Visibility of arrays in javascript ("undefined")

Question

I tried to calculate the sum of two matrizes (multi-dimensional-arrays), but I get the following error-message:

Uncaught TypeError: Cannot set properties of undefined (setting '0')
    at matAdd (matrixCalc.js:28)

when I do this code↓. I don't understand why "matSum[0][0]" is undefined.

// M1 + M2
function matAdd(m1, m2){        
    let matSum = new Array(m1.length);
    for (let i=0; i<m1.length; i++){       //create a blanco-matrix
        matSum=new Array(m1[0].length);
    }
    
    if (m1.length == m2.length && m1[0].length==m2[0].length){
        for (let i=0; i<m1.length; i++){
            for (let j=0; j<m1[0].length; j++){
                matSum[i][j]=m1[i][j]+m2[i][j];                  //HERE THE ERROR OCCURS
            }
        }
    }
    else console.log("Dimension-Error")
    return matSum;
} 

the code with line-numbers

Thx for the help:)

Answer 1

Try to initiate your matrix like this

function matAdd(m1, m2){        
    let matSum = new Array(m1.length);
    for (let i=0; i<m1.length; i++){       //create a blanco-matrix
        matSum [i] =new Array(m1[0].length); // for each column
    }
    
    if (m1.length == m2.length && m1[0].length==m2[0].length){
        for (let i=0; i<m1.length; i++){
            for (let j=0; j<m1[0].length; j++){
                matSum[i][j]=m1[i][j]+m2[i][j];                  //HERE THE ERROR OCCURS
            }
        }
    }
    else console.log("Dimension-Error")
    return matSum;
}