1 变成number

1.1 Number()

Number("1");            //1
Number(null);           //0
Number(undefined);      //NaN
Number(true);           //1
Number(false);          //0
Number({name:'jack'});  //NaN

1.2 parseInt()

parseInt("1",10);           //1
parseInt(null,10);          //NaN
parseInt(undefined,10);     //NaN
parseInt(true,10);          //NaN
parseInt(false,10);         //NaN
parseInt({name:'jack'},10); //NaN

1.3 -0

"1" - 0;            //1
null - 0;           //0
undefined - 0;      //NaN
true - 0;           //1
false - 0;          //0
{name:'jack'} - 0;  //-0

1.4 parseFloat()

2 变成string

2.1 toString()

var a = 1;
a.toString();       //"1"
var a = null;
a.toString();       //Uncaught TypeError:Cannot read property 'toString' of null(…)
var a = undefined;
a.toString();       //Uncaught TypeError:Cannot read property 'toString' of null(…)
var a = true;
a.toString();       //"true"
var a = false;
a.toString();       //"false"
var a = {name:'Jack'};
a.toString();       //"[object Object]"

2.2 +空字符串（注意是空字符串不是空格字符串，中间没有空格）

1 + "";                 //"1"
null + "";              //"null"
undefined + "";         //"undefined"
true + "";              //"true"
false + "";             //"false"
var a = {name:"Jack"};
a + ""                  //"[object Object]"

3 变成Boolean

3.1 Boolean()

Boolean(1)      //true

3.2 两次取反

!!1             //true

关于具体值在https://blog.csdn.net/qq_37298706/article/details/81672507中有说明

4 关于parseInt()

parseInt("1a1")         //1
parseInt("11a")         //11

一直读到不能转成int的字符为止