traverse-json.js

const o = { 
  foo:"bar",
  bar: {
    foo: "bar"
  },
  a: [1, 2, 3],
};

function traverse(json) {
  for (const key in json) {
    if (json.hasOwnProperty(key)) {
      const value = json[key];
      
      console.log(key, value);
                 
      if (typeof value === 'object') {
        traverse(value);
      }
    }
  }
}

traverse(o);