Creating unions for union find using Node.js readline

index.js

const readline = require('readline');

const unions = [];

const rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
});

let number = undefined, i = 0;

rl.on('line', (line) => {
    if(typeof(number) != 'undefined') {
        line = line.split(" ");
        
        let a = parseInt(line[0].trim()),
            b = parseInt(line[1].trim());
            
        union(a, b);
        
        if(++i >= number) {
            console.log("unions", unions);
            rl.close();
        }
    }
    else {
        number = parseInt(line);
    }
});

function union(a, b) {
    if(unions.length == 0) {
        unions.push([a, b]);
    } else {
        const val1 = unions.findIndex((val) => {
            return val.indexOf(a) !== -1;
        });
        const val2 = unions.findIndex((val) => {
            return val.indexOf(b) !== -1;
        });
        
        if(val1 == -1 && val2 == -1) {
            unions.push([a, b]);
        } else if(val1 !== -1 && unions[val1].indexOf(b) == -1) {
            unions[val1].push(b);
        } else if(val2 !== -1 && unions[val2].indexOf(a) == -1) {
            unions[val2].push(a);
        }
    }
}