参考文章：《Truth, Equality and JavaScript》
《JavaScript 中的相等性判断》
也许你运行代码时会感到困惑，为什么字符串可以作为判断条件，字符串和true 进行 运算结果会是true
if ("potato") {
console.log("potato" == false); //false
console.log("potato" == true); //false
}
在JavaScript中，所有的条件语句和操作符遵循相同的强制转换规范。 比如说if（Expression）语句将使用抽象方法ToBoolean将表达式求值的结果强制为布尔值，为此，ES5规范定义了以下算法：
| Argument Type | Result |
|---|---|
| Undefined | false |
| Null | false |
| Boolean | The result equals the input argument (no conversion). |
| Number | The result is false if the argument is +0, −0, or NaN;otherwise the result is true. |
| String | The result is false if the argument is the empty String (its length is zero);otherwise the result is true. |
| Object | true |
所以 "potato" 作为表达式 转换结果将是 true
==那么 "potato" == false是怎么进行运算的，如果"potato" == 1呢？==
运算规则如下：
| Type(x) | Type(y) | Result |
|---|---|---|
| x and y are the same type | See Strict Equality (===) Algorithm | |
| null | Undefined | true |
| Undefined | null | true |
| Number | String | x == toNumber(y) |
| String | Number | toNumber(x) == y |
| Boolean | (any) | toNumber(x) == y |
| (any) | Boolean | x == toNumber(y) |
| String or Number | Object | x == toPrimitive(y) |
| Object | String or Number | toPrimitive(x) == y |
| otherwise… | false |
那么toNumber ,toPrimitive 是按照什么规则进行转换的
ToNumber
| Argument Type | Result |
|---|---|
| Undefined | NaN |
| Null | +0 |
| Boolean | The result is 1 if the argument is true.The result is +0 if the argument is false. |
| Number | The result equals the input argument (no conversion). |
| String | In effect evaluates Number(string);“abc” -> NaN;“123” -> 123 |
| Object | Apply the following steps:1. Let primValue be ToPrimitive(input argument, hint Number).2. Return ToNumber(primValue). |
ToPrimitive
| Argument Type | Result |
|---|---|
| Object | (in the case of equality operator coercion) if valueOf returns a primitive, return it. Otherwise if toString returns a primitive return it. Otherwise throw an error |
| otherwise… | The result equals the input argument (no conversion). |
