From 33058f2b292b3a581333bdfb21b8f671898c5060 Mon Sep 17 00:00:00 2001 From: Peter Bengtsson Date: Tue, 8 Dec 2020 14:40:17 -0500 Subject: initial commit --- .../global_objects/array/length/index.html | 149 +++++++++++++++++++++ 1 file changed, 149 insertions(+) create mode 100644 files/zh-cn/web/javascript/reference/global_objects/array/length/index.html (limited to 'files/zh-cn/web/javascript/reference/global_objects/array/length/index.html') diff --git a/files/zh-cn/web/javascript/reference/global_objects/array/length/index.html b/files/zh-cn/web/javascript/reference/global_objects/array/length/index.html new file mode 100644 index 0000000000..7b126495fb --- /dev/null +++ b/files/zh-cn/web/javascript/reference/global_objects/array/length/index.html @@ -0,0 +1,149 @@ +--- +title: Array.length +slug: Web/JavaScript/Reference/Global_Objects/Array/length +tags: + - JavaScript + - 参考 + - 属性 + - 数组 +translation_of: Web/JavaScript/Reference/Global_Objects/Array/length +--- +

{{JSRef}}

+ +

length 是Array的实例属性。返回或设置一个数组中的元素个数。该值是一个无符号 32-bit 整数，并且总是大于数组最高项的下标。

+ +
{{EmbedInteractiveExample("pages/js/array-length.html")}}
+ +

描述

+ +

length 属性的值是一个 0 到 232-1 的整数。

+ +
var namelistA = new Array(4294967296); // 2的32次方 = 4294967296
+var namelistC = new Array(-100) // 负号
+
+console.log(namelistA.length); // RangeError: 无效数组长度
+console.log(namelistC.length); // RangeError: 无效数组长度
+
+
+
+var namelistB = [];
+namelistB.length = Math.pow(2,32)-1; //set array length less than 2 to the 32nd power
+console.log(namelistB.length);
+
+// 4294967295
+
+ +

你可以设置 length 属性的值来截断任何数组。当通过改变length属性值来扩展数组时，实际元素的数目将会增加。例如：将一个拥有 2 个元素的数组的 length 属性值设为 3 时，那么这个数组将会包含3个元素，并且，第三个元素的值将会是 undefined 。

+ +
var arr = [1, 2, 3];
+printEntries(arr);
+
+arr.length = 5; // set array length to 5 while currently 3.
+printEntries(arr);
+
+function printEntries(arr) {
+  var goNext = true;
+  var entries = arr.entries();
+  while (goNext) {
+    var result = entries.next();
+    if (result.done !== true) {
+      console.log(result.value[1]);
+      goNext = true;
+    } else
+      goNext = false;
+  }
+  console.log('=== printed ===');
+}
+
+// 1
+// 2
+// 3
+// === printed ===
+// 1
+// 2
+// 3
+// undefined
+// undefined
+// === printed ===
+ +

但是， length 属性不一定表示数组中定义值的个数。了解更多：长度与数值下标属性之间的关系。

+ +

{{js_property_attributes(1, 0, 0)}}

+ + + +

示例 

+ +

遍历数组

+ +

下面的例子中，通过数组下标遍历数组元素，并把每个元素的值修改为原值的2倍。

+ +
var numbers = [1, 2, 3, 4, 5];
+var length = numbers.length;
+for (var i = 0; i < length; i++) {
+  numbers[i] *= 2;
+}
+// 遍历后的结果 [2, 4, 6, 8, 10]
+ +

截断数组

+ +

下面的例子中，如果数组长度大于 3，则把该数组的长度截断为 3 。

+ +
var numbers = [1, 2, 3, 4, 5];
+
+if (numbers.length > 3) {
+  numbers.length = 3;
+}
+
+console.log(numbers); // [1, 2, 3]
+console.log(numbers.length); // 3
+ +

规范

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
SpecificationStatusComment
{{SpecName('ES1')}}{{Spec2('ES1')}}Initial definition.
{{SpecName('ES5.1', '#sec-15.4.5.2', 'Array.length')}}{{Spec2('ES5.1')}} 
{{SpecName('ES6', '#sec-properties-of-array-instances-length', 'Array.length')}}{{Spec2('ES6')}} 
{{SpecName('ESDraft', '#sec-properties-of-array-instances-length', 'Array.length')}}{{Spec2('ESDraft')}} 
+ +

浏览器兼容性

+ +
+ + +

{{Compat("javascript.builtins.Array.length")}}

+
+ +

相关链接

+ + -- cgit v1.2.3-54-g00ecf