String.prototype.charAt()

var anyString = "  ";

console.log("   0    '" + anyString.charAt(0) + "'");
console.log("   1    '" + anyString.charAt(1) + "'");
console.log("   2    '" + anyString.charAt(2) + "'");
console.log("   3    '" + anyString.charAt(3) + "'");
console.log("   4    '" + anyString.charAt(4) + "'");
console.log("   5    '" + anyString.charAt(5) + "'");
console.log("   999  '" + anyString.charAt(999) + "'");

   0    ''
   1    ''
   2    ''
   3    ''
   4    ''
   5    ''
   999  ''

var str = "A \uD87E\uDC04 Z"; //    -  
for (var i = 0, chr; i < str.length; i++) {
  if ((chr = getWholeChar(str, i)) === false) {
    continue;
  }
  //        ,    
  //   ;    
  //  

  console.log(chr);
}

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

  if (isNaN(code)) {
    return ""; //   
  }
  if (code < 0xd800 || code > 0xdfff) {
    return str.charAt(i);
  }

  //     (     0xDB7F,
  //          
  //  )
  if (0xd800 <= code && code <= 0xdbff) {
    if (str.length <= i + 1) {
      throw "      ";
    }
    var next = str.charCodeAt(i + 1);
    if (0xdc00 > next || next > 0xdfff) {
      throw "      ";
    }
    return str.charAt(i) + str.charAt(i + 1);
  }
  //     (0xDC00 <= code && code <= 0xDFFF)
  if (i === 0) {
    throw "      ";
  }
  var prev = str.charCodeAt(i - 1);

  // (     0xDB7F,   
  //         )
  if (0xd800 > prev || prev > 0xdbff) {
    throw "      ";
  }
  //        ,
  //    
  return false;
}

var str = "A\uD87E\uDC04Z"; //    -  
for (var i = 0, chr; i < str.length; i++) {
  [chr, i] = getWholeCharAndI(str, i);
  //        ,    
  //   ;       
  //     'i' (     )

  console.log(chr);
}

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

  if (isNaN(code)) {
    return ""; //   
  }
  if (code < 0xd800 || code > 0xdfff) {
    return [str.charAt(i), i]; //  ,   'i' 
  }

  //     (     0xDB7F,
  //          
  //  )
  if (0xd800 <= code && code <= 0xdbff) {
    if (str.length <= i + 1) {
      throw "      ";
    }
    var next = str.charCodeAt(i + 1);
    if (0xdc00 > next || next > 0xdfff) {
      throw "      ";
    }
    return [str.charAt(i) + str.charAt(i + 1), i + 1];
  }
  //     (0xDC00 <= code && code <= 0xDFFF)
  if (i === 0) {
    throw "      ";
  }
  var prev = str.charCodeAt(i - 1);

  // (     0xDB7F,   
  //         )
  if (0xd800 > prev || prev > 0xdbff) {
    throw "      ";
  }
  //    (  )
  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))
  ) {
    //   ,       
    ret += str.charAt(idx + 1);
  }
  return ret;
}