javascript 数组长度不起作用
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Array length not working
提问by Tudor Ravoiu
Possible Duplicate:
Array length undefined
可能重复：
数组长度未定义
I have the following array but whenever I try to find out the length with categoryData.lengthit gives me only undefined. If I type console.log(categoryData)it gives me all the objects in the array.
我有以下数组，但是每当我试图找出它的长度时，categoryData.length它只会给我undefined. 如果我输入console.log(categoryData)它会给我数组中的所有对象。
var categoryData = {
animals: {
name: "Animals",
description: "All your favorites from aardvarks to zebras.",
items: [
{
name: "Pets"
},
{
name: "Farm Animals"
},
{
name: "Wild Animals"
}
]
},
colors: {
name: "Colors",
description: "Fresh colors from the magic rainbow.",
items: [
{
name: "Blue"
},
{
name: "Green"
},
{
name: "Orange"
},
{
name: "Purple"
},
{
name: "Red"
},
{
name: "Yellow"
},
{
name: "Violet"
}
]
},
vehicles: {
name: "Vehicles",
description: "Everything from cars to planes.",
items: [
{
name: "Cars"
},
{
name: "Planes"
},
{
name: "Construction"
}
]
}
};
};
回答by raina77ow
That's because categoryData is not an Array - it's an Object. And while some JS objects (arguments, for example) support lengthproperty, those created with object literal notation do not.
那是因为 categoryData 不是数组 - 它是一个对象。虽然一些 JS 对象（arguments例如）支持length属性，但那些使用对象文字符号创建的对象不支持。
You can count your object's length by yourself, with this:
您可以自己计算对象的长度，如下所示：
function countProps(obj) {
var count = 0;
for (var p in obj) {
obj.hasOwnProperty(p) && count++;
}
return count;
}
This can be done even in a more simple way, if your target environment supports (or has it shimmed) the Object.keysmethod:
这甚至可以以更简单的方式完成，如果您的目标环境支持（或已填充）该Object.keys方法：
function sizeObj(obj) {
return Object.keys(obj).length;
}
... and that's exactly how it's done in Underscore.js library method:
...这正是它在Underscore.js 库方法中的完成方式：
_.size = function(obj) {
if (obj == null) return 0;
return (obj.length === +obj.length) ? obj.length : _.keys(obj).length;
};
