message')
    if (existing) existing.remove()

    if (message) {
      const errorEl = document.createElement('div')
      errorEl.className = 'error-message'
      errorEl.textContent = message
      field.parentNode.appendChild(errorEl)
    }
  }
}

// 使用
const validator = new FormValidator(document.getElementById('myForm'))

const form = document.getElementById('myForm')

// 从表单创建
const formData = new FormData(form)

// 获取值
formData.get('username')
formData.getAll('hobbies') // 多个同名字段

// 设置值
formData.set('username', '新用户名')
formData.append('extra', '额外数据')

// 删除
formData.delete('password')

// 检查是否存在
formData.has('username')

// 遍历
for (const [key, value] of formData) {
  console.log(`${key}: ${value}`)
}

// 转为对象
const data = Object.fromEntries(formData)

// 转为 JSON（需处理多值情况）
function formDataToJson(formData) {
  const obj = {}
  for (const [key, value] of formData) {
    if (obj[key]) {
      // 处理多值
      if (!Array.isArray(obj[key])) {
        obj[key] = [obj[key]]
      }
      obj[key].push(value)
    } else {
      obj[key] = value
    }
  }
  return JSON.stringify(obj)
}

const form = document.getElementById('uploadForm')

form.addEventListener('submit', async (e) => {
  e.preventDefault()

  const formData = new FormData(form)
  const fileInput = form.querySelector('input[type="file"]')

  // 检查文件
  const file = fileInput.files[0]
  if (file) {
    console.log('文件名:', file.name)
    console.log('大小:', file.size)
    console.log('类型:', file.type)

    // 验证文件类型
    const allowedTypes = ['image/jpeg', 'image/png', 'image/gif']
    if (!allowedTypes.includes(file.type)) {
      alert('只支持 JPG、PNG、GIF 图片')
      return
    }

    // 验证文件大小（2MB）
    if (file.size > 2 * 1024 * 1024) {
      alert('文件不能超过 2MB')
      return
    }
  }

  // 上传
  try {
    const response = await fetch('/api/upload', {
      method: 'POST',
      body: formData, // 不要设置 Content-Type
    })
    const result = await response.json()
    console.log('上传成功:', result)
  } catch (error) {
    console.error('上传失败:', error)
  }
})

function uploadWithProgress(url, formData, onProgress) {
  return new Promise((resolve, reject) => {
    const xhr = new XMLHttpRequest()

    xhr.upload.addEventListener('progress', (e) => {
      if (e.lengthComputable) {
        const percent = (e.loaded / e.total) * 100
        onProgress(percent)
      }
    })

    xhr.addEventListener('load', () => {
      if (xhr.status >= 200 && xhr.status < 300) {
        resolve(JSON.parse(xhr.responseText))
      } else {
        reject(new Error(`HTTP ${xhr.status}`))
      }
    })

    xhr.addEventListener('error', () => reject(new Error('网络错误')))

    xhr.open('POST', url)
    xhr.send(formData)
  })
}

// 使用
const formData = new FormData(form)
uploadWithProgress('/api/upload', formData, (percent) => {
  progressBar.style.width = `${percent}%`
  progressText.textContent = `${Math.round(percent)}%`
}).then((result) => {
  console.log('完成:', result)
})

function serializeForm(form) {
  const data = {}

  for (const field of form.elements) {
    if (!field.name || field.disabled) continue

    switch (field.type) {
      case 'checkbox':
        if (field.checked) {
          if (data[field.name]) {
            data[field.name] = [].concat(data[field.name], field.value)
          } else {
            data[field.name] = field.value
          }
        }
        break

      case 'radio':
        if (field.checked) {
          data[field.name] = field.value
        }
        break

      case 'select-multiple':
        data[field.name] = [...field.selectedOptions].map((o) => o.value)
        break

      case 'file':
        // 跳过文件字段
        break

      default:
        data[field.name] = field.value
    }
  }

  return data
}

class FormState {
  constructor(form) {
    this.form = form
    this.initialState = this.getState()
  }

  getState() {
    return serializeForm(this.form)
  }

  isDirty() {
    const current = this.getState()
    return JSON.stringify(current) !== JSON.stringify(this.initialState)
  }

  reset() {
    this.form.reset()
    this.initialState = this.getState()
  }

  save() {
    this.initialState = this.getState()
  }
}

// 使用
const formState = new FormState(form)

// 离开页面前检查
window.addEventListener('beforeunload', (e) => {
  if (formState.isDirty()) {
    e.preventDefault()
    e.returnValue = ''
  }
})

// 动态添加字段
function addField(form, name, type = 'text') {
  const wrapper = document.createElement('div')
  wrapper.className = 'form-field'
  wrapper.innerHTML = `
    <input type="${type}" name="${name}" placeholder="${name}">
    <button type="button" class="remove-field">删除</button>
  `

  wrapper.querySelector('.remove-field').addEventListener('click', () => {
    wrapper.remove()
  })

  form.insertBefore(wrapper, form.querySelector('[type="submit"]'))
}

// 动态字段验证
document.getElementById('addFieldBtn').addEventListener('click', () => {
  const fieldCount = form.querySelectorAll('.form-field').length
  addField(form, `field_${fieldCount + 1}`)
})