i was tryng to write a js script to calculate the determinant of a matrix
to do so i created a function to give me a matrix of a specific dimension filled with a certain value.
as i was tryng to debug my unfunctional code i run into a paradox, wich you can experience at line 118.
while creating a 4×4 of zeors js creates a 4×4 whith every rows that starts with a 1.
i tried changing the nature of the variable, the name, and running the same piece of code in other parts of the script, i just wanted a 4×4 filled with zeros.
as you can see in the last line this doesn’t happen in any other part of the code.
can someone please explain what am i doing worng? thanks in advance and sorry for my poor english.

/* implement matrix visualizer

implement fast matrix creation

implement matrix multiplication

implement matrix inversion

convert a letter in a column vector with te one  */

function matrix_visualizer(M){
    n=M.length;
    M_str='';
    for(let i=0; i<n; i++){
        m=M[i].length;
        
        for(let j=0; j<m; j++){
            M_str+=M[i][j].toString()+' ';
        }
        M_str+='<br> '; /* <br> è lo n degli output innerHTML */
    }
    document.getElementById("matrix").innerHTML=M_str;
    return
} /* put a preatty matrix visualization */

function M_of_a(n,m,a){
    a=parseInt(a);
    M=[];
    for(let i=0; i<n; i++){
        M.push(Array(m).fill(a));
    }
    return M;
} /* creates an n x m matrix full of a elements  */

function eye(n){
    M=M_of_a(n,n,0);
    for(let i=0; i<n; i++){
        M[i][i]=1;
    }
    return M;
} /* creates an identy square matrix */

function matrix_product(A, B){
    /* check if both the matrix have consistent dimension */
    /* A */
    let nA=A.length;
    let mA=A[0].length;
    for(let i=1; i<nA;i++){
        if(A[i].length!=mA){
            alert('matrix A not well written');
            return [0];
        }
    }
    /* B */
    let nB=B.length;
    let mB=B[0].length;
    for(let i=1; i<nB;i++){
        if(B[i].length!=mB){
            alert('matrix B not well written');
            return [0];
        }
    }
    /* check if the matrix are compatible for product */

    if(mA!=nB){
        alert('matrix size non compatible for multiplication');
        return [0];
    }
    /* do the actual product */
    let C=M_of_a(nA,mB, 0);
    for(let i=0; i<nA; i++){
        for (let j=0; j<mB; j++){
            for (let k=0; k<mA; k++){
                C[i][j]=parseInt(C[i][j]);
                C[i][j]+=parseInt(A[i][k])*parseInt(B[k][j]);
            }
        }
    }
    return C;
} /* product of the A and B matrix */

function matrix_det(M){
    /* check if M is a consistent squared matrix */
    console.log(M);
    let det=0;
    n=M.length;
    m=M[0].length;
    
    for (let i=1; i<n; i++){
        if(M[i].length!=m){
            alert('matrix not consistent');
            console.log('matrix not consistent');
            return 0;
        }
    }

    if(n!=m){
        alert('not squared matrix');
        console.log('not squared matrix');
        return 0;
    }
    
    /* easy determinats */

    /* n=1 */
    if(n==1){
        det=M[0][0];
        return det;
    } else if(n==2){
        det=(parseInt(M[0][0])*parseInt(M[1][1]))-(parseInt(M[1][0])*parseInt(M[1][0]));
        return det;
    } else{
        for(let k=0; k<n; k++){
            if(parseInt(M[0][k])!=0){
                console.log(parseInt(M[0][k]));
                /* this should optimize the process by a lot */
                let Sub_matrix=M_of_a(n-1,n-1,0);
                console.log(Sub_matrix);
                console.log(M_of_a(n-1,n-1,0));
               
                /* WHAT IS HAPPENING? */
                /* buld A */
                for(i=1; i<n; i++){
                    for(j=0; j<n; j++){
                        let jj=0;
                        if(j!=k){
                            Sub_matrix[i-1][jj]+=parseInt(M[i][j]);
                            jj++;
                        }
                    }
                }

                det+=parseInt(M[1][k])*matrix_det(Sub_matrix);
            }
        }
    }
    return det;
}

console.log(matrix_det(eye(5))); 

let A=M_of_a(4,4,0);
console.log(A);