The array in JavaScript is a simple zero-based structure. The array.length returns the n + 1 where n is the maximum index in an array.
That's just how it works - when you assign 90'th element and this array's length is less than 90, it expands an array to 90 and sets the 90-th element's value. All missing values are interpreted as null.
If you try the following code:
var a = [];
a[21] = {};
a[90] = {};
a[13] = {};
console.log[JSON.stringify[a]];
You will get the following JSON:
[null,null,null,null,null,null,null,null,null,null,null,null,null,{},null,null,null,null,null,null,null,{},null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,null,{}]
Moreover, array.length is not a readonly value.
If you set a length value less than the current, then the array will be resized:
var arr = [1,2,3,4,5];
arr.length = 3;
console.log[JSON.stringify[arr]];
// [1,2,3]
If you set a length value more than the current, then the array will be expanded as well:
var arr = [1,2,3];
arr.length = 5;
console.log[JSON.stringify[arr]];
// [1,2,3,null,null]
In case you need to assign such values, you can use JS objects.
You can use them as associative array and assign any key-value pairs.
var a = {};
a[21] = 'a';
a[90] = 'b';
a[13] = 'c';
a['stringkey'] = 'd';
a.stringparam = 'e'; // btw, a['stringkey'] and a.stringkey is the same
console.log[JSON.stringify[a]];
// returns {"13":"c","21":"a","90":"b","stringkey":"d","stringparam":"e"}
console.log[Object.keys[a].length];
// returns 5
The length property of an object which is an instance of type Array sets or returns the number of elements in that array. The value is an unsigned, 32-bit integer that is always numerically greater than the highest index in the array.
Try it
Description
The value of the length property is an integer with a positive sign
and a value less than 2 to the 32nd power [2^32].
const listA = [1,2,3];
const listB = new Array[6];
console.log[listA.length];
// 3
console.log[listB.length];
// 6
listB.length = 4294967296; //2 to the 32nd power = 4294967296
// RangeError: Invalid array length
const listC = new Array[-100] //negative sign
// RangeError: Invalid array length
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 increases; for example, if you set length to 3 when it is currently 2, the array now contains 3 elements, which causes the third element to be a non-iterable empty slot.
const arr = [1, 2];
console.log[arr];
// [ 1, 2 ]
arr.length = 5; // set array length to 5 while currently 2.
console.log[arr];
// [ 1, 2, ]
arr.forEach[[element] => console.log[element]];
// 1
// 2
As you can see, the length property does not necessarily indicate the
number of defined values in the array. See also Relationship between length and numerical properties.
Writable: If this attribute set tofalse, the value of the property cannot be changed.Configurable: If this attribute set tofalse, any attempts to delete the property or change its attributes [Writable,Configurable, orEnumerable] will fail.Enumerable: If this attribute set totrue, the property will be iterated over duringfororfor...inloops.
Examples
Iterating over an array
In
the following example, the array numbers is iterated through by looking at the length property. The value in each element is then doubled.
const numbers = [1, 2, 3, 4, 5];
const length = numbers.length;
for [let i = 0; i 3] {
numbers.length = 3;
}
console.log[numbers]; // [1, 2, 3]
console.log[numbers.length]; // 3
Create empty array of fixed length
const numbers = [];
numbers.length = 3;
console.log[numbers]; // [empty x 3]
Specifications
| ECMAScript Language Specification # sec-properties-of-array-instances-length |
Browser compatibility
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