JavaScript if(x)，==和===解析(翻译整理)

一、if()中的布尔判断

if ( Expression ) 表达式会通过ES5定义的ToBoolean方法强制把Expression 转换成布尔值。

示例：

var trutheyTester = function(expr) {
    return expr ? "truthey" : "falsey";
}

trutheyTester({}); //truthey (an object is always true)

trutheyTester(false); //falsey
trutheyTester(new Boolean(false)); //truthey (an object!)

trutheyTester(""); //falsey
trutheyTester(new String("")); //truthey (an object!)

trutheyTester(NaN); //falsey
trutheyTester(new Number(NaN)); //truthey (an object!)

二、==

会对不同类型的数据进行强制转换以比较，记住两点即可：

1. undefined和null永远互相相等
2. 当两个不同类型的值进行比较，Object会被toPrimitive()转化为原始值，String和Boolean会被toNumber()转换为数字或NaN

toNumber()的转换规则：

ToPrimitive()转换规则：

示例：

[0] == true;

 1 //EQUALITY CHECK...
 2 [0] == true;
 3
 4 //HOW IT WORKS...
 5 //convert boolean using toNumber
 6 [0] == 1;
 7 //convert object using toPrimitive
 8 //[0].valueOf() is not a primitive so use...
 9 //[0].toString() -> "0"
10 "0" == 1;
11 //convert string using toNumber
12 0 == 1; //false!

“potato” == true;

//EQUALITY CHECK...
"potato" == true;

//HOW IT WORKS...
//convert boolean using toNumber
"potato" == 1;
//convert string using toNumber
NaN == 1; //false!

“potato” == false;

//EQUALITY CHECK...
"potato" == false;

//HOW IT WORKS...
//convert boolean using toNumber
"potato" == 0;
//convert string using toNumber
NaN == 0; //false!

object with valueOf;

//EQUALITY CHECK...
crazyNumeric = new Number(1);
crazyNumeric.toString = function() {return "2"};
crazyNumeric == 1;

//HOW IT WORKS...
//convert object using toPrimitive
//valueOf returns a primitive so use it
1 == 1; //true!

object with toString;

//EQUALITY CHECK...
var crazyObj  = {
    toString: function() {return "2"}
}
crazyObj == 1;

//HOW IT WORKS...
//convert object using toPrimitive
//valueOf returns an object so use toString
"2" == 1;
//convert string using toNumber
2 == 1; //false!

三、===

值和类型都必须严格相等：

1. 不同类型永远不相等
2. 对象必须引用自同一个对象才相等
3. 字符串必须包含相同的字符集合
4. 原型(primitives)共享同一个值才相等
5. NaN, null and undefined永远不和其他类型相等
6. NaN甚至不会和自己相等

参考资料：

1. 《Truth, Equality and JavaScript  》:https://javascriptweblog.wordpress.com/2011/02/07/truth-equality-and-javascript/