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);