在javascript中未定义Object.length

Question

我有一个像数组的javascript对象,

var coordinates = {
     "a": [
         [1, 2],
         [8, 9],
         [3, 5],
         [6, 1]
     ],

         "b": [
         [5, 8],
         [2, 4],
         [6, 8],
         [1, 9]
     ]
 };

但coordinates.length返回undefined. 小提琴就在这里.

Answer 1

因为coordinates是Object不Array,使用for..in

var coordinates = {
     "a": [
         [1, 2],
         [8, 9],
         [3, 5],
         [6, 1]
     ],

     "b": [
         [5, 8],
         [2, 4],
         [6, 8],
         [1, 9]
     ]
 };

for (var i in coordinates) {
  console.log(coordinates[i]) 
}

要么 Object.keys

var coordinates = {
  "a": [
    [1, 2],
    [8, 9],
    [3, 5],
    [6, 1]
  ],

  "b": [
    [5, 8],
    [2, 4],
    [6, 8],
    [1, 9]
  ]
};

var keys = Object.keys(coordinates);

for (var i = 0, len = keys.length; i < len; i++) {
  console.log(coordinates[keys[i]]);
}

Answer 2

http://jsfiddle.net/3wzb7jen/2/

 alert(Object.keys(coordinates).length);

Answer 3

coordinates是一个对象.默认情况下,javascript中的对象没有length属性.某些对象具有length属性:

"a string - length is the number of characters".length

['an array', 'length is the number of elements'].length

(function(a, b) { "a function - length is the number of parameters" }).length

您可能正在尝试查找keys对象中的数量,这可以通过Object.keys()以下方式完成:

var keyCount = Object.keys(coordinates).length;

要小心,因为length属性可以添加到任何对象:

var confusingObject = { length: 100 };