JS类型隐式转换-ToPrimitive

本篇仅对隐式转换中的知识点做记录,不深入展开介绍.

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注意: PreferredType默认为Number,当input为Date类型数据时,PreferredType为String

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1. valueOf()

var num = new Number('123');
num.valueOf(); // 123

var str = new String('12df');
str.valueOf(); // '12df'

var bool = new Boolean('fd');
bool.valueOf(); // true

var a = new Date();
a.valueOf(); // 1532234132475

var a = new Array();
a.valueOf();  //[]
a.valueOf() === a;  // true

var b = new Object({});
b.valueOf(); //{}
b.valueOf() === b; // true

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2.toString()

var num = new Number('123sd');
num.toString(); // 'NaN'

var str = new String('12df');
str.toString(); // '12df'

var bool = new Boolean('fd');
bool.toString(); // 'true'

var arr = new Array(1,2);
arr.toString(); // '1,2'

var d = new Date();
d.toString(); // "Sun Jul 22 2018 12:38:42 GMT+0800 (中国标准时间)"

var func = function () {}
func.toString(); // "function () {}"

var obj = new Object({});
obj.toString(); // "[object Object]"

Math.toString(); // "[object Math]"

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type(x)	type(y)	result(x==y)
null	undefined	true
String	Number	Number(x)==y
Boolean	any	Number(x)==y
Object	String or Number	toPrimitive(x)==y
NaN	any	false

[]==0; //true

1.[]=>ValueOf([])=>[];
2.[]=>toSring([])=>""

""==0

3.""=>Number("")=>0

0==0

[]=="0" //false

1.[]=>ValueOf([])=>[];
2.[]=>toSring([])=>""

""=="0"

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type(x)	type(y)	result(x+y)
String	any	x+toString(y)
Object	any	toString(x)+toString(y)
Boolean	Number or Boolean	Number(x)+Number(y)

"1ac"+true   // "1actrue"
"1ac"+[]    // "1ac"
"1ac"+[1,'a']   // "1ac1,a"
"1ac"+{}   // "1ac[object Object]"

{}+true   // "[object Object]true"
{}+[]   //  "[object Object]"

true+true   // 2
true+1   // 2
false+1   // 1