Index Signatures

You met map types like {[string]: number} back in the tables chapter: an index signature: any string key maps to a number value. They shine when the keys aren't known while you write the code: counters, settings, caches, scores: anything built up at runtime.

type Counts = {[string]: number}

local counts: Counts = {}
counts["potion"] = 3   -- any string key is allowed
counts["sword"] = 1
counts["potion"] += 1  -- values are checked as numbers

Two runtime facts matter when working with maps. First, reading a key that was never set returns nil: the classic counting pattern handles that with or, turning "missing" into 0:

counts[item] = (counts[item] or 0) + 1

Second, pairs() iterates a map's keys in no guaranteed order: running the same program twice can visit keys differently. Whenever output must be predictable, collect the keys into an array, table.sort them, and iterate the sorted array instead:

local keys: {string} = {}
for key in pairs(counts) do
    table.insert(keys, key)
end
table.sort(keys)
for _, key in ipairs(keys) do
    print(`{key}: {counts[key]}`)
end