Example of printing a source code line at an error position (based on an index within the entire source code)

printerror.js

// Example of printing a source code line at an error position
// (based on an index within the entire source code)
//
// By Andrew Kingdom
// MIT license
// (and use at own risk)
//
// Example prints:
//
//    DEFGH
//    ..^
//

var s='AB\nDEFGH\nJK';  // Dummy source code.
var errpos = 6;  // Supposed index of the first character of the error within entire source string.

function printableError(s,errpos) {
  let maxPrintable = 127;
  var p=errpos-1;
  if(p < 0 || p >= s.length) return "(out-of-range)\n";
  var extraindent = 0;
  if(s[p] === '\n') {
    if(p === 0) return "\n^";
    p -= 1;
    if(s[p] === '\n') return "\n^";
    extraindent = 1;
  }
  var l0=s.lastIndexOf('\n',p);
  var l1=s.indexOf('\n',l0+1);
  var wholeline=s.substring(l0+1,l1);  // only the line of source code for the specified position
  var errindent='.'.repeat(p-l0-1 + extraindent);  // 
  if (wholeline.length > maxPrintable) {
    wholeline = wholeline.slice(-maxPrintable)
    errindent = errindent.slice(-maxPrintable-extraindent+1);
  }
  return wholeline + '\n' + errindent + '^';
}

console.log( printableError(s,errpos));