Take this [short course](https://www.codecademy.com/learn/introduction-to-javascript/modules/learn-javascript-arrays) on Codecademy to familiarize yourself with common array functions.
## Operate on a column
This method (from [this Stack Overflow post](https://stackoverflow.com/questions/66554777/how-to-change-one-column-in-2d-array-by-mapping-function-in-google-scripts)) operates over a column in a 2-d array using using the [`comma operator`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator).
```javascript
var array = [
[1, 2, 3],
[11, 12, 13]
]
array.map(row => (row[1] = row[1] * 10, row))
```
To avoid mutating the original, make a copy with [`slice()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/slice) or by using [spread syntax](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax) (or by building an entirely new array). Once copied, you can simply index into the new row to apply changes.
```javascript
var newarray = array.map(row => {
const newRow = [...row]; // copy row to avoid mutating original array
newRow[1] = newRow[1] * 10;
return newRow
})
```
#rough