jump logic in pygame

import pygame
pygame.init()
win = pygame.display.set_mode((500, 500))
pygame.display.set_caption("My First Game")
clock = pygame.time.Clock()
x = 0
y = 300
width = 40
height = 60
vel = 5
j_vel = 10 ## The Velocity of the jump
run = True
isJump = False
##main game loop
while run:
    clock.tick(60)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False
    ##moving the rectangle
    key = pygame.key.get_pressed()
    if key[pygame.K_LEFT] and x > 0:
        x -= vel
    if key[pygame.K_RIGHT] and x<500-width:
        x += vel
    if key[pygame.K_SPACE]:
        isJump = True
    if isJump:
        y -= j_vel
        j_vel -= 1
        if j_vel < -10:
            isJump = False
            j_vel = 10
    ##drawing the character
    win.fill((0,0,0))
    pygame.draw.rect(win, (255, 0, 0), (x, y, width, height))
    pygame.display.update()
pygame.quit()