String.prototype.charAt()

const sentence = "The quick brown fox jumps over the lazy dog.";

const index = 4;

console.log(`The character at index ${index} is ${sentence.charAt(index)}`);
// Expected output: "The character at index 4 is q"

str.charAt(index);

var anyString = "Brave new world";
console.log("The character at index 0   is '" + anyString.charAt() + "'");
// No index was provided, used 0 as default

console.log("The character at index 0   is '" + anyString.charAt(0) + "'");
console.log("The character at index 1   is '" + anyString.charAt(1) + "'");
console.log("The character at index 2   is '" + anyString.charAt(2) + "'");
console.log("The character at index 3   is '" + anyString.charAt(3) + "'");
console.log("The character at index 4   is '" + anyString.charAt(4) + "'");
console.log("The character at index 999 is '" + anyString.charAt(999) + "'");

The character at index 0   is 'B'
The character at index 1   is 'r'
The character at index 2   is 'a'
The character at index 3   is 'v'
The character at index 4   is 'e'
The character at index 999 is ''

var str = "A \uD87E\uDC04 Z"; //       .
for (var i = 0, chr; i < str.length; i++) {
  if ((chr = getWholeChar(str, i)) === false) {
    continue;
  }
  // Adapt this line at the top of each loop, passing in the whole string and
  // the current iteration and returning a variable to represent the
  // individual character

  console.log(chr);
}

function getWholeChar(str, i) {
  var code = str.charCodeAt(i);

  if (Number.isNaN(code)) {
    return ""; // Position not found
  }
  if (code < 0xd800 || code > 0xdfff) {
    return str.charAt(i);
  }

  // High surrogate (could change last hex to 0xDB7F to treat high private
  // surrogates as single characters)
  if (0xd800 <= code && code <= 0xdbff) {
    if (str.length <= i + 1) {
      throw "High surrogate without following low surrogate";
    }
    var next = str.charCodeAt(i + 1);
    if (0xdc00 > next || next > 0xdfff) {
      throw "High surrogate without following low surrogate";
    }
    return str.charAt(i) + str.charAt(i + 1);
  }
  // Low surrogate (0xDC00 <= code && code <= 0xDFFF)
  if (i === 0) {
    throw "Low surrogate without preceding high surrogate";
  }
  var prev = str.charCodeAt(i - 1);

  // (could change last hex to 0xDB7F to treat high private
  // surrogates as single characters)
  if (0xd800 > prev || prev > 0xdbff) {
    throw "Low surrogate without preceding high surrogate";
  }
  // We can pass over low surrogates now as the second component
  // in a pair which we have already processed
  return false;
}

var str = "A\uD87E\uDC04Z"; // We could also use a non-BMP character directly
for (var i = 0, chr; i < str.length; i++) {
  [chr, i] = getWholeCharAndI(str, i);
  // Adapt this line at the top of each loop, passing in the whole string and
  // the current iteration and returning an array with the individual character
  // and 'i' value (only changed if a surrogate pair)

  console.log(chr);
}

function getWholeCharAndI(str, i) {
  var code = str.charCodeAt(i);

  if (Number.isNaN(code)) {
    return ""; // Position not found
  }
  if (code < 0xd800 || code > 0xdfff) {
    return [str.charAt(i), i]; // Normal character, keeping 'i' the same
  }

  // High surrogate (could change last hex to 0xDB7F to treat high private
  // surrogates as single characters)
  if (0xd800 <= code && code <= 0xdbff) {
    if (str.length <= i + 1) {
      throw "High surrogate without following low surrogate";
    }
    var next = str.charCodeAt(i + 1);
    if (0xdc00 > next || next > 0xdfff) {
      throw "High surrogate without following low surrogate";
    }
    return [str.charAt(i) + str.charAt(i + 1), i + 1];
  }
  // Low surrogate (0xDC00 <= code && code <= 0xDFFF)
  if (i === 0) {
    throw "Low surrogate without preceding high surrogate";
  }
  var prev = str.charCodeAt(i - 1);

  // (could change last hex to 0xDB7F to treat high private surrogates
  // as single characters)
  if (0xd800 > prev || prev > 0xdbff) {
    throw "Low surrogate without preceding high surrogate";
  }
  // Return the next character instead (and increment)
  return [str.charAt(i + 1), i + 1];
}

function fixedCharAt(str, idx) {
  var ret = "";
  str += "";
  var end = str.length;

  var surrogatePairs = /[\uD800-\uDBFF][\uDC00-\uDFFF]/g;
  while (surrogatePairs.exec(str) != null) {
    var li = surrogatePairs.lastIndex;
    if (li - 2 < idx) {
      idx++;
    } else {
      break;
    }
  }

  if (idx >= end || idx < 0) {
    return "";
  }

  ret += str.charAt(idx);

  if (
    /[\uD800-\uDBFF]/.test(ret) &&
    /[\uDC00-\uDFFF]/.test(str.charAt(idx + 1))
  ) {
    // Go one further, since one of the "characters" is part of a surrogate pair
    ret += str.charAt(idx + 1);
  }
  return ret;
}