How to assign a single element in a multidimensional array (JavaScript)

Question

I don't know why I'm seeing the below:

const sideLength = 4;
const multiArray = new Array(sideLength).fill(new Array(sideLength));
console.log(multiArray)
var counter = 1;
multiArray[3][1] = counter;
console.log('multiArray:', multiArray)

Console output:

[ [ , , ,  ], [ , , ,  ], [ , , ,  ], [ , , ,  ] ]
multiArray: [ [ , 1, ,  ], [ , 1, ,  ], [ , 1, ,  ], [ , 1, ,  ] ]

I would have expected the second line of output to be:

multiArray: [ [ , , ,  ], [ , , ,  ], [ , , ,  ], [ , 1, ,  ] ]

Why is that 1 being added to every array's index=1 element?

marvel308 · Accepted Answer · 2017-09-23 15:38:30Z

6

The fill() method fills all the elements of an array from a start index to an end index with a static value.

That means all elements would refer to the same array. Changing the value of of one element would change the value of all elements since they refer to the same static object

answered Sep 23, 2017 at 15:38

marvel308

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2 Comments

vnt Over a year ago

thanks for your explanation, @marvel308. could you please help me to understand why the first console.log(multiArray) in the example above also log updated array value out

vnt Over a year ago

thanks @marvel308. there must be something wrong with console dev tool :). image.prntscr.com/image/a6vTGz-CQWq40RFl8pbfdQ.jpg

guest271314 · Accepted Answer · 2017-09-23 15:44:46Z

2

You can use Array.from() to create an Array having a specific .length

const sideLength = 4;
const multiArray = Array.from({length:sideLength}, () => new Array(sideLength));
console.log(multiArray)
var counter = 1;
multiArray[3][1] = counter;
console.log('multiArray:', multiArray)

answered Sep 23, 2017 at 15:44

guest271314

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