Template for creating a snippet plugin with one argument in Roblox Studio.

new_class.lua

local ScriptEditorService = game:GetService("ScriptEditorService")

local SNIPPET_NAME = "new_class"
local SNIPPET_DESC = "Create a new prototype class with an export type defined!"

local SNIPPET = table.concat({
	"local ${1:%s} = {}",
	"${1}.__index = ${1}",
	"",
	"export type Class = typeof(setmetatable({} :: {",
	"	",
	"}, ${1}))",
	"",
	"function ${1}.new(): Class",
	"	return setmetatable({",
	"		",
	"	}, ${1})",
	"end",
	"",
	"return ${1}"
}, "\n")

type AutocompleteItem = {
	label: string,
	kind: Enum.CompletionItemKind?,
	tags: {Enum.CompletionItemTag}?,
	detail: string?,

	documentation: {
		value: string,
	}?,

	overloads: number?,
	learnMoreLink: string?,
	codeSample: string?,
	preselect: boolean?,

	textEdit: {
		newText: string,
		replace: { 
			start: { line: number, character: number }, 
			["end"]: { line: number, character: number } 
		},
	}?
}

type AutocompleteResponse = {
	items: { AutocompleteItem }
}

type AutocompleteRequest = {
	position: {
		line: number,
		character: number
	},

	textDocument: {
		document: ScriptDocument?,
		script: LuaSourceContainer?
	}
}

local function snippetCallback(request: AutocompleteRequest, response: AutocompleteResponse)
	local textDoc = request.textDocument
	local scriptDoc = textDoc.document
	local luaSource = textDoc.script
	
	local pos = request.position
	local id = "Module"

	if luaSource then
		id = luaSource.Name
	end
	
	if scriptDoc then
		local char = pos.character - 1
		local line = scriptDoc:GetLine(pos.line)
		
		if line:sub(1, char) ~= SNIPPET_NAME:sub(1, char) then
			-- Line doesn't start with the snippet, peace out.
			return response
		end
	end
	
	table.insert(response.items, {
		label = SNIPPET_NAME,
		kind = Enum.CompletionItemKind.Snippet,

		documentation = {
			value = SNIPPET_DESC
		},

		textEdit = {
			newText = SNIPPET:format(id),

			replace = {
				start = {
					line = pos.line,
					character = 1
				},

				["end"] = pos
			}
		}
	})

	return response
end

pcall(function ()
	-- If testing, deregister the existing callback.
	ScriptEditorService:DeregisterAutocompleteCallback(SNIPPET_NAME)
end)

ScriptEditorService:RegisterAutocompleteCallback(SNIPPET_NAME, 1000, snippetCallback)

Cool stuff!