Array Length in Javascript

Question

I am an absolute beginner to programming and Javascript and was watching one of Douglas crockford's videos where he says the following

Arrays unlike objects have a special length member

It is always 1 larger than the highest integer subscript

In this array

var a = [1,2,3,4,5,6,7]

a.length equals to 7.

So I am not quite sure what 1 larger than highest integer subscript means...? Is it just an outdated piece of info from a older version of Javascript or am i missing something ?

Answer 1

length equals to no of elements in the array and index starts from 0 , understand it like you are not starting with 1 instead with 0 so you will have 1 less than total elements(length). so

length = total elements

and

last index = length-1;

Answer 2

数组从0（第一个元素）开始索引，但长度从1（一个元素）开始（如果没有元素，则数组的长度为-1，如果没有元素，则长度为0；如果有2个元素，第二个元素的索引为1，长度为2）。

Answer 3

It just means that the length of an array is always * its largest index + 1.

Consider:

var a = [];
a[6] = 'foo';
console.log(a.length) //7
a[20] = 'bar';
console.log(a.length) //21

*actually not always, like for example when you use Array constructor with number argument: var a = new Array(5) , the array is empty, but it has its length explicitely set to 5

Answer 4

Indexes and lengths are different. While a JavaScript array will start at 0, the length will always be the true number. This is helpful since on a slice, the last number is NOT inclusive.