mars-exploration.js

function main() {
    var S = readLine();
    const expectedSignal = 'SOS';
    let errorCount = 0;
    
    for (i = 0, len = S.length; i <len; i += 3) {
        const currentSignal = S.slice(i, i + 3);
        if (currentSignal === expectedSignal) continue;
        if (currentSignal[0] !== expectedSignal[0]) errorCount += 1;
        if (currentSignal[1] !== expectedSignal[1]) errorCount += 1;
        if (currentSignal[2] !== expectedSignal[2]) errorCount += 1;
    }
    console.log(errorCount);

}