how can I loop through a unciode json string in javascript?

I get the following string returned by the CBOE options api:

{u'inputs': {u'stock_price_max': 50.0, u'high_low_stock_max': None, u'time_frame': u'middle', u'hv30_max': None, u'high_low_stock_min': None, u'symbols': None, u'hv30_min': None, u'low_strike': 3.0, u'high_strike': 4.0, u'industry_codes': None, u'spread_ask_price_max': None, u'stock_price_min': 10.0}, u'output': [{u'stock_price': 43.2, u'stock_hi_lo_percent': 72.9651, u'symbol': u'EWZ', u'industry_code': 55501010, u'max_gain': 0.52, u'high_strike_otm_percent': 0.463, u'low_strike_otm_percent': 2.7778, u'spread_ask': 0.48, u'spread': u'43/42 Put', u'expiry': u'2019-04-18', u'max_gain_to_spread_ask_percent': 108.3333, u'hv30': 27.3836}, {u'stock_price': 41.37, u'stock_hi_lo_percent': 21.7957, u'symbol': u'FXI', u'industry_code': 55501010, u'max_gain': 0.26, u'high_strike_otm_percent': 0.8944, u'low_strike_otm_percent': 2.103, u'spread_ask': 0.24, u'spread': u'41/40.5 Put', u'expiry': u'2019-05-17', u'max_gain_to_spread_ask_percent': 108.3333, u'hv30': 20.2925}

I want to loop through it and place elements into cells in a Google spreadsheet. I have this code:

function myFunction() {
  var response = UrlFetchApp.fetch(endpoint);
  var data = response.getContentText();
  sheet.getRange("A8").setValue(data);
}

This puts the entire string into cell A8.

I have tried to loop through the string with

for (i = 0; i < jsonlen; i++) {
    sheet.getRange("A:A").setValaue(data['output']['symbol']);
}

This returns "undefined". So problems are: 1) how can I extract the elements I need form the "output" part of the string 2) put the symbols into A3, A4 etc then stock_price into B3, B4 etc 3) how to identify the length of the string in order to make the loop work correctly until the string has been entirely looped over?

Many thanks!