Functions that output the transpose of a matrix - a new matrix where the columns and rows of the original are swapped. I'm trying to figure out why the second function outputs incorrect results?

function transpose(matrix) {
    let res = [];
    for(let i = 0;  i < matrix[0].length; i++) {
        res[i] = [];
        for(let j = 0;  j < matrix.length; j++) {
            res[i][j] = matrix[j][i];
        }
    }
    return res; 
}

function transpose(matrix) {
    let res = Array(matrix[0].length).fill([]);
    for(let i = 0;  i < res.length; i++) {
        for(let j = 0;  j < matrix.length; j++) {
            res[i][j] = matrix[j][i];
        }
    }
    return res; 
}