Double jump help

Question

I'm trying to make double jumps for my FPS game in Unity. I have written so that if I press "Jump", (the space bar) and that I'm not grounded, I will jump again and that I wont be able to double jump any more by turning off a public bool. Normally if I'm falling and I press jump, I will jump and it works, but when I jump the first time, the canDoublejump bool just turns off, so could you someone maybe help me with it?

Code:

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

public class PlayerMovement : MonoBehaviour
{
    //Character Controller new name
    public CharacterController controller;
    //Basic Starting values
    public float speed = 12f;
    public float gravity = -9.81f;
    public float jumpHeight = 2.5f;
    public float doublejumpMultiplier = 0.5f;
    public bool canDoublejump = true;
    public bool grounded = true;

    //Assingables
    public Transform playerCam;

    //Other
    private void Jump()
    {
        velocity.y = Mathf.Sqrt(jumpHeight * -2f * gravity);
    }


    //New names for stuff and assignables
    private Rigidbody rb;

    //Not falling forever lol
    public Transform groundCheck;
    public float groundDistance = 0.4f;
    public LayerMask groundMask;

    Vector3 velocity;
    bool isGrounded;

    // Update is called once per frame
    void Update()
    {
        //Random Bullshit
        if (isGrounded) grounded = true;
        if (!isGrounded) grounded = false;

        //Gravity stuff
        velocity.y += gravity * Time.deltaTime;

        controller.Move(velocity * Time.deltaTime);

        //Walking
        isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);

        if (isGrounded && velocity.y < 0)
        {
            velocity.y = -2f;
        }

        float x = Input.GetAxis("Horizontal");
        float z = Input.GetAxis("Vertical");

        Vector3 move = transform.right * x + transform.forward * z;

        controller.Move(move * speed * Time.deltaTime);

        //Jumping
        if (Input.GetButtonDown("Jump") && isGrounded)
        {
            Jump();
            canDoublejump = true;
        }

        //Running
        if (Input.GetButton("LeftShift")) speed = 12f;
        if (Input.GetButtonUp("LeftShift")) speed = 6f;
        if (Input.GetButton("LeftShift")) jumpHeight = 4.25f;
        if (Input.GetButtonUp("LeftShift")) jumpHeight = 2.5f;
        //Crouching
        if (Input.GetButton("LeftCtrl")) controller.height = 1.2f;
        if (Input.GetButtonUp("LeftCtrl")) controller.height = 2f;
        if (Input.GetButton("LeftCtrl")) speed = 3f;
        if (Input.GetButtonUp("LeftCtrl")) speed = 6f;
        //Crawling
        if (Input.GetButton("C")) controller.height = 0.25f;
        if (Input.GetButtonUp("C")) controller.height = 2f;
        if (Input.GetButton("C")) speed = 1.75f;
        if (Input.GetButtonUp("C")) speed = 6f;
        //DoubleJump
        if (isGrounded) canDoublejump = true;
        if (!isGrounded && canDoublejump == true)
        {
            if(Input.GetButton("Jump"))
            {
                Jump();
            }
        }
        if (!isGrounded && Input.GetButton("Jump"))
        {
            canDoublejump = false;
        }
    }
}

Answer 1

Compare the three places you check the Jump button.

In one you use "GetButtonDown" to detect the press only on the frame when the button transitions from up to down.

In the other two, you use "GetButton" - this will also fire as long as the button is held down. Including on the frame after you successfully jump from the ground, immediately burning your double jump.

You can also simplify your code to something like...

if (isGrounded) {        
    canDoublejump = true;
} 

if (Input.GetButtonDown("Jump") && canDoubleJump) {
    ​canDoubleJump = isGrounded;
    Jump();
}

Handling both single and double-jumps in one place like this avoids code duplication, so you can't accidentally use the right function in one place and the wrong function in a different place: there's only one place where the functionality lives, making it leaner and easier to reason about.