为什么if（" string"）评估" string"如果为真，但if（" string" == true）不是？ [英] why does if("string") evaluate "string" as true but if ("string"==true) does not?

问题描述

给出以下代码：

if ("string") {
    console.log('true!');
}
//logs "true" to the console
if ("string"==true) {
    console.log('true!');
}
//doesn't log anything

为什么会这样？我认为string被强制转换为数字，布尔值也是如此。所以 true 变为 1 ，而string变为的NaN 。第二个if语句有意义，但我不明白为什么第一个语句会导致内部循环被评估。这里发生了什么？

Why does this happen? I thought "string" was being cast to a number, as is the boolean. So true becomes 1, and "string" becomes NaN. The second if statement makes sense, but I don't see why the first statement causes the inner loop to be evaluated. What's going on here?

推荐答案

它被强制转换为布尔值。任何非空字符串的计算结果为true。

It is being cast to Boolean. Any non-empty string evaluates to true.

来自ECMAScript语言规范：

12.5 if 声明

语义

生产 IfStatement ： if（ Expression ） Statement else Statement 评估如下：

12.5 The if statement

Semantics

The production IfStatement: if ( Expression ) Statement else Statement is evaluated as follows:

1. 让 exprRef 是评估表达式的结果。


2. 如果ToBoolean（GetValue（ exprRef ））是 true ，然后
   
   
   
   • 返回评估第一个 Statement 的结果。
   
   

1. Let exprRef be the result of evaluating Expression.
2. If ToBoolean(GetValue(exprRef)) is true, then
   • Return the result of evaluating the first Statement.

• 返回评估第二个语句的结果。

• Return the result of evaluating the second Statement.

9.2 ToBoolean

抽象操作ToBoolean根据表11将其参数转换为Boolean类型的值：

9.2 ToBoolean

The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:

未定义： false

Null： false

布尔值：结果等于输入参数（无转换） ）。$
数字：如果参数为 +0 ， -0 或，则结果为 false 为NaN ;否则结果为 true 。

字符串：如果参数为空字符串（其长度为零），则结果为 false ;
否则结果为 true 。

对象： true

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

---

就 == 运算符而言，它很复杂，但它的要点是如果将数字与非数字进行比较，后者将转换为数字。如果将布尔值与非布尔值进行比较，则首先将布尔值转换为数字，然后应用前一句子。

---

As far as the == operator is concerned, it's complicated, but the gist of it is that if you compare a number to a non-number the latter is converted into a number. If you compare a boolean against a non-boolean, the boolean is first converted to a number, and then the previous sentence applies.

有关详细信息，请参阅第11.9.3节。

See section 11.9.3 for details.

// Call this x == y.
if ("string" == true) 

// Rule 6: If Type(y) is Boolean,
//         return the result of the comparison x == ToNumber(y).
if ("string" == Number(true))

// Rule 5: If Type(x) is String and Type(y) is Number,
//         return the result of the comparison ToNumber(x) == y.  
if (Number("string") == Number(true))

// The above is equivalent to:
if (NaN == 1)

// And NaN compared to *anything* is false, so the end result is:
if (false)

这篇关于为什么if（" string"）评估" string"如果为真，但if（" string" == true）不是？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！