Sort() is mutating my original array. Is there a different workaround? [duplicate]

Question

I've searched other threads and I couldn't find a working solution to my problem. I tried slice() for the array but no luck. I've got a array, first I create a copy of the original array and after a few second I overwrite one value [0][1]=555. And this affects my copy/original array, the value 1000 gets overwritten with 555. Any ideas on how to overcome this? I need to compare the two tables after the value gets changed. Here is my javascript code:

let intialstationtablevalues = [
["station name 1",75,100,100,100,100,100,100,0,100,100,100,100,1],
["station name 2",200,220,100,100,100,100,100,2,100,100,100,100,2],
["station name 3",300,110,100,100,100,100,100,0,100,100,100,760,3],
["station name 4",400,100,109,100,100,100,100,3,100,100,100,100,4],
["station name 5",500,100,100,100,100,100,100,0,100,100,100,100,5],
["station name 6",600,130,134,100,100,100,100,2,100,100,100,340,6],
["station name 7",700,100,100,100,100,100,100,0,100,100,100,100,7],
["station name 8",800,200,340,100,100,100,100,5,100,100,100,10,8],
["station name 9",900,100,100,100,100,100,100,0,100,100,100,6,9],
["station name 10",1000,100,900,100,100,100,100,2,100,100,100,100,10],
];
let old = []
let copy = []


setInterval(read,2000)
setTimeout(changevalue,10000)

function changevalue(){
  console.log("value changed")
  intialstationtablevalues[0][1]=555;
}


function read(){
        if(copy==""){
            copy = intialstationtablevalues.slice()
      }
        sortnow = intialstationtablevalues.sort(function(a,b){
            return b[1]-a[1];
        })

        console.log("sortnow",sortnow)

        if(old==""){
            copy.forEach(function(item,index){
                old.push(item)
            })
            old.sort(function(a,b){
                return b[1]-a[1];
            })
        }

        console.log("old",old)
}

You make a copy with .slice() but then you proceed to sort the original array anyway. The .sort() method sorts the array for which it's invoked. — Pointy
– Pointy, Commented Nov 8, 2019 at 16:10
.slice is the correct solution but you've fallen into shallow vs deep copy confusion — Aluan Haddad
– Aluan Haddad, Commented Nov 8, 2019 at 16:12
if(copy=="") - That's not how you should test for an empty array. It works but only because JavaScript has truthy/falsy values. — Andreas
– Andreas, Commented Nov 8, 2019 at 16:14
Aluan Haddad so is there a way of doing this right to get what I want ? — ukulele
– ukulele, Commented Nov 8, 2019 at 16:17

Viktor Garba · Accepted Answer · 2019-11-08 16:31:29Z

2

sortedArray = [...arrayYouWantToSort].sort() Spreading the array you want to copy into a new array and then sort that array. Fun way to do it!

sortedArray = [...arrayYouWantToSort];
sortedArray[0][1] = whatever // whatever you wanna change
sortedArray.sort()

The main point is that spreading the arrayYouWantToSort into a new array literal creates a new array in memory. So arrayYouWantToSort and sortedArray are now total different physical parts of memory, but they have the same 'values'. So you can change values in one without affecting the other.

So you first create the copied array, and then mutate the new array.

edited Nov 8, 2019 at 16:31

answered Nov 8, 2019 at 16:14

Viktor Garba

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9 Comments

Viktor Garba Over a year ago

Ah didn't read the whole thing sorry. Still just use the spread operator. Ill update my response

ukulele Over a year ago

Well I think I've done it correctly if(copy.length===0){ copy = [...intialstationtablevalues] }

Viktor Garba Over a year ago

should do it. And then use the copy

ukulele Over a year ago

it still throws the 555 value in the copy array or maybe I'm doing somethind wrong...;)

Viktor Garba Over a year ago

Hmm. The only way that could happen is either intialStationableValues has 555 set before you spread, or you set it after. What is the point of this function exactly?

|

Ahmed · Accepted Answer · 2023-12-27 11:26:02Z

1

As of 2023 .toSorted() can be used to return sorted array without mutating original array

answered Dec 27, 2023 at 11:26

Ahmed

314 bronze badges

Comments

M.Amer · Accepted Answer · 2019-11-08 16:21:55Z

-1

I think .slice() is working fine here, try to replace the if statment with

if(copy.length ===0)

to check the empty array

answered Nov 8, 2019 at 16:21

M.Amer

7087 silver badges13 bronze badges

1 Comment

ukulele Over a year ago

I'm still getting the same 555 result in the copy array

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