console.log outputs a Value different when using debugger

Question

I have been making a sudoku solver in JavaScript. There is a very weird behaviour. Here is the code and context:

solveSudoku = ()=>{
      let sectionsCopy_ =sectionsCopy

      for(let sect =0; sect< 9; sect++){
        for(let box =0; box <9; box++){
          if(sectionsCopy[sect][box] === ''){
            for(let num=1; num<=9;num++){
              if(this.isPossible(sect,box,num)){
                sectionsCopy[sect][box]=''+num
                this.solveSudoku()
                sectionsCopy[sect][box]=''

              }
            }
            return
          }
        }
      }
      debugger
      console.log(sectionsCopy)

  }

This is inside a class

sectionsCopy is a global variable

sectionsCopy_ is just for debugging (to see the value of the global var easily)

isPossible() is a function that determines if num is valid in that place

sect is the index of the 3x3 square in Sudoku

box is the index of the unit box in Sudoku

The problem:

Consider this Sudoku:

When you click solve the sectionsCopy variable will store :

[
     ["2","6","" ,"4","7","" ,"5","8","1"],
     ["" ,"" ,"3","" ,"" ,"" ,"" ,"" ,"4"],
     ["" ,"1","5","" ,"" ,"8","7","6","3"],
     ["" ,"3","" ,"" ,"" ,"6","" ,"" ,"8"],
     ["4","8","9","" ,"" ,"2","3","1","" ],
     ["" ,"7","" ,"8","3","" ,"" ,"" ,"" ],
     ["6","9","" ,"3","" ,"" ,"" ,"1","" ],
     ["" ,"" ,"8","" ,"9","" ,"5","" ,"" ],
     ["" ,"" ,"7","2","" ,"" ,"" ,"9","6"]
]

When I run it without the debugger being on I get this result:

If I turn on the debugger, I see this:

Here is side by side, the results when I dont use debugger vs when I use debugger:

I was expecting behaviour under debugger to be the same without debugger (or else it will be useless like this case)

I tried console.log(sectionsCopy_) but I know arrays are immutable so it didnt matter

It says: "This value was evaluated upon expanding. it may have changed since then" What does that mean? — Mina Eskandar
– Mina Eskandar, Commented Nov 9, 2022 at 18:30
Means the value there is not the same value as it was when you logged it. console lazy logs data. stackoverflow.com/questions/23392111/console-log-async-or-sync or stackoverflow.com/questions/4057440/… — epascarello
– epascarello, Commented Nov 9, 2022 at 18:37

Mustafa Poya · Accepted Answer · 2022-11-13 04:52:10Z

1

Thanks to @epascarello note.

seems like console.log is outputting data while it's being changed.

The solution I did was

let solution = JSON.parse(JSON.stringify(sections))
console.log(solution)

instead of

console.log(sections)

edited Nov 13, 2022 at 4:52

Mustafa Poya

3,1236 gold badges30 silver badges48 bronze badges

answered Nov 9, 2022 at 19:11

Mina Eskandar

334 bronze badges

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