为什么逻辑运算符（&&和||）总是返回布尔结果？ [英] Why don't logical operators (&& and ||) always return a boolean result?

问题描述

为什么这些逻辑运算符返回一个对象而不是布尔值？

Why do these logical operators return an object and not a boolean?

var _ = (obj.fn && obj.fn() ) || obj._ || ( obj._ = {} );

var _ = obj && obj._;

我想了解为什么它会返回 obj.fn（）<的结果/ code>（如果已定义）或 obj ._ 但不是布尔结果。

I want to understand why it returns result of obj.fn() (if it is defined) OR obj._ but not boolean result.

推荐答案

var _ = ((obj.fn && obj.fn() ) || obj._ || ( obj._ == {/* something */}))? true: false 

将返回布尔值。

更新

请注意，这是基于我的测试。我不能完全依赖。

Note that this is based on my testing. I am not to be fully relied upon.

这是一个表达式不分配 true 或 false 值。而是指定计算值。

It is an expression that does not assign true or false value. Rather it assigns the calculated value.

让我们来看看这个表达式。

Let's have a look at this expression.

示例表达式：

var a = 1 || 2;
// a = 1

// it's because a will take the value (which is not null) from left
var a = 0 || 2;
// so for this a=2; //its because the closest is 2 (which is not null)

var a = 0 || 2 || 1;    //here also a = 2;

您的表达式：

var _ = (obj.fn && obj.fn() ) || obj._ || ( obj._ = {} );

// _ = closest of the expression which is not null
// in your case it must be (obj.fn && obj.fn())
// so you are gettig this

另一个表达式：

var a = 1 && 2;
// a = 2

var a = 1 && 2 && 3;
// a = 3 //for && operator it will take the fartest value
// as long as every expression is true

var a = 0 && 2 && 3;
// a = 0

另一个表达式：

var _ = obj && obj._;

// _ = obj._

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