Javascript unset array - javascript

How do i unset an array in javascript?
i just want to empty it - so it has nothing in it keys or anything

you can assign a new array to it:
var array = ["element1","element2","element3"];
...
array = new Array();
OR
array = [];

Assign an empty array to it
x = []

array.length = 0
should work.

What about delete?
delete array
Or to delete a single index:
delete array[x]

var array = ["elem", "item"];
...
array = []; // Empty!

FOR REFERENCE (I know its old and answered)
If you want to empty the array, that has been answered already.
If you want to unset the array, you could assign an unset variable...
var undef;
var myArr = [];
...
myArr = undef;
// typeof myArr == undefined

Related

JSON stringify does not convert array [duplicate]

In the example below, the array2.length is only 10, while in my mind, it should be 13.
Why does the "string keyed" indexes not increase the length of the array?
I can store things and still access it, and the VS debugger shows that those arrays are being stored properly. So why is the length not increased?
var array2 = new Array();
array2["a"] = new Array();
array2["b"] = new Array();
array2["c"] = new Array();
for (var i = 0; i < 10; ++i)
array2[i] = new Array();
var nothing = "";
for (var i = 0; i < array2.length; ++i)
nothing = "";
Javascript arrays cannot have "string indexes". A Javascript Array is exclusively numerically indexed. When you set a "string index", you're setting a property of the object. These are equivalent:
array.a = 'foo';
array['a'] = 'foo';
Those properties are not part of the "data storage" of the array.
If you want "associative arrays", you need to use an object:
var obj = {};
obj['a'] = 'foo';
Maybe the simplest visualization is using the literal notation instead of new Array:
// numerically indexed Array
var array = ['foo', 'bar', 'baz'];
// associative Object
var dict = { foo : 42, bar : 'baz' };
Because the length is defined to be one plus the largest numeric index in the array.
var xs = [];
xs[10] = 17;
console.log( xs.length ); //11
For this reason, you should only use arrays for storing things indexed by numbers, using plain objects instead if you want to use strings as keys. Also, as a sidenote, it is a better practice to use literals like [] or {} instead of new Array and new Object.
You're not adding items to the array; you're adding properties to the Array object.
As said above, use object for associative arrays.
If you don't you won't necessarily notice you're doing it wrong, until you innocently use "length" as an array index :
var myArray = [];
myArray["foo"] = "bar"; //works
console.log(myArray["foo"]) //print "bar"
myArray["length"] = "baz" //crash with a "RangeError: Invalid array length"
That is because you are replacing the length attribute of an array with a String, which is invalid.
"string keyed" indexes are not indexes at all, but properties. array2["a"] is the same as saying array2.a. Remember that you can set properties on any kind of variable in javascript, which is exactly what you're doing here.
You can push object to array, it will automatically get indexed (integer). If you want to add index as you want then you want to make it as object
If you want to use an object's properties as if they were like instances of a string indexed array, the work around for the length is:
var myArray = new Array();
myArray["a"] = 'foo';
myArray["b"] = 'bar';
myArray["c"] = 'baz';
let theLength = Object.keys(myArray).length

javascript re-populate an array

I am using:
var myArry = [];
as a global variable. I then populate the array with some data from the database.
If I then want to replace the array data with new data do I have to empty / reset the original array.
Or what is the correct method?
Here are some ways to empty an array:
myArray = [];
myArray = new Array();
myArray.length = 0;
myArray.splice(0, myArray.length);
These will all work.
myArray.length = 0; will empty the array. You can then repopulate it as needed.
If it comes from the database, it would be better to clear your array since some data may be deleted.
To do so, you can write myArray = [];

Why does a string index in an array not increase the 'length'?

In the example below, the array2.length is only 10, while in my mind, it should be 13.
Why does the "string keyed" indexes not increase the length of the array?
I can store things and still access it, and the VS debugger shows that those arrays are being stored properly. So why is the length not increased?
var array2 = new Array();
array2["a"] = new Array();
array2["b"] = new Array();
array2["c"] = new Array();
for (var i = 0; i < 10; ++i)
array2[i] = new Array();
var nothing = "";
for (var i = 0; i < array2.length; ++i)
nothing = "";
Javascript arrays cannot have "string indexes". A Javascript Array is exclusively numerically indexed. When you set a "string index", you're setting a property of the object. These are equivalent:
array.a = 'foo';
array['a'] = 'foo';
Those properties are not part of the "data storage" of the array.
If you want "associative arrays", you need to use an object:
var obj = {};
obj['a'] = 'foo';
Maybe the simplest visualization is using the literal notation instead of new Array:
// numerically indexed Array
var array = ['foo', 'bar', 'baz'];
// associative Object
var dict = { foo : 42, bar : 'baz' };
Because the length is defined to be one plus the largest numeric index in the array.
var xs = [];
xs[10] = 17;
console.log( xs.length ); //11
For this reason, you should only use arrays for storing things indexed by numbers, using plain objects instead if you want to use strings as keys. Also, as a sidenote, it is a better practice to use literals like [] or {} instead of new Array and new Object.
You're not adding items to the array; you're adding properties to the Array object.
As said above, use object for associative arrays.
If you don't you won't necessarily notice you're doing it wrong, until you innocently use "length" as an array index :
var myArray = [];
myArray["foo"] = "bar"; //works
console.log(myArray["foo"]) //print "bar"
myArray["length"] = "baz" //crash with a "RangeError: Invalid array length"
That is because you are replacing the length attribute of an array with a String, which is invalid.
"string keyed" indexes are not indexes at all, but properties. array2["a"] is the same as saying array2.a. Remember that you can set properties on any kind of variable in javascript, which is exactly what you're doing here.
You can push object to array, it will automatically get indexed (integer). If you want to add index as you want then you want to make it as object
If you want to use an object's properties as if they were like instances of a string indexed array, the work around for the length is:
var myArray = new Array();
myArray["a"] = 'foo';
myArray["b"] = 'bar';
myArray["c"] = 'baz';
let theLength = Object.keys(myArray).length

Javascript multidimentional array undefined object error

I am trying to make a two dimensional array out of two one dimentional arrays with this code:
var PassAssoArr = new Array();
for(k in PassPourcentNames) {
PassAssoArr[k][0] = PassPourcentNames[k]
PassAssoArr[k][1] = PassPourcentValue[k]
}
However, I get the error message: " 'undefined' is null or not an object " and it points to the first line after the for statement.
PassPourcentNames and PassPourcentValue have the same number of elements and none of the values are null. The first one contain strings and the second one integers.
Any help is greatly apreciated.
var PassAssoArr = new Array();
for(k in PassPourcentNames) {
PassAssoArr[k] = new Array();
PassAssoArr[k][0] = PassPourcentNames[k]
PassAssoArr[k][1] = PassPourcentValue[k]
}
Also instead of new Array() you can use []
var PassAssoArr = [];
for(k in PassPourcentNames) {
PassAssoArr[k] = [];
PassAssoArr[k][0] = PassPourcentNames[k]
PassAssoArr[k][1] = PassPourcentValue[k]
}
I believe this is actually faster in most JS engines.
First define PassAssoArr[k] = []; before assigning to [0] and [1].
Javascript does not support true multi-dimensional arrays.
You're trying to use nested arrays without creating the inner arrays.
You need to put an array into each element of the outer PassAssoArr:
PassAssoArr[index] = []; //Empty array literal
You're only defining one dimension of PassAssoArr - you need to set PassAssoArr[k] = new Array();
Try just doing:
PassAssoArr[k] = new Array(PassPourcentNames[k], PassPourcentValue[k]);

javascript find if value is NOT IN array

My problem with this is that the loop keeps going into the if statement even for duplicate barcodes. I'm trying to enter the if statement only for unique barcodes but at the end of the loop myArray has duplicates in it....why?
var myArray = new Array(); var i = 0;
$("li.foo").each(function(){
var iBarCode = $(this).attr('barcode');
if( !( iBarCode in myArray ) ){
myArray[i++] = iBarCode;
//do something else
}
});
Jquery has an inArray() function.
var myArray = new Array(); var i = 0;
$("li.foo").each(function(){
var iBarCode = $(this).attr('barcode');
if( $.inArray(iBarCode, myArray) == -1 ){
myArray[i++] = iBarCode;
//do something else
}
});
The in keyword search for properties, for instance when you want to know if an object has some method available. Since you are looking for values, it always returns false.
You should instead use an array search function as Gazler advises.
2021 Update
let myArray = [...new Set([...document.querySelectorAll('li.foo')].map(a => a.dataset.barcode))]
Working backwards: Create an array using the spread syntax from the matching elements, which Map only the data-barcode attribute. Use that to create a new Set, then create an array from that set

Categories