How to i get stdin value in javascript (it works, but help me to understand)

I have a coding challenge using HackerRank, which I am unfamiliar with.

The challenge is simple but i couldn't understand the input from stdin.

when i try to put var score = 0 as first default value, it returns wrong answer

but when i removed var score = 0, it works, and it said stdin 11,

'use strict';

process.stdin.resume();
process.stdin.setEncoding('utf-8');

let inputString = '';
let currentLine = 0;

process.stdin.on('data', inputStdin => {
    inputString += inputStdin;
});

process.stdin.on('end', _ => {
    inputString = inputString.trim().split('\n').map(string => {
        return string.trim();
    });

    main();    
});

function readLine() {
    return inputString[currentLine++];
}

function getGrade(score) {

    let grade;

    if ((score > 25) && (score <= 30)) {
        grade = "A";
    }
    else if ((score > 20) && (score <= 25)) {
        grade = "B";
    }
    else if ((score > 15) && (score <= 20)) {
        grade = "C";
    }
    else if ((score > 10) && (score <= 15)) {
        grade = "D";
    }
    else if ((score > 5) && (score <= 10)) {
        grade = "E";
    }
    else if ((score > 0) && (score <= 5)) {
        grade = "F";
    }
    else{}

    return grade;

}



function main() {
    const score = +(readLine());

    console.log(getGrade(score));
}

What i need to know is where's 11 coming from?