I have an array like this
var list =['lemon', 'apple'];
list['yuck'] = 'durian';
list.forEach(function(i){
console.log(i);
});
The out put is
lemon
apple
So where is durian?
This question is not about how use array properly, I just find it odd that we able to do that, what's really going on here?
forEach iterates on an array's elements, that is the properties identified by a positive integer. 'yuck' is a string which can't be converted to an integer.
If you really want to iterate on all values, you can do this:
for (var key in list) {
console.log(list[key]);
}
But it breaks all the optimizations of arrays. You should probably use a Set or an object instead.
Because that's not how you add a value to a javascript array. You have to use the method Array.prototype.push()
var list = ['lemon', 'apple'];
list.push('durian');
list.forEach(function(i){
console.log(i);
});
list['yuck'] = 'durian';
Here the syntax says add the text durain to a property yuck.
Here you are adding key values. This is how you assign values to a object.
The above line will throw an exception as you cannot add key values to array. So your array is never modified.
And this one
list.forEach(function(i){
console.log(i);
});
You are looping the initial array. Hence same values are printed.
On other hand you can use.
list.push('durian')
Or
list[2] = 'durian'
Related
I've been teaching myself OOJS by creating a blackjack game. Everything is going great, but since I randomly create cards, sometimes I create and then deal the same card twice in a round. I'm trying to avoid that by writing some logic that gets rid of duplicate cards.
So I found this discussion.
https://stackoverflow.com/a/840849/945517
and the code below:
function eliminateDuplicates(arr) {
var i,
len=arr.length,
out=[],
obj={};
for (i=0;i<len;i++) {
obj[arr[i]]=0;
}
for (i in obj) {
out.push(i);
}
return out;
}
var a=[];
var b=[];
a[0]="fish";
a[1]="fishes";
a[2]="fish";
a[3]="1";
a[4]="fishes";
b=eliminateDuplicates(a);
console.log(a);
console.log(b);
I understand what's going on in general and on almost every line except:
for (i=0;i<len;i++) {
obj[arr[i]]=0;
}
It seems like it's looping through the array and setting the key of obj to zero. What's going on here and how does this help get rid of duplicate entries in the array being passed at the start?
Thanks!
{} = {key: value, key2:value2,....}
The above is essentially a key value map. The code iterates through the array and adds the key into the map with a value of zero. When the map tries to access an existing value, all it does is reset the value to zero. A useless operation, but it avoids an if..else or other more complex logic.
Once the array is done being iterated across, you only need to iterate across the key value map to get the keys. Since the keys are guaranteed to be unique, you have a list of unique items.
The main thing to realize here is that an object can only have one property per unique string, so when you set obj.fish to 0 when i = 0, you don't add a second "fish" property to obj when i = 2.
Then you can just loop through all of the properties of obj, each one is guaranteed to be unique, and thus you have stripped duplicates.
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]
I have some simple Javascript looping through an array of items (Tridion User Groups) to check if the user is a member of a specific group.
I can easily code around the issue shown below ( see && extensionGroup !== 'true') but I want to understand why the isArray = true is counted as a value in the array - any ideas?
The screenshot below demonstrates that the value extensionGroups has been set thus
var extensionGroups = ["NotEvenARealGroup", "Author", "ExampleGroupAfterOneUserIsActuallyIn"];
but returns the isArray value as a 4th value?
updated to show images a little clearer
You're using for in to iterate an array; don't do that. Use for (or forEach):
for(var i = 0; i < extensionGroups.length; i++) {
var extensionGroup = extensionGroups[i];
// ...
}
The reason this fails is because for in is used to iterate over an object's properties in JavaScript. Iterating over an array in this way means you get anything else assigned to it, such as this property or length.
And if you're able to use Array#forEach, it's probably most appropriate here:
extensionGroups.forEach(function(extensionGroup) {
// ...
});
For..in, technically speaking, doesn't iterate through values. It iterates through property names. In an array, the values ARE properties, under the hood. So when you iterate over them with for..in you get funky stuff like that happening.
Which highlights my next point: don't use for..in. Don't use it for arrays -- don't use it for anything, really. Ok -- maybe that's going a bit too far. How about this: if you feel the need to use for..in, think hard to see if it's justifiable before you do it.
Doing so
var x = new Array();
x['length']=5;
will make x an array of 5 undefined items, but I actually want to have the value '5' stored at key 'length'.
Is that possible?
In javascript arrays do not have keys. You are looking for objects:
var x = {}
x.length = 5;
I have to parse a file containing many words and store the number of occurences of each word
Use an object, and make the words the keys. You aren't storing sequential / ordered data, so you shouldn't use an array.
var word_count = {};
for (var i; i < words.length; i++) {
var word = words[i];
if (word_count[word]) {
word_count[word]++;
} else {
word_count[word] = 1;
}
If you want to do this you'd be better off creating an object rather than an array. This should give you what you want.
var x = {};
x['length'] = 5;
You can call methods on a javascript object using two different syntaxes. The familiar 'dot' syntax with parens to invoke the method, and the square bracket syntax. You can 'call' a method on a javascript object using the syntax myObj["methodname"](args). This is handy when you want to construct the method name dynamically using strings. Remember, objects in javascript are very much like a hash table (dictionary) where keys denote property and function names. If a key's value holds a function, it can be invoked (using parentheses).
In your example, Array has a method called 'length'. You are inadvertently calling its setter (which sets the length of the array to empty values, i.e., undefined).
Putting that all aside, you really do want a hash (associative array) in this case. An array is an offset indexed data structure.
A simple object literal like myObj = {} will suffice to give you hash semantics (again, objects in javascript are already like hashes) and you can then call myObj.whatever = "some value"
You could use objects instead of arrays to store your data. But if you need to use Arrays (you might need to use their functionality), You could cripple the words and store them as array keys.
Use some kind of simple rule to follow to bypass all the possible keywords. For example prefix all your array keys with a "_" character. This way you could always restore the original words from the keys, by simply removing their first character, and you are sure you are not referencing any specific property of the Array objects (like the length property).