构造 x = x || 是什么?你的意思是?

2022-01-29 00:00:00 parameters javascript optional-parameters or-operator

我正在调试一些 JavaScript，但无法解释这个 || 的作用:

I am debugging some JavaScript and can't explain what this || does:

function (title, msg) {
  var title = title || 'Error';
  var msg   = msg || 'Error on Request';
}

为什么这个人使用 var title = title ||'错误'?我有时也会看到它没有 var 声明.

Why is this guy using var title = title || 'ERROR'? I sometimes see it without a var declaration as well.

推荐答案

这意味着 title 参数是可选的.因此，如果您调用不带参数的方法，它将使用默认值 "Error".

It means the title argument is optional. So if you call the method with no arguments it will use a default value of "Error".

这是写作的简写:

if (!title) {
  title = "Error";
}

这种布尔表达式的速记技巧在 Perl 中也很常见.用表达式:

This kind of shorthand trick with boolean expressions is common in Perl too. With the expression:

a OR b

如果 a 或 b 为 true，则计算结果为 true.因此，如果 a 为真，则根本不需要检查 b.这称为短路布尔评估，因此:

it evaluates to true if either a or b is true. So if a is true you don't need to check b at all. This is called short-circuit boolean evaluation so:

var title = title || "Error";

基本上检查 title 的计算结果是否为 false.如果是，则返回"Error"，否则返回title.

basically checks if title evaluates to false. If it does, it "returns" "Error", otherwise it returns title.

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