blocks|key|3054760|text|对我来说，从逻辑上讲，|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3054761|+-1也是arr的一部分。|blockquote|3054762|3054763|3054764|是的，但不是你想的那样。|3054765|您可以为数组分配任意属性(就像JavaScript中的任何其他对象一样)，这就是您在-1为数组“索引”并赋值时所做的事情。由于这不是数组的成员，而只是一个任意属性，因此您不应期望length会考虑该属性。|offset|length|style|CODE|3054766|换句话说，下面的代码做同样的事情：|3054767|​var+arr+=+[1,+2,+3];

​arr.cookies+=+4;

alert(arr.length)+//+3;|code-block|syntax|javascript|3054768|entityMap^0|0|0|0|0|0|16|2|2H|6|0|0|0^^$0|@$1|2|3|4|5|6|7|X|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Y|8|@]|9|@]|A|$]]|$1|E|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|10|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|11|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|12|8|@$K|13|L|14|M|N]|$K|15|L|16|M|N]]|9|@]|A|$]]|$1|O|3|P|5|6|7|17|8|@]|9|@]|A|$]]|$1|Q|3|R|5|S|7|18|8|@]|9|@]|A|$T|U]]|$1|V|3|-4|5|6|7|19|8|@]|9|@]|A|$]]]|W|$]]

<blockquote>
 SO for me logically it seems like arr[-1] is also a part of arr.
</blockquote>

Yes it is, but not in the way you think it is.

You can assign arbitrary properties to an array (just like any other Object in JavaScript), which is what you're doing when you "index" the array at <code>-1</code> and assign a value. Since this is not a member of the array and just an arbitrary property, you should not expect <code>length</code> to consider that property.

In other words, the following code does the same thing:

<pre><code>​var arr = [1, 2, 3];

​arr.cookies = 4;

alert(arr.length) // 3;
</code></pre>

blocks|key|3054792|text|JavaScript中的数组实际上是对象。它们只是从Array构造器中生成原型。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3054793|数组索引实际上是hashmap中的键，并且所有键都被转换为字符串。您可以创建任何键(即"-1")，但array上的方法被裁剪成类似数组的行为。所以length不是对象的大小，而是只保证大于最大的整数索引。类似地，打印arr将仅列出具有整数键>=+0的值。|offset|length|style|CODE|3054794|More+info+here.|3054795|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/JavaScript_syntax#Array^0|0|21|6|30|3|0|0|F|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|S|8|@$D|T|E|U|F|G]|$D|V|E|W|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|X|8|@]|9|@$D|Y|E|Z|1|10]]|A|$]]|$1|J|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|K|$L|$5|M|N|O|A|$P|Q]]]]

Arrays in JavaScript are actually objects. They are simply prototyped from the Array constructor. 

Array indices are actually keys in a hashmap, and all keys are converted to strings. You can create any key (i.e. "-1"), but the methods on Array are tailored to act like an array. So <code>length</code> isn't the size of the object, it's rather only guaranteed to be larger than the largest integer index. Similarly, printing <code>arr</code> will only list values with integer keys >= 0.

<a href="http://en.wikipedia.org/wiki/JavaScript_syntax#Array" rel="nofollow">More info here.</a>

blocks|key|206060|text|当您使用非正索引或非数字索引时，该数组的行为类似于具有键-值对的关联数组。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|206061|要遍历数组，您可以使用|206062|for(var+index+in+myArray)+{
++document.write(+index+%2B+"+:+"+%2B+myArray[index]+%2B+"<br+/>");
}|code-block|syntax|javascript|206063|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

When you use non-positive or non numeric indexes the array behaves as an associative array which has key-value pairs.

To iterate through the array you can use

<pre><code>for(var index in myArray) {
 document.write( index + " : " + myArray[index] + "&lt;br /&gt;");
}
</code></pre>

blocks|key|2841808|text|length属性将返回一个大于最高赋值“索引”的数字1，其中数组“索引”是大于或等于零的整数。请注意，JS允许“稀疏”数组：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2841809|var+someArray+=+[];
someArray[10]+=+"whatever";
console.log(someArray.length);+//+"11"|code-block|syntax|javascript|2841810|当然，如果没有元素，那么length就是0。还要注意，如果使用delete删除最高的元素，则length不会更新。|2841811|但数组是对象，因此您可以使用包括负数或分数在内的其他任意属性名称来分配属性：|2841812|someArray[-1]+=+"A+property";
someArray[3.1415]+=+"Vaguely+Pi";
someArray["test"]+=+"Whatever";|2841813|请注意，在幕后，JS会将属性名称转换为字符串，即使您提供了像-1这样的数字。(正整数索引也会变成字符串。)|2841814|数组方法，如.pop()、.slice()等，只适用于零或更高的整数“索引”，而不适用于其他属性，因此length在这一点上是一致的。|2841815|entityMap^0|0|6|0|0|C|6|K|1|V|6|1A|6|0|0|0|U|2|0|6|6|D|8|1F|6|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|10|8|@$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]|$9|17|A|18|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|19|8|@]|D|@]|E|$]]|$1|O|3|P|5|H|7|1A|8|@]|D|@]|E|$I|J]]|$1|Q|3|R|5|6|7|1B|8|@$9|1C|A|1D|B|C]]|D|@]|E|$]]|$1|S|3|T|5|6|7|1E|8|@$9|1F|A|1G|B|C]|$9|1H|A|1I|B|C]|$9|1J|A|1K|B|C]]|D|@]|E|$]]|$1|U|3|-4|5|6|7|1L|8|@]|D|@]|E|$]]]|V|$]]

The <code>length</code> property will return a number one higher than the highest assigned "index", where Array "indexes" are integers greater than or equal to zero. Note that JS allows "sparse" arrays:

<pre><code>var someArray = [];
someArray[10] = "whatever";
console.log(someArray.length); // "11"
</code></pre>

Of course if there are no elements then <code>length</code> is <code>0</code>. Note also that the <code>length</code> doesn't get updated if you use <code>delete</code> to remove the highest element.

But arrays are objects, so you can assign properties with other arbitrary property names including negative numbers or fractions:

<pre><code>someArray[-1] = "A property";
someArray[3.1415] = "Vaguely Pi";
someArray["test"] = "Whatever";
</code></pre>

Note that behind the scenes JS converts the property names to strings even when you supply a number like <code>-1</code>. (The positive integer indexes also become strings, for that matter.)

Array methods, like <code>.pop()</code>, <code>.slice()</code>, etc., only work on the zero-or-higher integer "indexes", not on other properties, so <code>length</code> is consistent on that point.

blocks|key|206510|text|请注意，当您使用位置(或0)索引时，值被放置在数组中：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|206511|var+array+=+[];

array[0]+=+"Foo";
array[1]+=+"Bar";

//+Result:+["Foo",+"Bar"]
//+Length:+2|code-block|syntax|javascript|206512|当您添加非索引值(不是0-9%2B)时，情况并非如此：|206513|var+array+=+[];

array[0]++=+"Foo";
array[1]++=+"Bar";
array[-1]+=+"Fizzbuzz";+//+Not+a+proper+array+index+-+kill+it

//+Result:+["Foo",+"Bar"]
//+Length:+2|206514|仅当您按规则进行游戏时，值才会放入数组中。如果你不这样做，他们就不会被接受。然而，它们在数组对象本身上是被接受的，JavaScript中的几乎任何东西都是这种情况。即使["Foo",+"Bar"]是数组中唯一的值，我们仍然可以访问"Fizzbuzz"|offset|length|style|CODE|206515|array[-1];+//+"Fizzbuzz"|206516|但请再次注意，这不是数组值的一部分，因为它的"index“是无效的。相反，它只是作为另一个成员添加到数组中。我们可以用同样的方式访问其他数组成员：|206517|array["pop"];+//+function+pop()+{+[native+code]+}|206518|注意，这里我们访问的是数组上的pop方法，它告诉我们这包含本机代码。我们访问的不是带有“pop键”的任何数组值，而是数组对象本身的一个成员。我们可以通过遍历对象的公共成员来进一步确认这一点：|206519|for+(var+prop+in+array)+
++++console.log(prop,+array[prop]);|206520|它显示了以下内容：|206521|+0+Foo
+1+Bar
-1+Fizzbuzz|206522|再说一次，它在对象上，但不在数组中。|206523|这是一个很棒的问题！这让我肯定做了一次重复拍摄。|206524|entityMap^0|0|0|0|0|2C|E|37|A|0|0|0|0|F|3|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1A|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|1B|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|1C|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|1D|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|1E|8|@$M|1F|N|1G|O|P]|$M|1H|N|1I|O|P]]|9|@]|A|$]]|$1|Q|3|R|5|D|7|1J|8|@]|9|@]|A|$E|F]]|$1|S|3|T|5|6|7|1K|8|@]|9|@]|A|$]]|$1|U|3|V|5|D|7|1L|8|@]|9|@]|A|$E|F]]|$1|W|3|X|5|6|7|1M|8|@$M|1N|N|1O|O|P]]|9|@]|A|$]]|$1|Y|3|Z|5|D|7|1P|8|@]|9|@]|A|$E|F]]|$1|10|3|11|5|6|7|1Q|8|@]|9|@]|A|$]]|$1|12|3|13|5|D|7|1R|8|@]|9|@]|A|$E|F]]|$1|14|3|15|5|6|7|1S|8|@]|9|@]|A|$]]|$1|16|3|17|5|6|7|1T|8|@]|9|@]|A|$]]|$1|18|3|-4|5|6|7|1U|8|@]|9|@]|A|$]]]|19|$]]

Note that when you use a position (or 0) index, values are placed within the array:

<pre><code>var array = [];

array[0] = "Foo";
array[1] = "Bar";

// Result: ["Foo", "Bar"]
// Length: 2
</code></pre>

This is not the case when you add non-index values (not 0-9+):

<pre><code>var array = [];

array[0] = "Foo";
array[1] = "Bar";
array[-1] = "Fizzbuzz"; // Not a proper array index - kill it

// Result: ["Foo", "Bar"]
// Length: 2
</code></pre>

Values are only placed in the array when you play by the rules. When you don't, they aren't accepted. They are however accepted on the Array object itself, which is the case with just about anything in JavaScript. Even though <code>["Foo", "Bar"]</code> are the only values in our array, we can still access <code>"Fizzbuzz"</code>:

<pre><code>array[-1]; // "Fizzbuzz"
</code></pre>

But note again that this isn't part of the array values, since its "index" isn't valid. It was instead added onto the array as just another member. We could access other array members in the same fashion:

<pre><code>array["pop"]; // function pop() { [native code] }
</code></pre>

Note here that we're accessing the <code>pop</code> method on the array, which informs us that this contains native code. We're not accessing any of the array values with a key of "pop", but rather a member on the array object itself. We can further confirm this by cycling over the public members of the object:

<pre><code>for (var prop in array) 
 console.log(prop, array[prop]);
</code></pre>

Which spits out the following:

<pre><code> 0 Foo
 1 Bar
-1 Fizzbuzz
</code></pre>

So again, it's on the object, but it's not in the array.

Awesome question! Caused me to do a double-take for sure.

blocks|key|3054971|text|根据MDN：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3054972|3054973|长度属性的值是一个具有正号且值小于2的32次方的整数(232)|blockquote|3054974|3054975|3054976|在Javascript中，您可以在创建的任何对象上设置属性。|3054977|var+array+=+new+Array();
array+=+[1,2,3];
array["boom"]+=+"pow";|code-block|syntax|javascript|3054978|同样，当您设置负索引时，它会将其存储为数组中的属性，而不是索引的一部分。|3054979|array[-1]+=+"property+does+not+add+to+array+length";|3054980|这就是为什么长度不能反映它，而for..in循环可以显示它的原因。|3054981|entityMap^0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|X|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|Y|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]|$1|G|3|-4|5|6|7|10|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|11|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|12|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|13|8|@]|9|@]|A|$]]|$1|Q|3|R|5|L|7|14|8|@]|9|@]|A|$M|N]]|$1|S|3|T|5|6|7|15|8|@]|9|@]|A|$]]|$1|U|3|-4|5|6|7|16|8|@]|9|@]|A|$]]]|V|$]]

According to MDN:

<blockquote>
 The value of the length property is an integer with a positive sign and a value less than 2 to the 32 power (232)
</blockquote>

In Javascript you can set a property on any object you create.

<pre><code>var array = new Array();
array = [1,2,3];
array["boom"] = "pow";
</code></pre>

In the same way when you set a negative index it stores it as a property on the array rather than part of the index.

<pre><code>array[-1] = "property does not add to array length";
</code></pre>

This is why the length doesn't reflect it but a for..in loop shows it.

blocks|key|206570|text|如果您确实希望在长度中包含负索引和其他索引，那么您可以自己创建该功能：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|206571|function+getExtendedArray()+{
++++var+a+=+[];

++++Object.defineProperty(a,+'totalLength',+{+
++++++++get+:+function()+{+return+Object.keys(a).length;+}
++++});

++++return+a;
}|code-block|syntax|javascript|206572|然后举个例子：|206573|var+myArray+=+getExtendedArray();
console.log(myArray.totalLength);+//+0
myArray.push("asdf");
myArray[-2]+=+"some+string";
myArray[1.7777]+=+"other+string";
console.log(myArray.totalLength);+//+3|206574|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

If you really want negative indexes and other indexes to be included in the length, then create the functionality yourself:

<pre><code>function getExtendedArray() {
 var a = [];

 Object.defineProperty(a, 'totalLength', { 
 get : function() { return Object.keys(a).length; }
 });

 return a;
}
</code></pre>

Then for example:

<pre><code>var myArray = getExtendedArray();
console.log(myArray.totalLength); // 0
myArray.push("asdf");
myArray[-2] = "some string";
myArray[1.7777] = "other string";
console.log(myArray.totalLength); // 3
</code></pre>

In javascript I define an array like this

<pre><code>var arr = [1,2,3];
</code></pre>

also I can do 

<pre><code>arr[-1] = 4;
</code></pre>

Now if I do

<pre><code>arr = undefined;
</code></pre>

I also lose the reference to the value at arr[-1].

SO for me logically it seems like arr[-1] is also a part of arr.

But when I do following (without setting arr to undefined)

<pre><code>arr.length;
</code></pre>

It returns 3 not 4;

So my point is if the arrays can be used with negative indexes, these negative indexes should also be a part of their length**.
I don't know may be I am wrong or I may be missing some concept about arrays.

Should negative indexes in JavaScript arrays contribute to array length?

在javascript中，我定义了如下数组var arr = [1,2,3];我也可以做arr[-1] = 4;现在如果我这么做了arr = undefined;我还丢失了对arr-1处的值的引用。所以对我来说，从逻辑上讲，arr-1似乎也是arr的一部分。但当我执行以下操作时(没有将arr设置为未定义)arr.len...

问JavaScript数组中的负索引是否应该影响数组长度？
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JavaScript数组中的负索引是否应该影响数组长度？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JavaScript数组中的负索引是否应该影响数组长度？
EN