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control flow

if / else

if (player.health < 5) {
    player.sendMessage("&clow health!")
} else if (player.health < 10) {
    player.sendMessage("&egetting low...")
} else {
    player.sendMessage("&ayoure ok :D")
}

braces are always required, even for a single statement:

// this will not compile: no braces
if (x > 0)
    doSomething()
// always do this
if (x > 0) {
    doSomething()
}

when

when compares one value against several possibilities, top to bottom, and runs the first match (like a switch statement):

when (color) {
    "red" -> {
        player.sendMessage("&cred team!")
    }
    "blue" -> {
        player.sendMessage("&9blue team!")
    }
    else -> { // this else statement is optional
        player.sendMessage("&7unknown team")
    }
}

return

function clamp(value, min, max) {
    if (value < min) {
        return min
    }
    if (value > max) {
        return max
    }
    return value
}

a function with no return returns null. return with no value is also valid:

function startGame() {
    if (gameRunning) {
        return // already running, do nothing
    }
    gameRunning = true
    ...
}

this is especially important inside Katton.after/Katton.every callbacks, see Katton.after and Katton.every for why you almost always want a return

hi :D