Unity make player only jump when it's grounded

I recently started messing around in Unity and I’m trying to make a 2D platformer game. I tried making it so that my player is only able to jump when it is grounded and this is the code I came up with, but it isn’t working for some reason and I don’t understand why.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Movement : MonoBehaviour
{
    public float moveSpeed = 5;
    public float jumpSpeed =  5;
    public Rigidbody2D rb;
    public GameObject gameObject;
    bool isGrounded;
    Vector3 movement;
    // Start is called before the first frame update
    void Start()
    {
        rb = GetComponent<Rigidbody2D>();
    }

    // Update is called once per frame
    void Update()
    {
        if (isGrounded)
        {
            Jump();
        } else
        {
            // do nothing
        }
        Vector3 movement = new Vector3(Input.GetAxis("Horizontal"), 0f, 0f);
        transform.position += movement * Time.deltaTime * moveSpeed;
    }

    void Jump()
    {
        if (Input.GetKeyDown(KeyCode.Space))
        {
            rb.AddForce(new Vector2(0f, jumpSpeed), ForceMode2D.Impulse);
        }
    }

    private void OnCollisionEnter2D(Collision2D collision)
    {
        if(collision.gameObject.CompareTag("Ground"))
        {
            isGrounded = true;
        } else
        {
            isGrounded = false;
        }
    }
}

>Solution :

Instead of having the else { isGrounded = false;} inside of your collision check, add isGrounded = false; inside of your Jump() function so that you’re guaranteed to only be able to jump while you’re on the ground;
Additionally you could have Jump(); without the if statement in your Update() function and have it inside Jump() like this if (isGrounded && Input.GetKeyDown(KeyCode.Space))