javascript类型转换

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基本数据类型

共有String、Number、Boolean、null/undefined、Object、Symbol(es6新增)6种类型

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typeof undefined // "undefined"
typeof 0 // "number"
typeof true // "boolean"
typeof "foo" // "string"
typeof Symbol("id") // "symbol"
typeof Math // "object"  (1)
typeof null // "object"  (2)
typeof alert // "function"  (3)

String类型转换

当需要字符串形式的值时会发生转换，如：alert(value)

Number转换

Numeric conversion happens in mathematical functions and expressions automatically.如:

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when division / is applied to non-numbers:
alert( "6" / "2" ); // 3, strings are converted to numbers
Number('123') //转为123

  • 规则
    ‘+’ 计算时一边是string，则会把另一边也转为string
value Becomes…
undefined NaN
null 0
true/false 1 and 0
string Whitespaces from the start and end are removed. If the remaining string is empty, the result is 0. Otherwise, the number is “read” from the string. An error gives NaN.

Boolean转换

  • 空字符串、0、null、undefined、NaN转换为false
  • 其它为true

Object到基本类型转换

ObjectTo基本类型

  • 如果定义了Symbol.toPrimitive则按该方法转换
  • 如果没定义，且hint是string则, 按obj.toString,obj.valueOf顺序
  • 如果没定义，hint(number或default)，则obj.valueOf,obj.toString顺序