'push' element to the subarray within nested array

Question

I have listed several ways to create nested array and add value to sub array:

Way 1: works!

// Create empty nested array:    
var arrs = [[],[],[]];
arrs[0].push(1);
arrs[1].push(2);
arrs[2].push(3);

Then, not surprisingly, arrs is updated:

[[1],[2],[3]]

Way 2: works!

var arrs = [];
for (var i = 0; i < 3; i++) {
   arrs[i] = [];
 }

arrs[0].push(1);
arrs[1].push(2);
arrs[2].push(3);

arrs:

 [[1],[2],[3]]

Way 3: ?

var arrs = [];
var subArr = [];
for (var i = 0; i < 3; i++) {
   arrs[i] = subArr;
 }

arrs[0].push(1);
arrs[1].push(2);
arrs[2].push(3);

arrs:

[[1,2,3],[1,2,3],[1,2,3]]

My question: why the way 3 leads to arrs result as [[1,2,3],[1,2,3],[1,2,3]] rather than [[1],[2],[3]]?

Interesting observation, you are making changes to subArr thrice and the same changed object is referred by 3 objects in arr — absin
– absin, Commented Feb 3, 2018 at 21:33
Array.from({length: 3}, (_, i) => ([i + 1])); ... just saying — Jonas Wilms
– Jonas Wilms, Commented Feb 3, 2018 at 21:36

Sébastien Renauld · Accepted Answer · 2018-02-03 21:29:52Z

1

In your third way, you are assigning each element of arrs to the same array by reference: subArr. There is no copying, all three items in arrs point to the same value in memory.

Naturally, when you push to arrs[0], you are then pushing to subArr. Repeat 3 times, and you then have the result you are experiencing.

answered Feb 3, 2018 at 21:29

Sébastien Renauld

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Comments

Terry Lennox · Accepted Answer · 2018-02-03 21:30:31Z

1

Because it's the same object!

If you did

arrs[i] = [];

You'd get a different result! You'd assign a new array each time instead of a reference to one array.

This is an easy trap to fall into in any programming language.

answered Feb 3, 2018 at 21:30

Terry Lennox

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2 Comments

juanli Over a year ago

Exactly. I start my first programming language as Python (in which assignment pass the variable values instead of reference). I didn't realize that problem. Thanks.

Terry Lennox Over a year ago

Yeah, different languages do this in their own way. Python is a great language too, it's nice to play about with all these different ways of doing things, it's fun!

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