Index Signature

interface Scores {
  [player: string]: number;
}

const scores: Scores = { alice: 10, bob: 7 };
scores["carol"] = 12;

console.log(scores["alice"]);            // 10
console.log(scores["carol"]);            // 12

let total = 0;
for (const name of Object.keys(scores)) {
  total += scores[name];
}
console.log(total);                      // 29

// The inline object-type form works the same way.
type Env = { [key: string]: string };
const env: Env = { HOME: "/root" };
env["SHELL"] = "/bin/sh";
console.log(env["HOME"] + " " + env["SHELL"]); // /root /bin/sh

// Number index signature: keys stringify (JS object keys are strings).
interface Sparse { [i: number]: string; }
const sparse: Sparse = {};
sparse[7] = "seven";
console.log(sparse[7]);

// Logical assignment works against a computed key (ADR-00600): ??= fills a key
// only when it's absent/null, ||= when the current value is falsy.
const cache: { [k: string]: string } = {};
cache["a"] = "hit";
cache["a"] ??= "miss";   // present → stays "hit"
cache["b"] ??= "miss";   // absent → "miss"
console.log(cache["a"], cache["b"]);