The arr variable in foreach has the same name, so arr2 is operated.
Delete the third parameter (arr)

That’s it, you can refer to the instructions on Yiha mdn:

in this code

arr2.forEach((item,index,arr)=>{ arr.push(item) console.log(arr) })

arr points to arr2.

You can make the following modifications

let arr1 = [] arr.push(1) let arr2 = [2,3,4,5] arr2.forEach((item,index)=>{ arr1.push(item) console.log(arr1) })