Index Signature

interface Cat {
  name: string;
  age: string;
}

type props = Record<string, string | undefined>;
function bb({ name, age }: props) {
  console.log("name is " + name + ", age is " + age);
}

const cat1: Cat = { name: "Fluffball", age: "6" };

const cat2 = { name: "Fluffball", age: "6" };

bb(cat1); // ❌ error
bb(cat2); // ✅ works

And Cat does NOT have an index signature.
Different contract, at type level. No guarantee random key a string. Even when undefined in Record.

Index signature - TS thing, not in JS. Nothing to do with iterators.

type Dictionary = {
  [key: string]: string;
};

this is the index signature

This means:
cat.name → allowed
cat.age → allowed
cat["random"] → ❌ error in TypeScript

Index signature is about:
Bracket property access (obj[someKey])

Without Index Signature (Structured Object)

---

type Translations = Record<string, string>;
// is syntactic sugar for
type Translations = {
  [key: string]: string;
};

Very Important Distinction

Structured object:

{
  name: string;
  age: string;
}

Dictionary object:

{
  [key: string]: string;
}

They are fundamentally different in TypeScript.

tip

Structured object:
{ name: string; age: string }
Means:
Only these keys are legal.

tip

Dictionary:
{ [key: string]: string }
Means:
Any key is legal.

Object typed with index signature is lying. The value is not always string, could be undefined. if you access

const val = dict[key];
if (val !== undefined) val.toUpperCase();

check of if not undefined

---

structured vs dynamic. Structured object.
index signature objects are for dynamic, dictionary-like objects