for (let i = 0; i < balls.length; ++i) b.display(); is wrong! 
Correct is for (let i = 0; i < balls.length; ++i) balls[i].display(); 
Other options: 
for (const b of balls) b.display();balls.forEach(b => b.display());
for (let i = 0; i < balls.length; ++i) b.display(); is wrong! 
Correct is for (let i = 0; i < balls.length; ++i) balls[i].display(); 
Other options: 
for (const b of balls) b.display();balls.forEach(b => b.display());