๋ดึงค่าproperty name ใน json array โดยไม่ใช้ Object.keys() ทำยังไงครับ

let peopleSalary =  [
    {"id":1001,"firstname":"Luke","lastname":"SkyWalker","company":"Walt Disney","salary":40000},
    {"id":1002,"firstname":"Tony","lastname":"Stark","company":"Marvel","salary":1000000},
    {"id":1003,"firstname":"Somchai","lastname":"Jaidee","company":"Love2work","salary":20000},
    {"id":1004,"firstname":"Monkey D","lastname":"Luffee","company":"One Piece","salary":9000000}
    ];
    
      for (var i in peopleSalary) { 
        
             }