How can you modify this function to correctly capitalize only the first letter of a string?

function capitalize(str) {
    return str.toUpperCase();
}

Given:

const arr = [1,2,3,4];
const sum = arr.reduce((a,b) => a+b, 0);

Context:

What happens after executing this code?

const div = document.createElement('div');
const copy = div;
div.style.backgroundColor = 'red';
console.log(copy.style.backgroundColor);

Given:

let x = 10;
let y = x;
y = 20;

What does this print?

const str = "World";
console.log(str[0]);

function getHumanChoice() {
  const choice = prompt("Enter rock, paper, or scissors:");
  return choice.trim().toLowerCase();
}

What happens after clicking a button with this code?

btn.addEventListener('click', function(e) {
  e.target.style.background = 'blue';
});