How to check if a given number is Fibonacci number ? | A utility function that returns true if x is perfect square ; Returns true if n is a Fibinacci Number , else false ; n is Fibinacci if one of 5 * n * n + 4 or 5 * n * n - 4 or both is a perferct square ; A utility function to test above functions
function isPerfectSquare ( x ) { let s = parseInt ( Math . sqrt ( x ) ) ; return ( s * s == x ) ; } function isFibonacci ( n ) { return isPerfectSquare ( 5 * n * n + 4 ) || isPerfectSquare ( 5 * n * n - 4 ) ; } for ( let i = 1 ; i <= 10 ; i ++ ) isFibonacci ( i ) ? document . write ( i + " " ) : document . write ( i + " " ) ;