What techniques that everyone should know?

[Cartread]

Using dt:

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Operator can be an [int] -- 1 = 1 second (dt+dt+dt)
   dtCountDown = dtCountDown - dt
   Body acts with [dt]
end

Do something once before dt hits an amount:

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if dtCountDown > 0 then
   if dtCountDown <= dt -- last call
else
end

Colors:

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MIDDLE_GRAY= {128,128,128,255}
thingDraw(x,y,MIDDLE_GRAY)

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function Critter:thingDraw(x,y,colorIn)
love.graphics.setColor(colorIn)
end

Save/Load tables:
http://lua-users.org/wiki/SaveTableToFile

[OttoRobba]

There is also the always useful variable swap!

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x,y = y,x

Which is a lot simpler than

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tempVariable = x
x = y
y = tempVariable
tempVariable = nil

[JovialFeline]

A small trick for quickly toggling a boolean. For not thinking of it myself, I felt like a fool when I first stumbled on it.

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local isFuzzy = false
isFuzzy = not isFuzzy
print( tostring(isFuzzy) ) -- outputs "true"

[Roland_Yonaba]

I use a similar trick, not not <value>, to convert any type to a boolean. A truthy value would return true, a falsy value would return false.

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print(not not {}) --> true
print(not not nil) --> false
print(not not false)  --> false
print(not not true)  --> true
print(not not 1)  --> true
print(not not function() end)  --> true
print(not not '')  --> true

[davisdude]

The LÖVE Blog (BLÖG?) also has some great techniques and such on it. I would recommend it.

[Ref]

I use a similar trick, not not <value>, to convert any type to a boolean. A truthy value would return true, a falsy value would return false.

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print(not not {}) --> true
print(not not nil) --> false
print(not not false)  --> false
print(not not true)  --> true
print(not not 1)  --> true
print(not not function() end)  --> true
print(not not '')  --> true

Couldn't resist!
Not not, who's there.
When not to use not not!

[Jasoco]

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print(not not "Who's there?")

> true

Hehe.

"True" who?

[Tanner]

I can't think of anywhere that I've used this with the Love2D api but there have been times when something requests a Lua callback and passes that callback something not super useful. For example, the VBA lua scripting interface function for `memory.registerwrite` passes the memory address to the callback as opposed to the value written to the memory address. So having a function that proxies a better callback with that is passed the result of a common operation can result in much better function signatures.

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function readbyte_proxy(callback)
  return function(address)
    callback(memory.readbyte(address), address)
  end
end

function print_direction(direction)
  print("direction", direction)
end

memory.registerwrite(memory_addresses.character_direction, readbyte_proxy(print_direction))

[substitute541]

Prevent your game from spazzing when you move the window:

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-- in love.update(dt)
dt = math.max(dt, 0.033333333)

Basically adds a lower limit of 30 fps to the framerate.

[Wojak]

Inverting an array – useful if you have an array that don't change to often, but you need to check it's content very often:

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local isValidOryginal = {'text1','text2','text3'}
locla isValid = {}
for i=1, # isValidOryginal do -- every time  isValidOryginal changes (once if it never changes)
	 isValid[isValidOryginal[i]] = true
end
isValidOryginal = nil -- optional if  isValidOryginal never changes

--checking content:
if  isValid['text1'] then --this is true
	-- do stuff
end
if  isValid['text4'] then-- this is nil (false)
	-- do stuff
end

And it works with non string types as well