function copysign( x, y ) {
	var hx;
	var hy;

	// Split `x` into higher and lower order words:
	toWords.assign( x, WORDS, 1, 0 );
	hx = WORDS[ 0 ];

	// Turn off the sign bit of `x`:
	hx &= ABS_MASK;

	// Extract the higher order word from `y`:
	hy = getHighWord( y );

	// Leave only the sign bit of `y` turned on:
	hy &= SIGN_MASK;

	// Copy the sign bit of `y` to `x`:
	hx |= hy;

	// Return a new value having the same magnitude as `x`, but with the sign of `y`:
	return fromWords( hx, WORDS[ 1 ] );
}
Returns a double-precision floating-point number with the magnitude of `x` and the sign of `y`.

@param {number} x - number from which to derive a magnitude
@param {number} y - number from which to derive a sign
@returns {number} a double-precision floating-point number

@example
var z = copysign( -3.14, 10.0 );
// returns 3.14

@example
var z = copysign( 3.14, -1.0 );
// returns -3.14

@example
var z = copysign( 1.0, -0.0 );
// returns -1.0

@example
var z = copysign( -3.14, -0.0 );
// returns -3.14

@example
var z = copysign( -0.0, 1.0 );
// returns 0.0