# For x=0, does it pass the vertical line test?

The line $x = 0$ does not pass the 'vertical line test' since the vertical line $x = 0$ intersects with it at more than one point (in fact at rather a lot of points). So $x = 0$ does not define a function.
The equation $x = 0$ defines a vertical line. It does not define a function of $x$, since there a multiple values of $y$ for the value $x = 0$.