text_table.js

test_data = [
    ['IP Source', 'Source Mask', 'IP Destination', 'Destination Mask'],
	['123.456.789.000', '/30', '111.111.111.111', '/30'],
	['123.23.45.233', '/31', '127.0.0.1', '/8'],
	['1.1.1.1', '/31', '1.2.3.4', '/30'],
	['n/a', '/31', '123.432.234.567', '/12'],
];

// List<List<String>> -> String
// Assumes a non-empty list of non-empty lists of Strings
function create_table(lst){
    retVal = "";
    size = lst[0].length;
    spaces = [lst[0].length];
    for(let x = 0; x < lst[0].length; x++){
        spaces[x] = 0;
    }
    
    //get spaces info
    for(let i = 0; i < lst.length; i++){
        for(let y = 0; y < size; y++){
            tmp = Math.floor(lst[i][y].length / 4) + 1;
            if(tmp > spaces[y]) {
                spaces[y] = tmp;
            }
            tmp = 1;
        }
    }
    
    // start adding values to return String
    for(let i = 0; i < lst.length; i++){
        for(let y = 0; y < lst[i].length; y++) {
            retVal += lst[i][y];
            this_size = Math.floor(lst[i][y].length / 4);
            for(k = this_size; k < spaces[y]; k++) {
                retVal += '\t';
            }
        }
        retVal += '\n';
        if(i === 0) {
            sum = 0;
            for(let c = 0; c < lst[0].length; c++){
                sum += lst[0][c].length + 1;
            }
            sum += 6;
            for(let c = 0; c < sum; c++){
                retVal += '-';
            }
            retVal += '\n';
        }
    }
    return retVal;
}


result = create_table(test_data);

console.log(result);

Fixed for loop size problem