SOURCE

let number = parseInt('4');
const data = ['0 1','0 2','1 2','3 4'];
let i = 0;
const obj = {};
while(number--){
    let row = data[i].split(' ');
    let index = row[0];
    let value = parseInt(row[1]);
    console.log(`${index} ${value}`);
    if(Object.keys(obj).includes(index)){
        obj[index]+=value;
    }else{
        obj[index] = value;
    }
    i++;
}

for(let item of Object.entries(obj)){
    console.log(item.toString().replace(',',' '))
}

let ccc = ['1234','4556']
console.log(ccc.join(',,,'))