var json = {"america":"1234","india":"5678","britain":"789"};

function getValueByKey(obj, val) {
  // Looping on object keys
  return Object.keys(obj).filter(function (key) {
    return obj[key] === val
  })[0];
}

// For testing
console.log(getValueByKey(json, "1234")); // america

console.log(getValueByKey(json, "5678")); // india