Double Jump -- Roblox

local UserInputService = game:GetService("UserInputService")
local player = game:GetService("Players").LocalPlayer
local character = player.Character
local humanoid = character:WaitForChild("Humanoid")

local MAX_JUMPS = 2
local TIME_BETWEEN_JUMPS = 0.2
local numJumps = 0
local canJumpAgain = false

local function onStatChanged(oldState, newState)
    if Enum.HumanoidStateType.Landed == newState then
        numJumps = 0
        canJumpAgain = false
    elseif Enum.HumanoidStateType.Freefall == newState then
        wait(TIME_BETWEEN_JUMPS)
        canJumpAgain = true
    elseif Enum.HumanoidStateType.Jumping  ==  newState then
        canJumpAgain = false
        numJumps += 1
    end

end

local function onJumpRequest()
    if canJumpAgain and numJumps < MAX_JUMPS then
        humanoid:ChangeState(Enum.HumanoidStateType.Jumping)
    end
end

humanoid.StateChanged:Connect(onStatChanged)
UserInputService.JumpRequest:Connect(onJumpRequest)