JavaScript 变量回退
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JavaScript Variable fallback
提问by thugsb
Please can someone explain to me what this line of code does:
请有人向我解释这行代码的作用：
var list = calls[ev] || (calls[ev] = {});
My best guess:
我最好的猜测：
It's setting the variable "list" with the value of calls.xxx, where xxx is a variable, ev. If calls[ev] doesn't exist, then it's creating it as an empty object and assigning that empty object to "list". Is that right?
它使用calls.xxx 的值设置变量“list”，其中xxx 是一个变量ev。如果 call[ev] 不存在，那么它会将它创建为一个空对象并将该空对象分配给“列表”。是对的吗？
Why are the parenthesis being used? Where can I find out more info on using || when setting variables, and the use of parenthesis in this context? Thanks!
为什么要使用括号？我在哪里可以找到有关使用 || 的更多信息 设置变量时，以及在这种情况下使用括号？谢谢！
回答by Alexander Pavlov
This code is equivalent to
这段代码相当于
var list;
if (calls[ev])
list = calls[ev];
else {
calls[ev] = {};
list = calls[ev];
}
Two features of the language are used:
使用了该语言的两个特性：
- The shortcutcomputation of boolean expressions (consider
a || b. Ifaistruethenbis not evaluated). Thus, if you assignvar v = a || b;andaevaluates to something that can be cast totrue, thenbis not evaluated. - The assignment statement evaluates to the last assigned value (to enable
var a = b = c;)
- 布尔表达式的快捷计算（考虑
a || b. Ifaistruethenb不计算）。因此，如果您对可以转换为 的内容进行分配var v = a || b;和a评估true，则b不会对其进行评估。 - 赋值语句计算为最后一个赋值（以启用
var a = b = c;）
The parentheses are necessary to avoid this interpretation:
括号是必要的，以避免这种解释：
var list = (calls[ev] || calls[ev]) = {};
(which is an error).
（这是一个错误）。
回答by Alex Turpin
Your guess is right. This is a common way to declare "default" values for variables in JavaScript.
你的猜测是对的。这是在 JavaScript 中为变量声明“默认”值的常用方法。
function foo(bar) {
var bar = bar || 0; //This sets bar to 0 if it's not already set
console.log(bar);
}
The way this works is that in JavaScript, an undefined variable is falsy, meaning that in any boolean comparaison operation, it will evaluate to false. You can then use the OR operator to combine two values and it will return the first value that evaluates to true.
它的工作方式是在 JavaScript 中，未定义的变量是假的，这意味着在任何布尔比较运算中，它的计算结果为false。然后，您可以使用 OR 运算符组合两个值，它将返回计算结果为 的第一个值true。
回答by Paul Dixon
||or 'logical OR' has a higher precedencethan the assignment operator =, thus the parentheses are necessary to ensure this idiom evaluates in the right order
||或 'logical OR' 的优先级高于赋值运算符=，因此括号是必要的，以确保此习语以正确的顺序计算
The other thing to be aware of is that many languages, Javascript included, provide short-circuit evaluationof boolean operators like AND and OR. If the first operand of a logical-or evaluates true, there is no need to evaluate the second operand, as it would make no difference to the result.
另一件需要注意的事情是，包括 Javascript 在内的许多语言都提供了对 AND 和 OR 等布尔运算符的短路评估。如果逻辑或的第一个操作数计算为真，则无需计算第二个操作数，因为它不会对结果产生影响。
Understand this, and you'll see this isn't some special assignment syntax, but an idiom, or pattern, that exploits a language feature to provide a more compact representation of an idea.
理解这一点，您就会发现这不是一些特殊的赋值语法，而是一种习惯用法或模式，它利用语言功能来提供更紧凑的想法表示。
回答by joidegn
you are right about your first guess. This is a common pattern for initialising javascript namespaces. It serves to make sure you dont overwrite a previous object with the same name. Most popular libraries will do something similar in order to create their namespace objects.
你的第一个猜测是对的。这是初始化 javascript 命名空间的常见模式。它用于确保您不会覆盖具有相同名称的先前对象。大多数流行的库都会做类似的事情来创建它们的命名空间对象。
The parenthesis is there so that the expressions are evaluated in the proper order.
括号在那里，以便以正确的顺序评估表达式。
