Is it tail call or not?

tail_call_factorial.js

function fact(x) {
  return _f(1, x);
  function _f(acc, x) {
    return x === 0 ?
      acc :
      _f(acc * x, x - 1);
  }
}
/*
Yes, it is!

fact(3)
_f(1, 3) =>
_f(3, 2) =>
_f(6, 1) =>
6
*/


function inspectObj(obj) {
    
  return _inspect('', obj);

  function _inspect(accumulator, obj) {
    switch(typeof obj) {
      case 'object':
        var keys = Object.keys(obj);
        var length = keys.length;
        if (length === 0) {
          accumulator += '{}';
        } else {
          accumulator += '{';
          for (var i = 0; i < length; i++) {
            var key = keys[i];
            accumulator = _inspect(accumulator + key + ': ', obj[key]);
            if (i < length - 1) {
              accumulator += ', ';
            }
          }
          accumulator += '}';
        }
        break;

      case 'string':
        accumulator += JSON.stringify(obj);
        break;

      default:
        accumulator += obj.toString();
        break;
    }
    return accumulator;
  }
}
/*
No, it's not.

inspectObj({a: {b: {c: 1}}})
_inspect('', {a: {b: {c: 1}}})
_inspect('{a: ', {b: {c: 1}}) + '}'
_inspect('{a: {b: ', {c: 1}) + '}' + '}'
_inspect('{a: {b: {c: ', 1) + '}' + '}' + '}'
'{a: {b: {c: 1}}}'

How do I rewrite it using a tail call?
*/