Create 2D array using new Array().fill(0) bug? [duplicate]

Question

I would like to create a 2d array, but i found an interesting behavior

const column = 10;
const row = 10;

let matrix = new Array(row).fill(new Array(column).fill(0));

matrix[0][1] = 1;

console.log(matrix)

and to my surprise i get the result as below:

0 2 0 0 0 0 0 0 0 0 
0 2 0 0 0 0 0 0 0 0 
0 2 0 0 0 0 0 0 0 0 
0 2 0 0 0 0 0 0 0 0 
0 2 0 0 0 0 0 0 0 0 
0 2 0 0 0 0 0 0 0 0 
0 2 0 0 0 0 0 0 0 0 
0 2 0 0 0 0 0 0 0 0 
0 2 0 0 0 0 0 0 0 0 
0 2 0 0 0 0 0 0 0 0

the entire column number 1 is set to 1, may I know why am I getting this behavior?

new Array(column).fill(0) creates an array with length column and elements as 0. new Array(row).fill(....) which fills with the array you were created, which is not creating different array for each... instead the reference to the array act as element — Pranav C Balan
– Pranav C Balan, Commented Aug 4, 2016 at 7:33

Pranav C Balan · Accepted Answer · 2016-08-04 07:34:57Z

2

The Array#fill method filling with the same array that the elements are the reference to a single array. So you need to create separate arrays as elements. So updating one will reflect all other since they are not different array they are the same.

Using Array.from() you can create and generate values by map function.

const column = 10;
const row = 10;

let matrix = Array.from({
  // create array of length `row`
  length: row 
  // map function to generate values
}, () => new Array(column).fill(0));

matrix[0][1] = 1;

console.log(matrix)

edited Aug 4, 2016 at 7:34

answered Aug 4, 2016 at 7:29

Pranav C Balan

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3 Comments

Tim Over a year ago

sry, still dont get it..why is it reference to a single array? since it generates row times of that new array?

Pranav C Balan Over a year ago

@Tim : first it creates an array of length row afterward it filling with a single array..... the fill act like matrix[0] = matrix[1] = .. = matrix[row - 1] = new Array(column).fill(0); so all the array elemnts are refering to same array....

Pranav C Balan Over a year ago

@Tim : The array would be like ref=[.....];[ref,ref,ref,...,ref] ....

ventaquil · Accepted Answer · 2016-08-04 07:30:45Z

1

@PranavCBalan have right.

It's something like that:

let matrix = new Array(row);
var x = new Array(column).fill(0);
matrix[0] = matrix[1] = .. = matrix[column - 1] = x;

Your matrix is array of the one object.

answered Aug 4, 2016 at 7:30

ventaquil

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