Edit in JSFiddle

// http://stackoverflow.com/a/834529

function convert() {
  var escapable = /[\\\"\x00-\x1f\x7f-\uffff]/g,
    meta = {
      '\b': '\\b',
      '\t': '\\t',
      '\n': '\\n',
      '\f': '\\f',
      '\r': '\\r',
      '"': '\\"',
      '\\': '\\\\'
    };

  this.toASCII = function(string) {
    escapable.lastIndex = 0;
    return escapable.test(string) ?
      '"' + string.replace(escapable, function(a) {
        var c = meta[a];
        return typeof c === 'string' ? c :
          '\\u' + ('0000' + a.charCodeAt(0).toString(16)).slice(-4);
      }) + '"' :
      '"' + string + '"';
  };
}

var utf8 = new convert(),
  input = document.getElementById('input'),
  output = document.getElementById('output');

input.oninput = function() {
  output.value = utf8.toASCII(this.value).slice(1, -1);
};

<p>
  <textarea name="input" id="input" cols="30" rows="10"></textarea>
</p>
<p>
  <textarea name="output" id="output" cols="30" rows="10"></textarea>
</p>