I'm making a Javascript minimax algorithm for a Tic-tac-toe game, but the IA is letting me win and also sometimes she doesn't win when she could. I tried to test to find where is the problem but i don't know how because take so many steps to follow in the process.

Here is my code, the game is set in an array like this: [" ", " ", " ", " ", " ", " ", " ", " ", " "]

The checkGameOver is returning "o", "x", "tie" or undefined.

const minimax = function (array, index, isMaximizing, testMark) {
        let result = gameFlow.checkGameOver(array, index, testMark);
        let scores = {
            "o": 1,
            "x": -1,
            "tie": 0 
        }
        if (result) {
            return scores[result];
        }
        if (isMaximizing) {
            let bestScore = -2;
            array.forEach( (item, index, array) => {
                if (item === " ") {
                    array[index] = "o";
                    let score = minimax(array, index, false, "o");
                    if (score > bestScore) {
                        bestScore = score;
                    }
                    array[index] = " ";
                }
            })
            return bestScore
        } else {
            let bestScore = 2;
            array.forEach( (item, index, array) => {
                if (item === " ") {
                    array[index] = "x";
                    let score = minimax(array, index, true, "x");
                    if (score < bestScore) {
                        bestScore = score;
                    }
                    array[index] = " ";
                }
            })
            return bestScore
        }
    }
    const minimaxPlay = function (array) {
        let bestScore = -2;
        let bestMove;
        array.forEach( (item, index, array) => {
            if (item === " ") {
                array[index] = "o";
                let score = minimax(array, index, false, "o");
                if (score > bestScore) {
                    bestScore = score;
                    bestMove = index
                }
                array[index] = " ";
            }
        })
        array[bestMove] = "o";
        gameFlow.changeCurrentPlayer();
        gameBoard.renderPlays();
    }