countries.html

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Countries Array</title>
</head>

<body>
  <div id="wrapper">

  </div>
  <script>
    let input = ["America", "india", "Pakistan", "iceland", "Chile", "china","Spain", "Sri Lanka", "Nepal", "Russia", "Afghanistan", "Romania", "Nigeria"]
    let output = {}

    input.forEach((country) => {
      let firstLetter = country.slice(0,1).toLowerCase()
      let splittedCountry = country.split('')
      splittedCountry.splice(0,1, firstLetter.toUpperCase())

      output[firstLetter] = (Object.keys(output).includes(firstLetter)) 
      
      ? [...output[firstLetter], splittedCountry.join('')] 
      
      : output[firstLetter] = [splittedCountry.join('')]
    })
    console.log(output)
  </script>
</body>

</html>