Are js array indexes set if not defined

Question

I am working on a script in which i need some data based on a lot of timestamps.

Below is just an example

var timestampData1 = [1555486016,1555486017,1555486018...]; 
var timestampData2 = [1555486016,1555486017,1555486018...];

var data = [];
data[1] = [];
$.each(timestampData1,function(index,value) {
    data[1][value] = 1;
});
data[2] = [];
$.each(timestampData2,function(index,value) {
    data[2][value] = 1;
});
console.log(data);

The example above will output the following in the console

However if i examine the data in the console, i see a lot of empty sets counting from 0 up to the very last timestamp

So my question is:

Will javascript set all of these indexes, or is it simply an indication in the console? If not, i guess that it is very bad for performance, doing it like above ?

Express the index as a string if you don't want this to happen. — Diodeus - James MacFarlane, May 03 '19 at 16:24
@GetOffMyLawn Although this: `data[1555486016]` is different from this: `data['1555486016']` — GrafiCode, May 03 '19 at 16:24
What's your end goal here? There may be a much simpler answer... — Heretic Monkey, May 03 '19 at 16:31
Your array is a sparse array [See this question](https://stackoverflow.com/questions/17295056/array-vs-object-efficiency-in-javascript) — Claudia Chersin, May 03 '19 at 16:53
@Diodeus-JamesMacFarlane Indexes cannot be strings. They must always be non-negative whole numbers. If you want strings, you need to create an object. — Scott Marcus, May 03 '19 at 16:53

score 0 · Accepted Answer · answered May 03 '19 at 16:28

Yes your assumption is correct. If you use objects which have key/value pairs whis won't be a problem, however you will you array methods like push and filter.

Example:

var timestampData1 = [1555486016,1555486017,1555486018...]; 
var timestampData2 = [1555486016,1555486017,1555486018...];

var data = [];
//Make data[1] an object not array
data[1] = {};
$.each(timestampData1,function(index,value) {
    data[1][value] = 1;
});
//Make data[1] an object not array
data[2] = {};
$.each(timestampData2,function(index,value) {
    data[2][value] = 1;
});
console.log(data);

Output:

Are js array indexes set if not defined

1 Answers1