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Convert a number (i. e. 5) to length of an array

I want to create a property on a polymer custom element that let's the user define a number of elements to repeat.
<my-element repeated-elements='5'></my-element>
This should tell my element to repeat the element inside of the component five times. For that I need an array with length of 5, so anything like this would do:
['','','','','']
Is there a way to do this in JavaScript? The number would be passed as a number, not a string. So somehow I would need to convert any number to the amount of array items in an array. Metaphorically speaking:
convertToArrayLength(5);
I'm completely lost here, I have no idea at all how this could be done.

The array constructor does exactly that:
var arr = Array(5);
console.log(arr, arr.length);
//=> Array [ <5 empty slots> ] 5
You don't need to put new before calling Array (just a peculiarity of the API).
Note though that arrays in JavaScript are dynamic, so most of the time you don't need to specify the length of your array beforehand and can simply initialize your variable with:
var arr = [];
// And then push to it as you go:
arr.push(x);

JavaScript will re size the array automatically as you push to it.
var array = new Array(5); // undefined
array.length; // 5
array.push("FOO"); // 6
array.length; // 6
All you have to do is make sure you assign the variable as an Array with either [] or new Array

Javascript array index

Sorry for asking a noob question but if I have an array:
MyArray["2cd"]="blah1";
MyArray["3cx"]="blah3";
MyArray["8cz"]="blah2";
And a string myStr="2cd";
And then I use MyArray[myStr] to get the value of blah, how can I get the number I am accessing in the object/array or 0 in this case?

If I may read between the lines, it sounds like you're thinking that the code you posted:
MyArray["2cd"] = "blah1";
MyArray["3cx"] = "blah3";
MyArray["8cz"] = "blah2";
will automatically become the equivalent of:
MyArray[0] = MyArray["2cd"] = "blah1";
MyArray[1] = MyArray["3cx"] = "blah3";
MyArray[2] = MyArray["8cz"] = "blah2";
and therefore you can get the string "blah1" either of these two ways:
var foo = MyArray[0]; // sets foo to "blah1"
var bar = MyArray["2cd"] // also sets bar to "blah1"
But that's not how JavaScript works.
You certainly can set things up so you can use my MyArray[0] and MyArray["2cd"] to fetch the same value, but you have to do it explicitly as in my example.
One thing you didn't mention is how you declared MyArray itself. Is it an Array or an Object? That is, before the code you posted, did you create MyArray with:
var MyArray = {}; // or equivalently, var Array = new Object;
or:
var MyArray = []; // or equivalently, var Array = new Array;
The first example creates an Object, the second an Array.
What is a bit confusing is that JavaScript has both of these two types, which in many cases you can use somewhat interchangeably. But it's customary to use an Object when you are using arbitrary (generally but not necessarily non-numeric) keys into the object, as in your example. Conversely, it's customary to use an Array when you are primarily using strictly numeric indexes.
In fact, JavaScript's Array type inherits from the Object type. An Array is simply an Object with some additional behavior:
An Array has additional methods such as .push() which appends an item to the array.
An Array has a .length property which is automatically updated when you add elements with .push() or a direct array[123] assignment, or when you remove elements with .pop() or other methods.
What JavaScript doesn't have, as Fabrício pointed out, is an "associative array" that behaves like what you might find in some other languages. It has Objects and it has Arrays (which inherit from Objects), and you have to deal with each of those on their own terms.

InArray returning -1 with matching value

var x = new Array(10);
$.inArray(10,x);
#Returns -1
Fiddle
I've come across this weird issue, checking a value in an array with $.inArray, really simple.
But, if the array only has one value in it, inArray returns -1.
If I add another value to the array, it works as expected.
This only happens with integers and not with strings.
What's going on?!

If you want to create an array with the one number(10) inside you should use bracket literal:
var x = [10];
$.inArray(10,x);
Or with push:
var x = new Array();
x.push(10);
Obviously the first one is more readable and faster to write.
A JavaScript array is initialized with the given elements, except in the case where a single argument is passed to the Array constructor and that argument is a number. Note that this special case only applies to JavaScript arrays created with the Array constructor, not with array literals created with the bracket syntax.
If the only argument passed to the Array constructor is an integer, a new, empty JavaScript array and its length is set to that number
MDN
Fixed fiddle

I suggest to check documentation for arrays in JavaScript, link: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Array
In your case, by using:
var x = new Array(10);
It creates an array with length 10 and each item of the array is undefined
I suggest to use:
var x = [10];
this is an array with one item at index 0 that has value 10.

var x = new Array(10); creates an array with 10 slots, not an array that contains "10"

Initialize an array whose indexes are scattered over integer range

How to initialize a string array (size<100 items) in javascript whose indexes are scattered over entire integer range, with data items.
If I do like this:
array1 = ["string1","string2","string3","string4"];
then I get array of length 4 with indices ranging 0 to 3
But in my case i want to keep my own indices, so that the array could be used like a high performance int-string hash table.
I'm preferably looking out for a single statement initialization.
The items of the array should be accessible like this: array1[23454]
Update From Comments
I'm restricted to initialize the array as a single statement since a dynamically prepared array initialization string is appended from server side like this: var array = <string from server here>

To create an array with a set number of indexes you can use
// Creates an array with 12 indexes
var myArray = new Array(12);
This isn't needed in javascript due to the way its array's work. There isn't an upper-bound for arrays. If you try to reference an item index in the array that doesn't exist, undefined is returned but no error is thrown
To create an array with perscribed indexes you can use something like array['index'] = value though this would force you to use multiple statements. Javascript doesn't have an array initalizer to allow for you to specify indexes and values all in a single statement though you can create a function to do as such
function indexArray(param) {
var a = [], i;
for (i=0; i<param.length; i+=1) {
a[param[i].index] = param[i].value;
}
return a;
}
var myArray = indexArray([
{ index: 123456, value : "bananas" },
{ index: 12, value : "grapes" },
{ index: 564, value : "monkeys" }
]);

var array1 = []
array1[23454] = 2
Just doing this should be fine. There's no set array size for javascript in the way there is for java.
If you really want to do this all in a single statement, you can make an object instead like this:
var object1 = {
"23454":2,
"123":1,
"50":3
};
and then retrieve the numbers like this:
object1["23454"] //2
I don't really recommend this though. The array method is a cleaner way of doing it even if it takes multiple lines since it doesn't require string conversion. I don't know enough about how these are implemented in browsers to comment on the performance impact.
Update
Since the 1 line requirement is based on something being passed to the server, I would recommend passing a JSON object to the server in the form:
"{"23454":2,"123":1,"50":3}"
then this code will parse it to an object:
var object1 = JSON.parse(jsonstringfromserver);
and if you like you can always convert that to an array by enumerating over the properties with a for in loop:
var array1 = []
for ( num in object1){
array1[num] = object1[num];
That is probably unnecessary though since object1[123] will already return 1. You only need this if you plan on doing array specific operations.

You don't have to pre-define the size of an array before you assign to it. For example:
var _array = [];
_array[0] = "foo";
_array[1000] = "bar"; // _array.length => 1001
_array[1] //undefined
No need to initialise the appropriate number of array elements before you assign to them.
Update
It already has been pointed out that you can use an object rather than an array. However, if you want to take advantage of array methods then this is still possible. Let me give you an example:
var obj = {
0: 15,
1: 10,
2: 5,
length: 3
};
If the object contains a length property then it can be treated as an array-like object. Although you can't call array methods directly from these objects you can use array methods.
Array.prototype.join.call( obj ); // 15,10,5
In fact using the ECMAScript 5 map function you can easily convert the above object to an array.
var _array = Array.prototype.map.call( obj, function( x ) { return x; } );
The map function does not exist in all browsers but you can use the following function if it doesn't.
Array.map = Array.map || function(a, f, thisArg) {
return Array.prototype.map.call(a, f, thisArg);
}

You can do what you want with an Object in this way:
var o = {23454: 'aaaa', 23473: 'bbb'};
You will lose the array methods/fields, e.g. length, but you will gain what you said you are looking for, and you will be able to add/remove members easily.

What’s the difference between "Array()" and "[]" while declaring a JavaScript array?

What's the real difference between declaring an array like this:
var myArray = new Array();
and
var myArray = [];

There is a difference, but there is no difference in that example.
Using the more verbose method: new Array() does have one extra option in the parameters: if you pass a number to the constructor, you will get an array of that length:
x = new Array(5);
alert(x.length); // 5
To illustrate the different ways to create an array:
var a = [], // these are the same
b = new Array(), // a and b are arrays with length 0
c = ['foo', 'bar'], // these are the same
d = new Array('foo', 'bar'), // c and d are arrays with 2 strings
// these are different:
e = [3] // e.length == 1, e[0] == 3
f = new Array(3), // f.length == 3, f[0] == undefined
;
Another difference is that when using new Array() you're able to set the size of the array, which affects the stack size. This can be useful if you're getting stack overflows (Performance of Array.push vs Array.unshift) which is what happens when the size of the array exceeds the size of the stack, and it has to be re-created. So there can actually, depending on the use case, be a performance increase when using new Array() because you can prevent the overflow from happening.
As pointed out in this answer, new Array(5) will not actually add five undefined items to the array. It simply adds space for five items. Be aware that using Array this way makes it difficult to rely on array.length for calculations.

The difference between creating an array with the implicit array and the array constructor is subtle but important.
When you create an array using
var a = [];
You're telling the interpreter to create a new runtime array. No extra processing necessary at all. Done.
If you use:
var a = new Array();
You're telling the interpreter, I want to call the constructor "Array" and generate an object. It then looks up through your execution context to find the constructor to call, and calls it, creating your array.
You may think "Well, this doesn't matter at all. They're the same!". Unfortunately you can't guarantee that.
Take the following example:
function Array() {
this.is = 'SPARTA';
}
var a = new Array();
var b = [];
alert(a.is); // => 'SPARTA'
alert(b.is); // => undefined
a.push('Woa'); // => TypeError: a.push is not a function
b.push('Woa'); // => 1 (OK)
In the above example, the first call will alert 'SPARTA' as you'd expect. The second will not. You will end up seeing undefined. You'll also note that b contains all of the native Array object functions such as push, where the other does not.
While you may expect this to happen, it just illustrates the fact that [] is not the same as new Array().
It's probably best to just use [] if you know you just want an array. I also do not suggest going around and redefining Array...

There is an important difference that no answer has mentioned yet.
From this:
new Array(2).length // 2
new Array(2)[0] === undefined // true
new Array(2)[1] === undefined // true
You might think the new Array(2) is equivalent to [undefined, undefined], but it's NOT!
Let's try with map():
[undefined, undefined].map(e => 1) // [1, 1]
new Array(2).map(e => 1) // "(2) [undefined × 2]" in Chrome
See? The semantics are totally different! So why is that?
According to ES6 Spec 22.1.1.2, the job of Array(len) is just creating a new array whose property length is set to the argument len and that's it, meaning there isn't any real element inside this newly created array.
Function map(), according to spec 22.1.3.15 would firstly check HasProperty then call the callback, but it turns out that:
new Array(2).hasOwnProperty(0) // false
[undefined, undefined].hasOwnProperty(0) // true
And that's why you can not expect any iterating functions working as usual on arrays created from new Array(len).
BTW, Safari and Firefox have a much better "printing" to this situation:
// Safari
new Array(2) // [](2)
new Array(2).map(e => 1) // [](2)
[undefined, undefined] // [undefined, undefined] (2)
// Firefox
new Array(2) // Array [ <2 empty slots> ]
new Array(2).map(e => 1) // Array [ <2 empty slots> ]
[undefined, undefined] // Array [ undefined, undefined ]
I have already submitted an issue to Chromium and ask them to fix this confusing printing:
https://bugs.chromium.org/p/chromium/issues/detail?id=732021
UPDATE: It's already fixed. Chrome now printed as:
new Array(2) // (2) [empty × 2]

Oddly enough, new Array(size) is almost 2x faster than [] in Chrome, and about the same in FF and IE (measured by creating and filling an array). It only matters if you know the approximate size of the array. If you add more items than the length you've given, the performance boost is lost.
More accurately: Array( is a fast constant time operation that allocates no memory, wheras [] is a linear time operation that sets type and value.

For more information, the following page describes why you never need to use new Array()
You never need to use new Object() in
JavaScript. Use the object literal {}
instead. Similarly, don’t use new Array(),
use the array literal []
instead. Arrays in JavaScript work
nothing like the arrays in Java, and
use of the Java-like syntax will
confuse you.
Do not use new Number, new String, or
new Boolean. These forms produce
unnecessary object wrappers. Just use
simple literals instead.
Also check out the comments - the new Array(length) form does not serve any useful purpose (at least in today's implementations of JavaScript).

In order to better understand [] and new Array():
> []
[]
> new Array()
[]
> [] == []
false
> [] === []
false
> new Array() == new Array()
false
> new Array() === new Array()
false
> typeof ([])
"object"
> typeof (new Array())
"object"
> [] === new Array()
false
> [] == new Array()
false
The above result is from Google Chrome console on Windows 7.

The first one is the default object constructor call. You can use it's parameters if you want.
var array = new Array(5); //initialize with default length 5
The second one gives you the ability to create not empty array:
var array = [1, 2, 3]; // this array will contain numbers 1, 2, 3.

I can explain in a more specific way starting with this example that's based on Fredrik's good one.
var test1 = [];
test1.push("value");
test1.push("value2");
var test2 = new Array();
test2.push("value");
test2.push("value2");
alert(test1);
alert(test2);
alert(test1 == test2);
alert(test1.value == test2.value);
I just added another value to the arrays, and made four alerts:
The first and second are to give us the value stored in each array, to be sure about the values. They will return the same!
Now try the third one, it returns false, that's because
JS treats test1 as a VARIABLE with a data type of array, and it treats test2 as an OBJECT with the functionality of an array, and
there are few slight differences here.
The first difference is when we call test1 it calls a variable without thinking, it just returns the values that are stored in this variable disregarding its data type!
But, when we call test2 it calls the Array() function and then it stores our "Pushed" values in its "Value" property, and the same happens when we alert test2, it returns the "Value" property of the array object.
So when we check if test1 equals test2 of course they will never return true, one is a function and the other is a variable (with a type of array), even if they have the same value!
To be sure about that, try the 4th alert, with the .value added to it; it will return true. In this case we tell JS "Disregarding the type of the container, whether was it function or variable, please compare the values that are stored in each container and tell us what you've seen!" that's exactly what happens.
I hope I said the idea behind that clearly, and sorry for my bad English.

There is no difference when you initialise array without any length. So var a = [] & var b = new Array() is same.
But if you initialise array with length like var b = new Array(1);, it will set array object's length to 1. So its equivalent to var b = []; b.length=1;.
This will be problematic whenever you do array_object.push, it add item after last element & increase length.
var b = new Array(1);
b.push("hello world");
console.log(b.length); // print 2
vs
var v = [];
a.push("hello world");
console.log(b.length); // print 1

There's more to this than meets the eye. Most other answers are correct BUT ALSO..
new Array(n)
Allows engine to reallocates space for n elements
Optimized for array creation
Created array is marked sparse which has the least performant array operations, that's because each index access has to check bounds, see if value exists and walk the prototype chain
If array is marked as sparse, there's no way back (at least in V8), it'll always be slower during its lifetime, even if you fill it up with content (packed array) 1ms or 2 hours later, doesn't matter
[1, 2, 3] || []
Created array is marked packed (unless you use delete or [1,,3] syntax)
Optimized for array operations (for .., forEach, map, etc)
Engine needs to reallocate space as the array grows
This probably isn't the case for older browser versions/browsers.

The first one is the default object constructor call.mostly used for dynamic values.
var array = new Array(length); //initialize with default length
the second array is used when creating static values
var array = [red, green, blue, yellow, white]; // this array will contain values.

The difference of using
var arr = new Array(size);
Or
arr = [];
arr.length = size;
As been discussed enough in this question.
I would like to add the speed issue - the current fastest way, on google chrome is the second one.
But pay attention, these things tend to change a lot with updates. Also the run time will differ between different browsers.
For example - the 2nd option that i mentioned, runs at 2 million [ops/second] on chrome, but if you'd try it on mozilla dev. you'd get a surprisingly higher rate of 23 million.
Anyway, I'd suggest you check it out, every once in a while, on different browsers (and machines), using site as such

As I know the diference u can find the slice(or the other funcitons of Array) like code1.and code2 show u Array and his instances:
code1:
[].slice; // find slice here
var arr = new Array();
arr.slice // find slice here
Array.prototype.slice // find slice here
code2:
[].__proto__ == Array.prototype; // true
var arr = new Array();
arr.__proto__ == Array.prototype; // true
conclusion:
as u can see [] and new Array() create a new instance of Array.And they all get the prototype functions from Array.prototype
They are just different instance of Array.so this explain why
[] != []
:)

There is no big difference, they basically do the same thing but doing them in different ways, but read on, look at this statement at W3C:
var cars = ["Saab", "Volvo","BMW"];
and
var cars = new Array("Saab", "Volvo", "BMW");
The two examples above do exactly the same. There is no need to use
new Array(). For simplicity, readability and execution speed, use the
first one (the array literal method).
But at the same time, creating new array using new Array syntax considered as a bad practice:
Avoid new Array()
There is no need to use the JavaScript's built-in array constructor
new Array().
Use [] instead.
These two different statements both create a new empty array named
points:
var points = new Array(); // Bad
var points = []; // Good
These two different statements both create a new array containing 6
numbers:
var points = new Array(40, 100, 1, 5, 25, 10); // Bad
var points = [40, 100, 1, 5, 25, 10]; // Good
The new keyword only complicates the code. It can also produce some
unexpected results:
var points = new Array(40, 100); // Creates an array with two elements (40 and 100)
What if I remove one of the elements?
var points = new Array(40); // Creates an array with 40 undefined elements !!!!!
So basically not considered as the best practice, also there is one minor difference there, you can pass length to new Array(length) like this, which also not a recommended way.

I've incurred in a weird behaviour using [].
We have Model "classes" with fields initialised to some value. E.g.:
require([
"dojo/_base/declare",
"dijit/_WidgetBase",
], function(declare, parser, ready, _WidgetBase){
declare("MyWidget", [_WidgetBase], {
field1: [],
field2: "",
function1: function(),
function2: function()
});
});
I found that when the fields are initialised with [] then it would be shared by all Model objects. Making changes to one affects all others.
This doesn't happen initialising them with new Array(). Same for the initialisation of Objects ({} vs new Object())
TBH I am not sure if its a problem with the framework we were using (Dojo)

Well, var x = new Array() is different than var x = [] is different in some features I'll just explain the most useful two (in my opinion) of them.
Before I get into expalining the differences, I will set a base first; when we use x = [] defines a new variable with data type of Array, and it inherits all the methods that belong to the array prototype, something pretty similar (but not exactly) to extending a class. However, when we use x = new Array() it initilizes a clone of the array prototype assigned to the variable x.
Now let's see what are the difference
The First Difference is that using new Array(x) where x is an integer, initilizes an array of x undefined values, for example new Array(16) will initialize an array with 16 items all of them are undefined. This is very useful when you asynchronously fill an array of a predefined length.
For example (again :) ) let's say you are getting the results of 100 competitiors, and you're receiving them asynchronously from a remote system or db, then you'll need to allocate them in the array according to the rank once you receive each result. In this very rare case you will do something like myArray[result.rank - 1] = result.name, so the rank 1 will be set to the index 0 and so on.
The second difference is that using new Array() as you already know, instanciates a whole new clone of the array prototype and assigns it to your variable, that allows you to do some magic (not recommended btw). This magic is that you can overwrite a specific method of the legacy array methods. So, for example you can set the Array.push method to push the new value to the beginning of the array instead of the end, and you can also add new methods (this is better) to this specific clone of the Array Prototype. That will allow you to define more complex types of arrays throughout your project with your own added methods and use it as a class.
Last thing, if you're from the very few people (that I truly love) that care about processing overhead and memory consumption of your app, you'd never tough new Array() without being desperate to use it :).
I hope that has explained enough about the beast new Array() :)

I found a difference while using promises. While using array of promises (say arr, initialised as arr=[]), got an error in Promise.all(arr). Whereas when declared as arr = Array(), did not get compilation issues. Hope this helps.

I've found one difference between the two constructions that bit me pretty hard.
Let's say I have:
function MyClass(){
this.property1=[];
this.property2=new Array();
};
var MyObject1=new MyClass();
var MyObject2=new MyClass();
In real life, if I do this:
MyObject1.property1.push('a');
MyObject1.property2.push('b');
MyObject2.property1.push('c');
MyObject2.property2.push('d');
What I end up with is this:
MyObject1.property1=['a','c']
MyObject1.property2=['b']
MyObject2.property1=['a','c']
MyObject2.property2=['d']
I don't know what the language specification says is supposed to happen, but if I want my two objects to have unique property arrays in my objects, I have to use new Array().

Using the Array constructor makes a new array of the desired length and populates each of the indices with undefined, the assigned an array to a variable one creates the indices that you give it info for.