javascript 你能在变量声明中添加条件吗？
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Can you add a condition to a variable declaration?
提问by Kayote
This doesn't make sense to me, but I have a feeling I saw a code using this:
这对我来说没有意义，但我有一种感觉，我看到了一个使用这个的代码：
var abc = def || ghi;
My question is, is this valid? Can we add a condition to a variable declaration? I imagine the answer is no but I have this at the back of my mind that I saw something similar in code once.
我的问题是，这有效吗？我们可以在变量声明中添加条件吗？我想答案是否定的，但我想我曾经在代码中看到过类似的东西。
回答by Denys Séguret
This gives to abcthe value of defif it isn't falsy (i.e. not false, null, undefined, 0or an empty string), or the value of ghiif not.
这得到abc的值def，如果它不是falsy（即不false，null，undefined，0或空字符串），或的值ghi如果不是。
This is equivalent to:
这相当于：
var abc;
if (def) abc = def;
else abc = ghi;
This is commonly used for options:
这通常用于选项：
function myfunc (opts) {
var mything = opts.mything || "aaa";
}
If you call myfunc({mything:"bbb"})it uses the value you give. It uses "aaa"if you provide nothing.
如果您调用myfunc({mything:"bbb"})它，则使用您提供的值。"aaa"如果你什么都不提供，它就会使用。
In this case, in order to let the caller wholly skip the parameter, we could also have started the function with
在这种情况下，为了让调用者完全跳过参数，我们也可以用
opts = opts || {};
回答by epascarello
The code var abc = def || ghi;
代码变量 abc = def || ghi;
is the same thing as
是一样的
if (def) { //where def is a truthy value
var abc = def;
} else {
abc = ghi;
}
You want a condition like an if statement?
你想要一个像 if 语句这样的条件吗？
if (xxx==="apple") {
var abc = def;
} else {
abc = ghi;
}
which as written as a ternary operator is:
写成三元运算符是：
var abc = (xxx==="apple") ? def : ghi;
回答by Jashwant
Yes, you can add condition to variable declaration
是的，您可以在变量声明中添加条件
You can use it like this,
你可以像这样使用它，
function greet(person) {
var name = person || 'anonymouse';
alert('Hello ' + name);
}
greet('jashwant');
greet();?
回答by Praveen Kumar Purushothaman
OKay, see, it is something like, you either check if one is true. The true one will be returned. :)
好吧，看，这就像，你要么检查一个是不是真的。真正的会回来的。:)
var abc = def || ghi;
Is equivalent to:
相当于：
var abc = return (def == true) or (ghi == true)
