I created an object which contains an array.
I noticed that when I dont enter any values into the array, it still has one - its size,
So how can I check if the array is actually empty?
Here's how I'm creating the array:
array = { text:[10] }
The size of the array is not an array entry (aka array "element"). It is a property, length, but it's not an entry. An empty array is exactly that: empty.
The code you posted in a comment:
array = { text:[10] }
does not create an empty array. It creates an array of length 1 with the entry 10 in it.
If you want to create an empty array with a specific length, you can't do that in a single statement with an array literal ([]). You have two ways you can do it:
Using an array literal and assigning to length:
var a = [];
a.length = 10;
or using new Array:
var a = new Array(10);
But there's virtually never any reason, in JavaScript, to predefine the length of the array, because standard JavaScript arrays aren't really arrays at all. It does make sense with the new typed arrays (Uint32Array and such), and to do it you have to use the array constructor (new Uint32Array(10)).
Related
In my code i initialize array then put a value inside it why the output be 0 ?in spite of this should be 1
var changedfields=[];
changedfields['product']="productname";
alert(changedfields.length);
You're creating an associative array (normal arrays have an numeric index) but are actually trying to build a HashMap (a key, value pair). Use Objects or ES6 Maps.
I hope the following example will help you out:
var changedfields = {}; // create an object
changedfields['product']="productname";
var keys = Object.keys(changedfields); // returns the keys of the object ['product']
alert(keys.length);
I would suggest to read more about datastructures in javascript and avoid associative arrays in general.
Length of a JavaScript object (that is, associative array)
associative array versus object in javascript
Your question is interesting. Following is the answer.
First of all Arrays in Javascript are object type.
The first line you wrote creates an empty array and it's type is object.
var changedfields=[];
The second line you wrote creates a property called product of changedfields and sets the value to productname. It allows you add the product property because Javascript Array type is object. If you just had var changedfields; you could not add this property.
changedfields['product']="productname";
The third line you wrote simply finds the length of the empty array.
alert(changedfields.length);
In Javascript associative arrays are achieved using object. But if you want to add the product name in changedfields array. You could use push method like below and check the length:
changedfields.push('productname');
console.log(changedfields.length);
Javascript numerical indexed array length
So the Javascript numerical indexed array length can be calculated this way:
console.log(array.length);
console.log(changedfields.length); // in your case
The Javascript associative array length
The Javascript associative array (object) length can be calculated following ways:
Option 1:
Object.len = function(obj) {
var objLen = 0;
for (i in obj) {
obj.hasOwnProperty(i) ? objLen++ : '';
}
return objLen;
};
console.log(Object.len(changedfields));
Option 2:
console.log(Object.keys(array).length);
console.log(Object.keys(changedfields).length); // in your case
Note: This has issues with Internet Explorer 8, Opera etc
I have a strange problem. I need a multidimensional javascript array that has numeric indexes e.g.:
MyArray = new Array();
$(".list-ui-wrapper").each(function(){
var unique = $(this).attr('id').replace(/[^\d.]/g,'');
MyArray ["'"+unique+"'"] = new Array();
});
The unique is a 8 digit integer. So if I dont wrap it inside the quotes, the ListUI_col_orders array will become extremly big, because if the unique = 85481726 then javascript will fill the array with 85481725 undefined elements before the the new empty array at the 85481726th index.
My problem is that later if i generate the unique again i cannot access the array anymore:
var unique = $(this).attr('id').replace(/[^\d.]/g,'');
console.log(MyArray [unique]); // undefined
console.log(MyArray ['"'+unique+'"']); // undefined
console.log(MyArray [unique.toString()]); // undefined
Any tips?
If you are going to use an array that's mostly sparse, then use a Hash table instead.
eg, set your variable as follows:
ListUI_col_Orders = {};
Then your indexing will be a key, so you don't have to worry about all the interstitial elements taking up space.
...because if the unique = 85481726 then javascript will fill the array with 85481725 undefined elements before the the new empty array at the 85481726th index.
No, it won't. JavaScript standard arrays are sparse. Setting an element at index 85481725 results in an array with one entry, and a length value of 85481726. That's all it does. More: A myth of arrays
The problem is that you're trying to retrieve the information with a different key than the key you used to store it. When storing, you're using a key with single quotes in it (actually in the key), on this line of code:
MyArray ["'"+unique+"'"] = new Array();
Say unique is 85481726. That line then is equivalent to this:
MyArray ["'85481726'"] = new Array();
Note that the key you're using (the text between the double quotes) has ' at the beginning and end. The actual property name has those quotes in it. Since it doesn't fit the definition of an array index, it doesn't affect length. It's a non-index object property. (How can you add a property to an array that isn't an array index? Arrays are not really arrays, see the link above.)
Later, you never use that key when trying to retrieve the value:
var unique = $(this).attr('id').replace(/[^\d.]/g,'');
console.log(MyArray [unique]); // undefined
console.log(MyArray ['"'+unique+'"']); // undefined
console.log(MyArray [unique.toString()]); // undefined
The keys you tried there were 85481726, "85481726" (with double quotes), and 85481726 again. Note that you never tried '85481726' (with single quotes), which is the key you used originally. Since you didn't use the same key, you didn't get the value.
Either use consistent quotes, or (much better) don't use quotes at all. Don't worry about the array length being a large number, JavaScript arrays are inherently sparse. Adding an entry with a large index does not create several thousand undefined entries in front of it.
All of that being said, unless you need the "array-ness" of arrays, you can just use an object instead. Arrays are useful if you use their features; if you're not using their array features, just use an object.
More about the sparseness of arrays: Consider this code:
var a = [];
a[9] = "testing";
console.log(a.length); // 10
Although the length property is 10, the array has only one entry in it, at index 9. This is not just sophistry, it's the actual, technical truth. You can tell using in or hasOwnProperty:
console.log(a.hasOwnProperty(3)); // false
console.log("3" in a); // false
When you try to retrieve an array element that doesn't exist, the result of that retrieval is undefined. But that doesn't mean there's an element there; there isn't. Just that trying to retrieve an element that doesn't exist returns undefined. You get the same thing if you try to retrieve any other property the object doesn't have:
var a = [];
a[9] = "testing";
console.log(a[0]); // undefined
console.log(a[200]); // undefined
console.log(a["foo"]); // undefined
Final note: All of this is true of the standard arrays created with [] or new Array(). In the next spec, JavaScript is gaining true arrays, created with constructors like Int32Array and such. Those are real arrays. (Many engines already have them.)
I haven't touched Javascript in a while and now I'm having trouble with basic arrays.
params=new Array();
params['return']='json';
alert(params.length);
This always returns 0 when I'm expecting 1. What's wrong with this?
Arrays use numerical indexes. When you use a string "index", it just adds a new property to the array object, but the new value isn't in the array.
Thus, the array is still empty, but you can access your value as you could with any other object.
Your code is equivalent to
var params = {}; //new object (not array)
params['return']='json'; //new property added to object
A few things:
You forgot var:
var params = new Array();
But an array takes numeric indices, so params['return'] is not really a member of the array but of the object that represents the array, so it doesn't affect the length.
You could use an object but objects have no length:
var params = {};
params['return'] = 'json';
To get the length though you can count the keys in that object and get the length of the resulting array:
var len = Object.keys(params).length;
Javascript arrays don't hold key value pairs like objects do, so the length isn't incremented when you assign a value. They are however objects themselves, so you can assign values to its properties (in this case the return property).
You probably want params to be a plain object: params = {}
You need to count the properties, like this for example:
function dictionarySize(dict){
var size = 0;
for (key in dict){
// In practice you'd probably want to filter using hasOwnProperty
size++;
}
return size
}
alert(dictionarySize(params));
Or using a library like underscore.js:
_.size(params);
Object.keys is also an option, although it won't work in IE8 and older.
If you don't need an key value pairs use params.push:
params=new Array();
params.push('json')
alert(params.length); // 1
You can create an array, push stuff on it, and assign properties to values of it like so:
var params=[];
params.push('firstly');
params[0]="jonson";
params['return']="fredy"
params.newone="json";
params.push('Carl');
NOW, if you do:
console.log(params.length,params);
the result of that is:
2 ["jonson", "Carl", return: "fredy", newone: "json"]
SO, you see "firstly" was replaced by "jonson" in the [0] - so the "pushed" value is addresed by the numerical [0]
It seems it's always empty :
var idStruttura=2;
var arrayMarkers=new Array();
arrayMarkers["sede_"+idStruttura] = "ciao";
alert(arrayMarkers.length);
Prints always 0. Why? And how can I fix it?
An array is not an associative array, however the array object is an object, and all objects are associative arrays. If you use a string as key when assigning items to the array, you are not using it as an array, you are using it as an object.
The length property of the array returns how many items you have stored in the array. If you also use the array object as an associative array, that won't affect how you use the array as an array.
What you've created is a regular array object and added a property to it called sede_.... JavaScript doesn't use associative arrays in the same way a language like PHP does. Arrays are objects that can have properties, but those properties are not among the numerically indexed array elements.
var idStruttura=2;
var arrayMarkers=new Array();
// Push an object onto the array having one property:
arrayMarkers.push({"sede_" + idStruttura : "ciao"});
// Or declare it as an object to begin with:
// This makes more sense....
var objMarkers = {};
objMarkers['sede_' + id] = 'ciao';
There is no length when you store objects, only when you use the array as intended.
Try this (DEMO)
var idStruttura=2;
var arrayMarkers={}; // creates a more appropriate object than []
arrayMarkers["sede_"+idStruttura] = "ciao";
arrayMarkers["sede_"+(++idStruttura)] = "espresso";
var len = 0;
for (var o in arrayMarkers) {
if (arrayMarkers.hasOwnProperty(o)) len++;
}
arrayMarkers.length=len
alert(arrayMarkers.length)
array's in javascript (unlike php) cannot have string key's, only numeric keys. If you want a string literal as a key, please use objects
What you need is a simple Object, not an Array. Arrays have numeric indexes.
var markers = {};
markers['sede_' + id] = 'ciao';
Also note that {} is the same as new Object() and [] is the same as new Array(). Always use the former ones. It's much clear to any JS developer.
I have to push elements in an associative array from another array after some processing and I'm doing something this:
for(var i in this.aNames) {
var a = this.aNames[i];
// some processing on a
aList[i] = a;
aList.push(i);
}
But it's not giving me the proper array.
EDIT :
Here aNames is an associative array like this
'1232':'asdasdasd',
'6578':'dasdasdas'
...... and so on of about 100 elements.
I'm using for here as I want to do some changes in every element of the array.
Then I'm displaying the result array on the page but it's showing the key value together with the array data.
I.e. it should only display asdasdasd or asdasdasd but it's displaying keys too, like 1232 asdasdasd 6578 dasdasdas.
There are multiple things which may go wrong...
Primarily, make sure that this is pointing to the correct context and that this.aNames is actually returning a complex object (associative array).
Also, what's aList? Is it an array? If it is, push should append your array with the key of the current member (the member's name).
If you want to append the values of the members on your source object, you need to do something like this:
var obj = {name: 'dreas'},
arr = []; // arr is an array
arr.push(obj["name"]); // arr now contains a single element, 'dreas'
In your for..in construct, you are both adding elements to an alleged array (aList) with push but also creating new members on your array (with the subscript notation, aList[i] = "asd" since i in this case (for..in iteration) refers to the member's name).
So, what you need to do is decide if you want to add elements to an array or members to an object, not both.
If you just want to clone an array, use a for loop. If on the other hand you want to clone an object, it's not that trivial because members can also be complex objects containing their own members, and simply doing arr[i] = obj.member will only copy a pointer to arr[i] if member is a complext object, not a value type.
Just to make sure my terminology is understandable:
var anObject = {name: "dreas"},
anArray = [1,2,3];
anObject["name"] <= member (results in "dreas")
anArray[1] <= element (results in 2)