Javascript merge to array with some identical value at certain index

What I understand from your question is you have two Arrays, arr1 and arr2, and you want to generate a new Array, arrfinal, with the folowing conditions

1. Items from arr1 should be copied to arrfinal
2. If there exists an arrfinal[i][0] === arr2[j][0] for some i, j, then items of arr[2][j] from index 1 should be appened to the end of the items from arr1[i] in arrfinal
3. If this does not happen, push a new Array to arrfinal with index 0 of arr2[j][0] padded by nulls before appending the other items from arr2[j]
4. All items in arrfinal should have the same length

var arrfinal = [],
    key_map = {},
    max_len_1 = 0,
    max_len_2 = 0,
    i, j;

for (i = 0; i < arr1.length; ++i) {
    max_len_1 = Math.max(max_len_1, arr1[i].length); // find maximum length of arr1
    arrfinal.push(arr1[i].slice()); // copy to final
    key_map[arr1[i][0]] = arrfinal.length - 1; // mapping for later
}

for (i = 0; i < arrfinal.length; ++i) { // fix lengths
    while (arrfinal[i].length < max_len_1) {
        arrfinal[i].push(null);
    }
}

for (i = 0; i < arr2.length; ++i) {
    max_len_2 = Math.max(max_len_2, arr2[i].length); // find maximum length of arr2
    if (key_map.hasOwnProperty(arr2[i][0])) { // entry exists from arr1
        j = key_map[arr2[i][0]];
    } else { // generate a new entry
        j = arrfinal.push([arr2[i][0]]) - 1;
        while (arrfinal[j].length < max_len_1) { // fix length
            arrfinal[j].push(null);
        }
    }
    Array.prototype.push.apply(
        arrfinal[j],
        arr2[i].slice(1)
    );
}

for (i = 0; i < arrfinal.length; ++i) { // fix lengths one last time
    while (arrfinal[i].length < max_len_1 + max_len_2 - 1) {
        arrfinal[i].push(null);
    }
}

It should be noted however, that this may give unexpected results if there exists multiple items which have the same value at index 0 per arr1 or arr2, for example

var arr1 = [
    [ 'foo',      1],
    ['fizz', 'buzz']
];
var arr2 = [
    ['foo', 'bar']
    ['foo', 'baz']
];

would result in

[
    [ 'foo',      1, 'bar', 'baz'],
    ['fizz', 'buzz',  null       ]
]

Further, if these are guaranteed to be unique then perhaps you should consider using an Object instead of an Array