Index Signatures & Mapped Types

Type Inference & Literal Types (const vs. let)

Discover how TypeScript infers types automatically and how let versus const declarations shape wide types versus narrow literal types.

1. Dynamic Key Dictionaries

In JavaScript, objects often act as lookup dictionaries with unknown dynamic property keys (for example, caching objects or user dictionaries).

2. Index Signatures

An Index Signature defines the data type of dynamic keys and values for objects whose exact property names are not known ahead of time:

// Index signature: key must be string, value must be number
interface UserScores {
  [username: string]: number;
}

const leaderboard: UserScores = {
  alex: 100,
  sarah: 250,
  maria: 180,
};

leaderboard.david = 300; // Valid dynamic key addition!

3. Mapped Types Syntax

A Mapped Type builds a new object shape by iterating over keys of an existing type using in keyof syntax:

type Permissions = "canRead" | "canWrite" | "canDelete";

// Mapped Type: Maps every permission string to a boolean!
type UserPermissionFlags = {
  [K in Permissions]: boolean;
};

/*
Equivalent to:
type UserPermissionFlags = {
  canRead: boolean;
  canWrite: boolean;
  canDelete: boolean;
};
*/

const myFlags: UserPermissionFlags = {
  canRead: true,
  canWrite: false,
  canDelete: false,
};

4. Modifying Flags in Mapped Types

You can add or remove readonly or optional ? modifiers inside mapped types using + or -:

// Make all properties optional
type Optional<T> = {
  [K in keyof T]?: T[K];
};

5. Common Mistakes

• Unchecked Index Access: Reading an index signature property (leaderboard["unknownUser"]) returns number by default, even though it evaluates to undefined at runtime! Enable noUncheckedIndexedAccess: true in tsconfig.json to force number | undefined safety!

6. Next Steps

Use mapped types for reusable type transformations. Next, let's enter Phase 3 and master Generics!