JavaScript 中没有“Else”的三元运算符

Ternary Operators in JavaScript Without an "Else"

提问by Oscar Godson

I've always had to put nullin the else conditions that don't have anything. Is there anyway around it? E.g.

我总是不得不放入null没有任何东西的 else 条件。反正周围有吗？例如

condition ? x = true : null;

basically, is there a way to do:

基本上，有没有办法做到：

condition ? x = true;

Now it shows up as a syntax error

现在它显示为语法错误

FYI, here is some real example code:

仅供参考，这是一些真实的示例代码：

!defaults.slideshowWidth ? defaults.slideshowWidth = obj.find('img').width()+'px' : null;

回答by Sean Kinsey

First of all, a ternary expression is not a replacement for an if/else construct - its an equivalent to an if/else construct that returnsa value. That is, an if/else clause is code, a ternary expression is an expression, meaning that it returns a value.

首先，三元表达式不是 if/else 结构的替代品——它等同于返回值的 if/else 结构。也就是说，if/else 子句是代码，三元表达式是表达式，意味着它返回一个值。

This mean several things:

这意味着几件事：

• use ternary expressions only when you have a variable on the left side of the =that is to be assigned the return value
• only use ternary expressions when the returned value is to be one of two values (or use nested expressions if that is fitting)
• each part of the expression (after ? and after : ) should return a value without side effects (the expression x = truereturns true as all expressions return the last value, but also changes x without x having any effect on the returned value)

• 仅当您在左侧有一个=要分配返回值的变量时才使用三元表达式
• 仅当返回值是两个值之一时才使用三元表达式（如果合适，则使用嵌套表达式）
• 表达式的每个部分（在 ? 之后和 : 之后）都应该返回一个没有副作用的值（表达式x = true返回 true 因为所有表达式都返回最后一个值，但也会改变 x 而 x 对返回值没有任何影响）

In short - the 'correct' use of a ternary expression is

简而言之 - 三元表达式的“正确”使用是

var resultofexpression = conditionasboolean ? truepart: falsepart;

Instead of your example condition ? x=true : null ;, where you use a ternary expression to set the value of x, you can use this:

代替您的 example condition ? x=true : null ;，您可以使用三元表达式来设置 的值x，您可以使用以下命令：

 condition && (x = true);

This is still an expression and might therefore not pass validation, so an even better approach would be

这仍然是一个表达式，因此可能无法通过验证，因此更好的方法是

 void(condition && x = true);

The last one will pass validation.

最后一个将通过验证。

But then again, if the expected value is a boolean, just use the result of the condition expression itself

但话又说回来，如果预期值是一个布尔值，只需使用条件表达式本身的结果

var x = (condition); // var x = (foo == "bar");

---

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UPDATEIn relation to your sample this is probably more appropriate:

更新关于您的样本，这可能更合适：

defaults.slideshowWidth = defaults.slideshowWidth || obj.find('img').width()+'px';

回答by In silico

No, it needs three operands. That's why they're called ternaryoperators.

不，它需要三个操作数。这就是为什么它们被称为三元运算符。

However, for what you have as your example, you can do this:

但是，对于您所拥有的示例，您可以这样做：

if(condition) x = true;

Although it's safer to have braces if you need to add more than one statement in the future:

如果将来需要添加多个语句，使用大括号会更安全：

if(condition) { x = true; }

Edit:Now that you mention the actual code in which your question applies to:

编辑：既然您提到了您的问题适用的实际代码：

if(!defaults.slideshowWidth)
    { defaults.slideshowWidth = obj.find('img').width()+'px'; }

回答by Eugene Tiurin

More often people get use of logical operators to shorten the statement syntax:

人们更多地使用逻辑运算符来缩短语句语法：

!defaults.slideshowWidth &&
  (defaults.slideshowWidth = obj.find('img').width()+'px');

But in your particular case the syntax can be even simpler

但是在您的特定情况下，语法可以更简单

defaults.slideshowWidth = defaults.slideshowWidth || obj.find('img').width()+'px';

This code will return the defaults.slideshowWidthvalue if the defaults.slideshowWidthis evaluated to true and obj.find('img').width()+'px'value otherwise.

defaults.slideshowWidth如果defaults.slideshowWidth被评估为真，则此代码将返回obj.find('img').width()+'px'值，否则返回值。

See the Short-Circuit Evaluationof logical operators for details.

有关详细信息，请参阅逻辑运算符的短路评估。

回答by philfreo

var x = condition || null;

回答by mdma

You could write

你可以写

x = condition ? true : x;

So that x is unmodified when the condition is false.

所以当条件为假时 x 未被修改。

This then is equivalent to

这则相当于

if (condition) x = true

EDIT:

编辑：

!defaults.slideshowWidth 
      ? defaults.slideshowWidth = obj.find('img').width()+'px' 
      : null 

There are a couple of alternatives - I'm not saying these are better/worse - merely alternatives

有几个替代方案——我不是说这些更好/更糟——只是替代方案

Passing in null as the third parameter works because the existing value is null. If you refactor and change the condition, then there is a danger that this is no longer true. Passing in the exising value as the 2nd choice in the ternary guards against this:

将 null 作为第三个参数传递有效，因为现有值为 null。如果您重构并更改条件，则存在不再为真的危险。传入现有值作为三元中的第二个选择可防止这种情况：

!defaults.slideshowWidth = 
      ? defaults.slideshowWidth = obj.find('img').width()+'px' 
      : defaults.slideshowwidth 

Safer, but perhaps not as nice to look at, and more typing. In practice, I'd probably write

更安全，但可能不那么好看，而且打字更多。在实践中，我可能会写

defaults.slideshowWidth = defaults.slideshowWidth 
               || obj.find('img').width()+'px'

回答by Yandiro

What about simply

简单的怎么样

    if (condition) { code if condition = true };

回答by Krasimir Kirilov

In your case i see the ternary operator as redundant. You could assign the variable directly to the expression, using ||, && operators.

在您的情况下，我认为三元运算符是多余的。您可以使用 ||、&& 运算符将变量直接分配给表达式。

!defaults.slideshowWidth ? defaults.slideshowWidth = obj.find('img').width()+'px' : null ;

will become :

会变成 ：

defaults.slideshowWidth = defaults.slideshowWidth || obj.find('img').width()+'px';

It's more clear, it's more "javascript" style.

更清楚，它更像是“javascript”风格。

回答by fogx

To use a Ternary Operator without else inside of an array or object declaration you can use the ES6 spread operator ...()

要在数组或对象声明中使用没有 else 的三元运算符，您可以使用 ES6 扩展运算符 ...()

const cond = false;
const arr = [
  ...(cond ? ['a'] : []),
  'b',
];
    // ['b']

And for objects:

对于对象：

const cond = false;
const obj = {
  ...(cond ? {a: 1} : {}),
  b: 2,
};
    // {b: 2}

original source

原始来源