JavaScript 类型转换:(true && 1) vs (true | | 1) [英] JavaScript type conversion: (true && 1) vs (true | | 1)

问题描述

JavaScript 是非严格类型的语言，例如 Java.

我们知道，它根据上下文转换结果的值:

"2" + "3" 结果 "23"

"2" * "3" 结果 6

这个比较清楚，可以理解.

我只是尝试了以下表达式并感到困惑:

真 &&1 结果 1
真||1 结果真

为什么第一个给出数字，第二个给出布尔值?

考虑到 JavaScript 转换规则，由于 boolean 表达式的上下文，我希望在这两种情况下都能得到 boolean 值.

解决方案

检查 Douglas Crockford 的网站，它说:

<块引用>

&&运算符通常称为逻辑与.它也可以称为警卫.如果第一个操作数为 false、null、未定义，"(空string) 或数字 0 则返回第一个操作数.否则，它返回第二个操作数.这提供了一种方便的编写方式空检查:

var value = p &&p.名称；/* 名称值只会从p 如果 p 有值，则避免错误.*/

<块引用>

||运营商通常是称为逻辑或.它也可以称为默认值.如果第一个操作数为 false、null、未定义、""(空字符串)或数字 0，然后它返回第二个操作数.否则，它返回第一个操作数.这提供了一种指定默认值的便捷方式:

值 = v ||10个；/* 使用 v 的值，但如果 v 没有值，改用 10.*/

JavaScript is non-strictly typed language as Java,for example.

As we know, it converts value of result dependently upon context:

"2" + "3" results "23"

"2" * "3" results 6

This is quite clear and OK for understanding.

I just tried following expressions and got confused:

true && 1 results 1
true || 1 results true

Why the first gives Number and the second gives boolean?

Considering JavaScript conversion rules,I expect to get boolean values in both cases,due to boolean context of expression.

解决方案

Check Douglas Crockford's site, it says:

The && operator is commonly called logical and. It can also be called guard. If the first operand is false, null, undefined, "" (the empty string), or the number 0 then it returns the first operand. Otherwise, it returns the second operand. This provides a convenient way to write a null-check:

var value = p && p.name; /* The name value will only be retrieved from
p if p has a value, avoiding an error. */

The || operator is commonly called logical or. It can also be called default. If the first operand is false, null, undefined, "" (the empty string), or the number 0, then it returns the second operand. Otherwise, it returns the first operand. This provides a convenient way to specify default values:

value = v || 10; /* Use the value of v, but if v doesn't have a value,
use 10 instead. */

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