blocks|key|4522877|text|因为它是有效的语法，甚至是需要解释的有效代码。您可以尝试访问任何对象的任何属性(在本例中，0将强制转换为Number-object)，如果存在，它将提供值，否则未定义。但是，尝试访问未定义的属性不起作用，因此0将导致运行时错误。尽管如此，这仍然被归类为有效语法。什么是有效语法，什么不会导致运行时/编译时错误，这是有区别的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4522878|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Because it is valid syntax, and even valid code to be interpreted. You can try to access any property of any object(and in this case 0 will be cast to a Number-object), and it will give you the value if it exists, otherwise undefined. Trying to access a property of undefined does not work however, so 0[0][0] would result in a runtime error. This would still be classified as valid syntax though. There's a difference of what is valid syntax and what won't cause runtime/compiletime errors.

blocks|key|15237|text|在某些情况下，您可以在Javascript中有效地为数字加上下标：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|15238|->+0['toString']
function+toString()+{+[native+code]+}|code-block|syntax|javascript|15239|虽然你想要这样做的原因并不是一目了然，但Javascript中的下标等同于使用点符号(尽管点符号限制了你使用标识符作为键)。|15240|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

There are situations where you could validly subscript a number in Javascript:

<pre><code>-&gt; 0['toString']
function toString() { [native code] }
</code></pre>

While not immediately apparent why you would want to do this, subscripting in Javascript is equivalent to using dotted notation (albeit the dot notation limits you to using identifiers as keys).

blocks|key|15253|text|在JavaScript中，一切都是对象，所以当解释器解析它时，它将0视为对象，并尝试将0作为属性返回。当您尝试访问true或“”(空字符串)的第0个元素时，也会发生同样的事情。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|15254|即使您设置了0=1，它也会在内存中设置该属性和它的值，但是当您访问0时，它将被视为一个数字(这里不要混淆将其视为对象和数字。)|15255|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

In JavaScript, everything is object, so when interpreter parse it, it treats 0 as a object and tries to return 0 as a property. The same thing happens when you try to access 0th element of true or ""(empty string). 

Even if you set 0[0]=1, it will set the property and its value in memory, but while you access 0 it treats as a number (Don't get confuse between treating as Object and number here.)

blocks|key|4522987|text|不仅语法有效，结果不一定是undefined，尽管在大多数情况下，即使不是所有正常的情况下，它也会是。JS是最纯粹的面向对象语言之一。大多数所谓的OO语言都是面向类的，因为一旦创建，就不能更改对象的形式(绑定到类)，只能更改对象的状态。在JS中，你可以改变对象的状态和形式，这比你想象的要频繁得多。如果您误用它，这种能力会导致一些相当晦涩的代码。数字是不可变的，所以你不能改变对象本身，不能改变它的状态，也不能改变它的形式|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4522988|0[0]+=+1;|code-block|syntax|javascript|4522989|这是一个有效的赋值表达式，返回1，但实际上没有赋值任何东西，数字0是不可变的。这本身就有点奇怪。你可以有一个有效的、正确的(可执行的)赋值表达式，它没有赋值任何东西(*)。然而，数字的类型是可变的对象，所以您可以改变类型，并且更改将沿着原型链向下层叠。|4522990|Number[0]+=+1;
//print+1+to+the+console
console.log(0[0]);
//will+also+print+1+to+the+console+because+all+integers+have+the+same+type
console.log(1[0]);+|4522991|当然，这与正常使用类别相去甚远，但语言被指定为允许这一点，因为在其他场景中，扩展对象功能实际上有很大的意义。这是jQuery插件如何挂钩到jQuery对象来给出一个例子。|4522992|(*)它实际上将值1赋值给对象的属性，但是您无法引用该(临时)对象，因此它将在nexx+GC传递时被收集|4522993|entityMap^0|D|9|0|0|W|1|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@$9|Z|A|10|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|12|8|@]|D|@]|E|$]]|$1|Q|3|R|5|6|7|13|8|@]|D|@]|E|$]]|$1|S|3|-4|5|6|7|14|8|@]|D|@]|E|$]]]|T|$]]

Not only is the syntax valid the result does not have to be <code>undefined</code> though in most, if not all sane cases it will. JS is one of the most pure object oriented languages. Most so called OO languages are class oriented, in the sense that you can't change the form (it's tied to the class) of the object once created, only the state of the object. In JS you can change the state as well as the form of the object and this you do more often than you think. This ability makes for some rather obscure code, if you misuse it. Numerals are immutable, so you can't change the object itself, not it's state nor it's form so you could do

<pre><code>0[0] = 1;
</code></pre>

which is an valid assignment expression that returns 1 but doesn't actually assign anything, The numeral <code>0</code> is immutable. Which in itself is somewhat odd. You can have a valid and correct (executable) assingment expression, that doesn't assign anything(*). However the type of the numeral is a mutable object so you can mutate the type, and the changes will cascade down the prototype chain.

<pre><code>Number[0] = 1;
//print 1 to the console
console.log(0[0]);
//will also print 1 to the console because all integers have the same type
console.log(1[0]); 
</code></pre>

of course it's a far cry from the sane use category but the language is specified to allow for this because in other scenarios, extending the objects capabilities actually makes a lot of sense. It's how jQuery plugins hook into the jQuery object to give an example.

(*) It does actually assign the value 1 to the property of an object, however there's no way you can reference that (transcient) object and it will thus be collected at the nexx GC pass

Why is this line valid in javascript ?

<pre><code>var a = 0[0];
</code></pre>

After that, <code>a</code> is <code>undefined</code>.

Why is 0[0] syntactically valid?

解释器

jQuery

为什么这一行在javascript中是有效的？var a = 0[0];在那之后，a就是undefined。

问为什么0[0]在语法上是有效的？
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问为什么0[0]在语法上是有效的？EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问为什么0[0]在语法上是有效的？
EN