Merging 2-dimensional arrays in JavaScript

Let's assume that you have a pair of 2-dimensional arrays (matrices):

javascript
let a = [
    ['Erik', 'Andrea', 'Paula'],
    [1990,   1993,     2005],
    ['male', 'female', 'female']
];

let b = [
    ['Barcelona', 'Barcelona', 'Calella'],
    ['motorbike', 'car',       'car']
];

Let's assume that you want to merge both of them at [2, 1]. You expect the following result:

javascript
[
    ['Erik', 'Andrea',    'Paula',     '']
    [1990,   1993,        2005,        '']
    ['male', 'Barcelona', 'Barcelona', 'Calella']
    ['',     'motorbike', 'car',       'car']
]

Creating the merge

If you want to have the same size in every row, first create an empty matrix:

javascript
const merge = (arr1, arr2, x, y) => {
    
    // Getting rows of both arrays
    let arr1_rows = arr1.length;
    let arr2_rows = arr2.length;
    
    // Getting cols of both arrays
    let arr1_cols = arr1[0].length;
    let arr2_cols = arr2[0].length;
    
    // Calculating rows and cols of the merged matrices
    let merg_rows = Math.max(arr1_rows, arr2_rows, x + arr2_rows);
    let merg_cols = Math.max(arr1_cols, arr2_cols, y + arr2_cols);
    
    // Filling the new matrice with blank spaces
    let merged = Array(merg_rows).fill('').map(e => Array(merg_cols).fill(''));
    
    // Adding first array to the matrix
    for(let i = 0; i < arr1_rows; i ++){
        
        merged[i].splice(0, arr1[i].length, ...arr1[i]);
        
    }
    
    // Adding second array to the matrix
    for(let i = x, j = 0; i < x + arr2_rows; i ++, j ++){    
        
        merged[i].splice(y, arr2[j].length, ...arr2[j]);
        
    }
    
    // Returning result
    return merged;
    
}

Result

javascript
let res = merge(a, b, 2, 1);

/*
[
    ['Erik', 'Andrea', 'Paula', '']
    [1990, 1993, 2005, '']
    ['male', 'Barcelona', 'Barcelona', 'Calella']
    ['', 'motorbike', 'car', 'car']
]
*/