SOURCE

var input = [
    'abcde123123123',
    'abc123123',
    'abcd',
    'ab123123'
];

var output = [];
var temp;

temp = input[0];
for(var i = 1; i < input.length; i++){
    checkSame(i)
    temp = output;
}


function checkSame(index){
    for(var i = 0; i < input[index].length; i ++){
        if(input[index][i] == temp[i]){
            output.push(input[index][i]);
        }
    }
}

console.log(output.join(''))