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?
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.
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
}
grab_array.constructor == Array will fail if grab_array origins from another document, really only use the isArray solution, see my answer for more details :)Just use grab_array.constructor == Array instead of typeof grab_array when checking the type.
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.
