var FOO = FOO ||如何Javascript中的{} idiom工作？ [英] How does the var FOO = FOO || {} idiom in Javascript work?

问题描述

从这个问题：什么是var FOO = FOO || {}" （将变量或空对象分配给该变量）是指Javascript？

我已经知道 var FOO = FOO || {} 本质上意味着如果存在FOO，则保持不变，否则将其设为空对象。

I've learned that var FOO = FOO || {} essentially means "If FOO exists, then leave it untouched, else make it an empty object".

但是如何？

这就是我解析这种语法的方法：

This is how I would parse this syntax:

var FOO =（FOO || { }）

所以：如果 FOO 存在并且评估为布尔值True，然后（FOO || {}）将返回True，因此最终 FOO 将被完全覆盖并保持布尔值从现在起的真值。

So: If FOO exists AND evaluates to Boolean value of True, then (FOO || {}) will return True, so eventually FOO will be completely overwritten and will hold the Boolean value of True from now on.

否则（FOO || {}）将返回任何布尔值 {} evalueates to。由于 {} 的空对象总是计算为True ...

Else (FOO || {}) will return to whatever Boolean value {} evalueates to. Since an empty object, which {} is, always evaluates to True...

然后在任何情况下（FOO || {}）应该评估为True，所以......

Then in ANY case (FOO || {}) should evaluate to True, so...

在任何可能的情况下，评估后 var FOO = FOO || {} ，FOO应该保持普通的布尔值True，无论之前持有什么。基本上，根据我的理解， var FOO = FOO || {} 应相当于 var FOO = True 。

In ANY POSSIBLE CASE, after evaluating var FOO = FOO || {}, FOO should hold the trivial Boolean value of True, regardless of whatever it was holding before. Essentially, to my understanding, var FOO = FOO || {} should be equivalent to var FOO = True.

我的错误在哪里？

推荐答案

如果FOO存在且评估为布尔值True，则（FOO || { }）将返回True

If FOO exists AND evaluates to Boolean value of True, then (FOO || {}) will return True

这不是 || 运算符的方式适用于JS。

That isn't how the || operator works in JS.

正确的解释是：

如果左侧是真值，请评估作为左侧（即 FOO ），否则评估为右侧（即 {} ）。

If the left hand side is a true value, evaluate as the left hand side (i.e. FOO), otherwise evaluate as the right hand side (i.e. {}).

var zero = 0;
var one = 1;
var two = 2;

console.log(zero || two);
console.log(one || two);

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