How to properly use the methods Roblox has provided, namely :FindFirstChild() and :WaitForChild()? [closed]

FindFirstChild is only necessary when you want to confirm that an object actually exists. If you directly index it via ., it will error. FindFirstChildwould just return nil. In your first example, you do

local box = game.Workspace:FindFirstChild("Model").box

This is rather pointless. Because you index box without any additional checks to verify Model exists, you are assuming that it will be there. The better way to write this is

local box = game.Workspace.ModelFindFirstChild("box")

The second example using brackets is fine, and its a valid point. Your third example can be simplified, although it is not recommended

local boxModel = game.Workspace:FindFirstChild("Box Model")
if boxModel then
    local boxHolder = boxModel:FindFirstChild("Box Holder")
    if boxHolder then
        box = boxHolder:FindFirstChild("box")
        if box then
            -- code
        end
    end
end

If you know the object exists but don't know when it will exist of you're afraid that it won't when you attempt to index, use WaitForChild. If you are this unsure about an object existing at every stage, you can do one of two things.

1) Rewrite something else so that you don't run into such a large issue 2) Use the second recursive parameter of FindFirstChild

local boxModel = game.Workspace:FindFirstChild("Box Model")
if boxModel then
    local box = boxModel:FindFirstChild("box", true)
    if box then
        --do code
    end
end

I don't recommend this because you could either have multiple instances with the same name or have a massive model which could make this time-consuming.

As for WaitForChild, up until recently, I would've only recommended that it be used on the server because the scripts load before ReplicatedStorage does. In a more recent update (i think this is what occurred), the player now loads before any scripts do. This changes a few things that I don't know a whole lot about yet, so take a look at the article here

Hope this answer explained some of what you were looking for!

I, myself am somewhat of a newbie when it comes to built in functions of Roblox Lua, but I think one of the purposes, or at least one of the ways I use :FindFirstChild() is when there is a bit a latency, or if I am waiting a set about of time after an event to do something, say damage a person with a sword after an animation has played, or checking if the player has a creator tag in them when they die: Ex1:

game.Players.PlayerAdded:Connect(function(plr)
    plr.CharacterAdded:Connect(function(char)
        local creator = plr:FindFirstChild("creator")       
        if creator then
            if creator.Value and  then
                local leaderstats = creator.Value:FindFirstChild("leaderstats")
                if leaderstats  and leaderstats:FindFirstChild("points") then
                    local points = leaderstats:FindFirstChild("points")
                    points.Value = Points.Value + 1
                end
            end 
        end
    end)
end)--the creator tag is an object value and the value of it is a plr object

Ex2:

--one script
wait(2)
local thing = Instance.new("Part")
thing.Parent = workspace
thing.Name = "brick"
--another script
local thing
repeat
thing = workspace:FindFirstChild("brick")
until thing ~= nil--not needed but to prove a point

This brings me to the second usage of FindFirstChild, it allows you find the child of some object without throwing an error if it isn't there, say I have a typo on a bit of code like this :

local a = Instance.new("Part")
a.Parent = workspace
a.Name = "thing"

if workspace.thkng then--throws an error

end

alternatively, if I have a typo on a bit of code like this

local a = Instance.new("Part")
a.Parent = workspace
a.Name = "thing"

if workspace:FindFirstChild("thkng") then--doesn't throw an error
    print("yay")
else
    print("uh oh")
end

In this aspect, it is superior to WaitForChild and the .as it doesn't infinitely wait for "thkng" and allows the rest of your script to go on undisturbed.

--if WaitForChild() is used without a timout argument
local a = Instance.new("Part")
a.Parent = workspace
a.Name = "thing"

if workspace:WaitForChild("thkng") then--waits forever and doesnt let the rest of the script run
    print("yay")
else
    print("uh oh")
end

However, In some areas, WaitForChild() is better, such as trying to get the humanoid of a player character in a local script;

local humanoid = game.Players.LocalPlayer.Character:WaitForChild("Humanoid")

or in the script i mentioned earlier on

--one script
wait(2)
local thing = Instance.new("Part")
thing.Parent = workspace
thing.Name = "brick"
--another script
local thing = workspace:WaitForChild("brick")