How to detect the mouse being held down?

ROBLOX doesn't have a built in way to determine this, but writing it yourself is very easy.

The intuition is that a button is held down for the time between pressing it and releasing it.

mouseDown = false

startedDown = 0

-- `mouse` a Mouse object from, e.g., theLocalPlayer:GetMouse()

mouse.Button1Down:connect( function()
    mouseDown = true
    -- From now on, held
    startedDown = tick() -- Current time, in seconds
end)

mouse.Button1Up:connect( function()
    mouseDown = false
    -- From now on, NOT held
end)

while wait() do
    if mouseDown then
        print(
            "Has been held for ",
            tick() - startedDown,
            -- Time between now and when `startedDown` was set
            "seconds"
        )
    else
        print("Not held.")
    end
end

Alright, so while BlueTaslem's code works, I think this way is a bit more elegant, although, me being a really bad coder, his is probably more efficient or whatever..

inptSrvc = game:GetService("UserInputService")

while true do
    if inptSrvc:IsMouseButtonPressed(0) then
        --Your script
    end
end

"--Your script" will run while the left mouse button is being held

As RarePlutonian said, The working script should be the following, Tho you have to cancel the script if the button isn't being held. That is very simple you can just do

inptSrvc = game:GetService("UserInputService")

while true do
    if inptSrvc:IsMouseButtonPressed(0) then
        --Your script
    else
        print('Not being held')
    end
end