How to get all key in JSON object (javascript)

I use Object.keys which is built into JavaScript Object, it will return an array of keys from given object MDN Reference

var obj = {name: "Jeeva", age: "22", gender: "Male"}
console.log(Object.keys(obj))

Try this

var s = {name: "raul", age: "22", gender: "Male"}
   var keys = [];
   for(var k in s) keys.push(k);

Here keys array will return your keys ["name", "age", "gender"]

var input = {"document":
  {"people":[
    {"name":["Harry Potter"],"age":["18"],"gender":["Male"]},
    {"name":["hermione granger"],"age":["18"],"gender":["Female"]},
  ]}
}

var keys = [];
for(var i = 0;i<input.document.people.length;i++)
{
    Object.keys(input.document.people[i]).forEach(function(key){
        if(keys.indexOf(key) == -1)
        {
            keys.push(key);
        }
    });
}
console.log(keys);