function tribonacci( n ) {
	if (
		isnan( n ) ||
		isInteger( n ) === false ||
		n < 0 ||
		n > FLOAT64_MAX_SAFE_NTH_TRIBONACCI
	) {
		return NaN;
	}
	return TRIBONACCI[ n ];
}
Computes the nth Tribonacci number.

@param {NonNegativeInteger} n - the Tribonacci number to compute
@returns {NonNegativeInteger} Tribonacci number

@example
var y = tribonacci( 0 );
// returns 0

@example
var y = tribonacci( 1 );
// returns 0

@example
var y = tribonacci( 2 );
// returns 1

@example
var y = tribonacci( 3 );
// returns 1

@example
var y = tribonacci( 4 );
// returns 2

@example
var y = tribonacci( 5 );
// returns 4

@example
var y = tribonacci( 6 );
// returns 7

@example
var y = tribonacci( NaN );
// returns NaN

@example
var y = tribonacci( 3.14 );
// returns NaN

@example
var y = tribonacci( -1.0 );
// returns NaN