• Object => String 输出’[object object]’ 字符串
  • ({}).toString() =>’[object object]’
• Array => String
  • [12,23].toString() => ‘12,23’，对数字格式化输出
  • [12].toString() => ‘12’
  • [].toString() =>’’
• Functon => String 输出函数代码字符串
• RegExp => String 输出正则代码字符串
• Number = > String 输出数字字符串
• Boolean => String 输出布尔字符串
• Null => String Error
• undefined => String Error
  

  Number()

• String => Number || 只有数字字符串才能转为Number

  • Number(‘’) => 0

  • Number(‘12.5’) => 12.5
  • Number(‘12.5px’) => NaN
• Boolean = > Number || 转为1,0
  +Number(true) => 1
  +Number(false) => 0
• null => Number || 转为0
• undefined => Number || 转为NaN
• Object(引用类型) => Number || 先toString(),在Number()
  

  Boolean(),!!，!

• 总结： 只有 0,undefined,null,’’,NaN 转为false，其他为true

• NaN => Boolean || false
• Null => Boolean || false
• undefined => Boolean || false
• String => Boolean || ‘’为false,其他为true

  • Boolean(‘’) => false

• Number => Boolean || 0为false,其他为true

  • Boolean(0) => false

isNaN()

• can Number() ? [=> Number()] : [=> toString() => Number()]

• isNaN(null) = > false

• isNaN(undefined) => true