Initializing new object with random values for each elem in 2D javascript array?

Question

I am trying to do this basic iteration and object initialization but it seems like every row is being created with the object containing same ids. Am I getting crazy?

function makeGrid (cols, rows) {
  const grid = new Array(cols).fill(new Array(rows))
  for (let i = 0; i < cols; i++) {
    for (let j = 0; j < rows; j++) {
      grid[i][j] = {id: random()}
    }
  }
  return grid
}

This is the result:

How do you know each row is a different object (as opposed to each being a reference to the same object)? — Scott Hunter
– Scott Hunter, Commented Mar 14, 2020 at 22:57
.fill(new Array(rows)) this is your problem. Use e.g. .fill().map(_ => new Array(rows)); — ASDFGerte
– ASDFGerte, Commented Mar 14, 2020 at 22:58
Thank you! I couldn't see the reference problem there. That made it. — dugong
– dugong, Commented Mar 14, 2020 at 23:15

dugong · Accepted Answer · 2020-03-14 23:19:47Z

2

Thanks to @Scott Hunter and @ASDFGerte (in the comments): The problem was new Array(cols).fill(new Array(rows)). This is filling the newly initiated array with references to another new array new Array(rows). What I wanted to do is creating new arrays for every element in the cols array. This solves the issue:

[...new Array(cols)].map(() => new Array(rows))

answered Mar 14, 2020 at 23:19

dugong

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DCR · Accepted Answer · 2020-03-14 23:55:03Z

0

this works but is not recommended. Id's must be unique and using random doesn't guarantee that they will be.

function makeGrid (cols, rows) {
  var grid = new Array(cols)
  for (let i = 0; i < cols; i++) {
     grid[i] = new Array(rows)      
    for (let j = 0; j < rows; j++) {
      grid[i][j]=  {id: Math.floor(Math.random()*1000 )  }  ;
    }
  }
  console.log(grid);
}

makeGrid(3,4)

answered Mar 14, 2020 at 23:55

DCR

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1 Comment

dugong Over a year ago

that was a mock code, IDs don't matter here, that was not the question nor the problem. but thanks!

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