1.Covert to Boolean

以下值会被转为false :

• null
• undefined
• 0
• -0
• NaN
• ""

[JavaScript]
var bl= !(!(null));
console.log(bl);  //=> "false"

2.Covert to Number

1. 以数字表示的字符串可以直接转换为数字，允许开始或结尾处带有空格，如“123”、“ 123.45 ”
2. 空格在数字中间，将造成字符串转为NaN，如“123 45”
3. 含有非数字字符的字符串，将造成字符串转为NaN，如“123T45”

[JavaScript]
var num = +"123.45";
console.log(num);  //=> 123.45
num = +"  123.45  ";
console.log(num);  //=> 123.45
num = +"123 45";
console.log(num);  //=> NaN
num = +"123T45";
console.log(num);  //=> NaN

3.Covert to String