In javascript is it possible to directly set length property of an array. For example I can shorten the array like this:
var arr = [1, 2, 3, 4];
arr.length = 2;
console.log(arr); // Array [ 1, 2 ]
// arr[2], arr[3] are deleted
I would expect the length to be read-only (e.g. as in java).
As I understand, array in javascript is also an object:
console.log(typeof [1, 2, 3]); // object
How the length property works under the hood?
Array.prototype.length is a writable property of array.
Adding description from Array#lengthMDN
You can set the length property to truncate an array at any time. When you extend an array by changing its length property, the number of actual elements does not increase; for example, if you set length to 3 when it is currently 2, the array still contains only 2 elements. Thus, the length property does not necessarily indicate the number of defined values in the array.
This can be used to shorten an array.
As I understand, array in javascript is also an object:
console.log(typeof [1, 2, 3]); // object
Check Why does typeof array with objects return "Object" and not "Array"?
Generally, the length property is determined based on the highest index in the array.
Reading the length
1) For a dense array, this means that the length corresponds strictly to the number of elements:
var fruits = ['orange', 'apple', 'banana']; //fruits is a dense array
fruits.length // prints 3, the real count of elements
fruits.push('mango');
fruits.length // prints 4, one element was added
var empty = [];
empty.length // prints 0, empty array
The dense array does not have empties and the number of items corresponds to highestIndex + 1. In [3, 5, 7, 8] the highest index is 3 of element 8, thus the array size is 3 + 1 = 4.
2) In a sparse array (which has empties), the number of elements does not correspond to length value, but still is determined by the highest index:
var animals = ['cat', 'dog', , 'monkey']; // animals is sparse
animals.length // prints 4, but real number of elements is 3
var words = ['hello'];
words[6] = 'welcome'; //the highest index is 6. words is sparse
words.length //prints 7, based on highest index
Modifying the length
1) Modifying the property leads to cut the elements (if the new value is smaller than the highest index):
var numbers = [1, 3, 5, 7, 8];
numbers.length = 3; // modify the array length
numbers // prints [1, 3, 5], elements 7 and 8 are removed
2) or creating a sparse array (if the new value is bigger than the highest index):
var osTypes = ['OS X', 'Linux', 'Windows'];
osTypes.length = 5; // creating a sparse array. Elements at indexes 3 and 4
// do not exist
osTypes // prints ['OS X', 'Linux', 'Windows', , , ]
Please read this post, which covers in details everything about the length of an array.
The necessary steps to be fulfilled when length is being set on an Array is described on section 9.4.2.4 ArraySetLength of ES 6.0 language specification.
It deletes index properties from the old length of the array to the new length of the array according to step 19:
While newLen < oldLen repeat,
Set oldLen to oldLen – 1.
Let deleteSucceeded be A.[[Delete]](ToString(oldLen)).
Assert: deleteSucceeded is not an abrupt completion.
If deleteSucceeded is false, then
Set newLenDesc.[[Value]] to oldLen + 1.
If newWritable is false, set newLenDesc.[[Writable]] to false.
Let succeeded be OrdinaryDefineOwnProperty(A, "length", newLenDesc).
Assert: succeeded is not an abrupt completion.
Return false.
Whenever you set up a property for the array, it checks first to see if the property is length and if it is, it's supposed to do the ArraySetLength operation. This operation is described in section 9.4.2.1 [[DefineOwnProperty]] (this for Array Exotic Objects).
When the [[DefineOwnProperty]] internal method of an Array exotic object A is called with property key P, and Property Descriptor Desc the following steps are taken:
Assert: IsPropertyKey(P) is true.
If P is "length", then
Return ArraySetLength(A, Desc).
If it's not length and it's an indexed property the other steps in the operation are performed. Basically sets the value of the length property to the value of the index property + 1. As far as I can tell, the ArraySetLength operation isn't being used here to set the new length of the array.
Is it possible ?
Yes, it's possibly to directly set length property of an array.
You could set length of array to be shorten or higher ( to create sparsed array) then existing array. That's the way you can achieve desired behaviour.
Information about that from First edition of EcmaScript Standart-262, 15.4 :
Specifically, whenever a property is added whose name is an array
index, the length property is changed, if necessary, to be one more
than the numeric value of that array index; and whenever the length
property is changed, every property whose name is an array index whose
value is not smaller than the new length is automatically deleted.
So when you assigned lower value of length than it was before for an array deleting items( they would be collected by garbage collector ECMA-262, 15.4.5.1) from that array.
How length determined ?
The length of an array is the highest index + 1. However you can update length of Array in both direction ( decrease to delete elements and increase to create sparsed Array).
How to create Array with defined length ?
To create Array with defined length, pass length value into Array constructor as in code below:
var length = 10;
var newArray = Array(length);
console.log(newArray.length);// 10
Use full links:
Mozzilla MDN Array
Delete JavaScript Array Elements
W3Schools JavaScript Array length Property
arr = arr.slice(0, 2); will give you the first 2 elements of the array as a new array.
I would say this method is much more clear than using array.length = number to set the length of an array.
From the specs :
Array objects give special treatment to a certain class of property
names.
...
whenever a property of an Array object is created or changed, other
properties are adjusted as necessary to maintain this invariant.
Basically since the built-in array methods like join, slice, indexOf, etc all get affected by the property change, it is also necessary to update the Array.
In this case, since length is changed, there will be a change to the keys in the array.
You may take a look to Paragraph 9.4.2: Array Exotic Objects, and subsequent paragraph "ArraySetLength" where is described the algorithm:
An Array object is an exotic object that gives special treatment to array index property keys (see 6.1.7). A
property whose property name is an array index is also called an element. Every Array object has a length
property whose value is always a nonnegative integer less than 2 32 . The value of the length property is
numerically greater than the name of every own property whose name is an array index; whenever an own
property of an Array object is created or changed, other properties are adjusted as necessary to maintain this
invariant. Specifically, whenever an own property is added whose name is an array index, the value of the
length property is changed, if necessary, to be one more than the numeric value of that array index; and
whenever the value of the length property is changed, every own property whose name is an array index
whose value is not smaller than the new length is deleted. This constraint applies only to own properties of an
Array object and is unaffected by length or array index properties that may be inherited from its prototypes.
Related
var myNumb = 5;
var myArr = Array(myNumb);
console.log(myArr.length); // 5
console.log(myArr[0]) // undefined, as well as for myArr[1] myArr[2] myArr[3] myArr[4]
These lines create an array myArray that has a length of 5, but it's also said "an empty array", in fact myArr[0] is undefined, for example.
I can't understand how is possible to have an empty array with a length different from zero. If the array has not items, how can it have a length different from zero?
Positions in an array are still there even if they have no value, they are empty. Say you have the following code:
var myArray = new Array(5);
myArray[4] = 'Hello';
Do you expect 'Hello' to be in position 0 or 4? The answer is, of course, 4. The array has a length of 5, and even if indices 0 to 3 are empty, index 4 has a value.
As per MDN documentation, using the Array constructor yields the following:
If the only argument passed to the Array constructor is an integer
between 0 and (2^32)-1 (inclusive), this returns a new JavaScript array
with its length property set to that number (Note: this implies an
array of arrayLength empty slots, not slots with actual undefined
values). If the argument is any other number, a RangeError exception
is thrown.
JavaScript Array
I am confused regarding javascript arrays and how the indices work in these arrays. The length of the array is always 0 no matter how many key value elements are there in the array. Also we can see that the array has only two elements provided that the key is string. Lets consider scenario 1, i have following code.
arr = [];
arr['home'] = 1234;
arr['cat'] = 12345;
console.log(arr.length);
console.log(arr);
Now consider second scenario, i have an empty array and i assign some number to the 20th index of the array. Then when i output the array, the length shows 21 with all the other locations being 'undefined'.
arr = [];
arr[20] = 20;
console.log(arr.length); // Length shows 20 Size of array increased to 21
Now in the third scenario i will assign both numbered indices and string indices and it shows another strange behavior. The length of the array doesn't count the value with string based index. Even though i have 4 number indices and one string based index. So the length should be 6 but the length shows 5.
arr = [];
arr[4] = 4;
arr['home'] = 'home';
console.log(arr);
Now i have two questions.Firstly, Why is the length function not considering string indexed item? Secondly, Why in the case of numbered indices, does the array size is increased to atleast that number?
You have to understand that JavaScript arrays are objects, with some additional behavior. Just like objects, you can assign properties to them.
If the property's name is a numeric string and there is no higher numeric key, the length property is updated. Non-numeric keys don't affect the length property.
The methods in Array.prototype (filter, slice, ...) only work with the numeric keys of the array.
Javascript doesn't have associative arrays like you see in PHP. It has arrays, which have a numerical index associated with the value, and then it has objects which are mapped key => value.
Javascript Arrays and Javascript Objects
const arr = [1, 2, 3, 4, 5, 6, 7];
console.log(arr.length); // outputs 7
const obj = {}
console.log(obj.length); // undefined
// set the key => value for obj from arr
arr.forEach(a => obj[a] = a);
console.log(obj[0]); // undefined
console.log(obj[1]); // 1
console.log(arr[0]); // 1
console.log(arr[1]); // 2
JS Bin
In the example above, you can see that arr is an array and its values are mapped index => value, while obj is an object and its values are mapped key => value. Objects can function as hash maps/associative arrays, but arrays cannot.
If you were to want to get the number of values in an object/hash map, you can always do...
Object.keys(obj).length
This is because Object.keys(...) returns the keys of an object as an array.
For an accurate answer, I should clarify that arrays are objects, however, they function differently than an object inherently does. Per the docs, arrays are "high-level, list-like objects"
Can Array.splice() be used to create a sparse array by adding an element at an index beyond the last element of the array?" I need this because in some situations I just want to push onto the array, but in other situations I need to splice into the array. But trying to use splice to make the array sparse did not work, though in my particular situation I was able to implement some code to test whether to use splice, or just assign an array element at an index beyond the array's length.
No. The ECMAScript specification does not allow a relative start position greater than the array length. From ES2015 on Array.prototype.splice, step 7:
...let actualStart be min(relativeStart, len).
The variable actualStart is what's actually used for the splice algorithm. It's produced by the minimum of relativeStart (the first argument to the function) and len (the length of the array). If len is less than relativeStart, then the splice operation will use len instead the actual argument provided.
In practical terms, this means that you can only append values onto the end of arrays. You cannot use splice to position a new element past the length index.
It should be noted the length of the array is not necessarily the index of the last item in the array plus 1. It can be greater.
Then, you can't use splice to insert elements beyond the length of the array, but if you make sure the length is large enough, you can insert beyond the last index plus 1.
var arrSplice = ['what', 'ever'];
arrSplice.length = 10; // Increase the capacity
arrSplice.splice(10, 0, 'foobar'); // Now you can insert items sparsely
console.log(arrSplice.length); // 10
console.log(arrSplice[arrSplice.length - 1]); // foobar
Array.splice() cannot be used to create sparse arrays. Instead, if the index argument passed to Array.splice() is beyond the length of the array, it seems that the element just gets appended to the array as if Array.push() had been used.
/* How you might normally create a sparse array */
var arrNoSplice = ['foo', 'bar'];
arrNoSplice[10] = 'baz';
console.log(arrNoSplice.length); // 11
/* Demonstrates that you cannot use splice to create a sparse array */
var arrSplice = ['what', 'ever'];
arrSplice.splice(10, 0, 'foobar');
console.log(arrSplice.length); // 3
console.log(arrSplice[arrSplice.length - 1]); // foobar
What would the length property of the array x be?
var x=new Array();
x[0]="Monday";
x[1]="Tuesday";
x[3]="Thursday";
It would be 4.
The .length property is defined to be one more than the numeric value of the largest integer-like property name. The largest (when interpreted as a number) such property name in the example code is 3, so the .length value is therefor 4.
If you set a property of an array, such that the property name is an integer (or a string that looks like an integer), then length is updated to be one more than that integer value. Symmetrically, if you set length to some value, then all properties whose names are integers greater than or equal to the new value are implicitly deleted.
if you want to know the length of your array 'x' then you can just do something like:
var length = x.length;
keep in mind that arrays are 0 based. So the length here will be 4, however, if you want to access the indexes of your array you will need to use 0, 1, 2, 3.
further, if you want to know the length, after getting your var length you could do either of the following:
console.log(length);
alert(length);
I am using the following pattern to index an injection from pairs of numbers to numbers:
var myHash = {};
...
for (... billion of iterations ...)
var x = someNum;
var y = otherNum;
myHash[x + "," + y] = z;
The problem with this code is that I'm using a string as the key of myHash, which has been tested to be much slower than integer keys. My question is: what is a more intelligent way to combine 2 numbers before using them as keys of an object? I.E., how to combine 2 doubles into an unique Integer?
There is the definition of an array in JavaScript:
Array objects give special treatment to a certain class of property names. A property name P (in the form of a String value) is an array index if and only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal to 232 - 1. A property whose property name is an array index is also called an element. Every Array object has a length property whose value is always a nonnegative integer less than 232. The value of the length property is numerically greater than the name of every property whose name is an array index; whenever a property of an Array object is created or changed, other properties are adjusted as necessary to maintain this invariant. Specifically, whenever a property is added whose name is an array index, the length property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the length property is changed, every property whose name is an array index whose value is not smaller than the new length is automatically deleted. This constraint applies only to own properties of an Array object and is unaffected by length or array index properties that may be inherited from its prototypes.
In other words, if the index you specify is a number representing an integer between 0 and 0xFFFFFFFE, then it is used as an array index. Any other value is taken as a string and it is used to create an object member instead of an array item.
So if you have constraints on your indices which would fit the valid range (0 to 0xFFFFFFFE) then you're good. Otherwise, what you have is probably the fastest.
So the following represents string indices which are members of object myHash:
myHash[x + "," + y] = z;
Someone mentioned using an array of arrays. That would not help you. You'd get many arrays instead of many strings. It would probably be about the same if not slower. The idea is something like this:
myHash[x] = []; // initialize the sub-array (must be done only once per value of 'x'
myHash[x][y] = z; // save z in that array
I do not recommend the double array because it will initialize one array for each value of 'x' on top of myHash and that probably not any faster than having the string concatenation (especially because you'll have to test whether the myHash[x] array was already defined or not...).
So... it is possible to write:
myHash[3.3] = "that worked?";
But if after that you check out the length, you'll notice it is zero:
console.log("Hash length = " + myHash.length);
This is because 3.3 is not an integer.