包含数组的JSON对象

Question

I have JSON,like,

var jsondata = {
  "an_array" : [[2,4],[1,2]]
} 

With JavaScript, I want to use a for loop to go through each element of the array in the array, like,

var grab_array = jsondata.an_array;
for (i = 0; i < grab_array.length; i++) {

 grab_array[i][0]; //doing stuff

}

However, and I know this is probably some serious noob stuff, when I run typeof() on grab_array, it says it is an object and has a length of 0. What should I be doing?

Answer 1

Well I can't follow the part with the length being 0 , did you actually run the loop? It should work just fine.

But concerning the typeof operator, that operator (together with instanceof operator) is probably the biggest design flaw of JavaScript. It is near of being completely broken .

Although instanceof still has its limited uses, typeof really has only one practical use case, which not happens to be checking the type of an object.

The JavaScript Typetable

Value               Class      Type
-------------------------------------
"foo"               String     string
new String("foo")   String     object
1.2                 Number     number
new Number(1.2)     Number     object
true                Boolean    boolean
new Boolean(true)   Boolean    object
new Date()          Date       object
new Error()         Error      object
[1,2,3]             Array      object
new Array(1, 2, 3)  Array      object
new Function("")    Function   function
/abc/g              RegExp     object (function in Nitro/V8)
new RegExp("meow")  RegExp     object (function in Nitro/V8)
{}                  Object     object
new Object()        Object     object

In the above table Type refers to the value the typeof operator returns. As you can see this is anything but consistent.

The Class refers to the value of the internal [[Class]] property of an object.

From the Specification: Class can be one of the following values: "Arguments" , "Array" , "Boolean" , "Date" , "Error" , "Function" , "JSON" , "Math" , "Number" , "Object" , "RegExp" , "String"

In order to retrieve the value of Class one can has to make use of the toString method of Object .

Checking the Class of an Object

function is(type, obj) {
    return Object.prototype.toString.call(obj).slice(8, -1) === type;
}

is('String', 'test'); // true
is('String', new String('test')); // true

In the above code Object.prototype.toString gets called with this being set to the object which its Class value should be retrieved.

Checking whether a variable has been defined

typeof foo !== 'undefined'

The above will check whether foo was actually declared or not, since just referencing it would result in a ReferenceError . This is the only thing typeof is actually useful for.

To Sum it up

Don't use typeof unless you're checking for the existence of a variable.

Answer 2

Well, as for me, I could easily run this code

var jsondata = {
  "an_array" : [[2,4],[1,2]]
}
var grab_array = jsondata.an_array, 
    max = grab_array.length;
for (i = 0; i < max; i++) {

 console.log(grab_array[i][0]); 

}

and the output was pretty expected: 2, 1 (each on new line). Concerning the second part of the question, you can easily test whether grab_array is an Array or not using following snippet:

if (grab_array.constructor == Array){
  //do stuff
}

Also in ECMAScript5, you can use has Array.isArray() function, but since you can encounter the case when the destination environment does not support ECMAScript5, you may want to add this universal function for testing if the Object is an Array:

if (typeof Array.isArray === "undefined") {
 Array.isArray = function (arg) {
  return Object.prototype.toString.call(arg) === "[object Array]";
 };
}

and then simply do:

if (Array.isArray()){
  //do stuff
}

Answer 3

检查类型时，只需使用grab_array.constructor == Array而不是typeof grab_array 。

Answer 4

I'm not sure what is going on here to be honest. Throwing your code as is through the jQuery JSON parser leads me to a result where an_array.length is 2. typeof still lists it as an object, but it has a length, and your loop should go through it. More code would probably be helpful.

As a debugging tip, trying doing a console.log call in Chrome or Firefox with Firebug of jsondata and see what comes out. Make sure that the JSON object is coming back correct. However, that said, you can also use jQuery utilities to go through each element instead of a for loop. Try using jQuery.each() on it.