Array Index Access with `[number]`

Overview

In TypeScript, you can access array element types using indexed access types. The [number] syntax is particularly powerful for extracting a union of all possible element types from an array or tuple.

Basic Concept

Just like you can access object properties with string keys, you can access array elements with numeric indices:

const roles = ["user", "admin", "anonymous"] as const;

// Access specific index
type FirstRole = (typeof roles)[0]; // "user"
type SecondRole = (typeof roles)[1]; // "admin"

// Access all elements with [number]
type AnyRole = (typeof roles)[number]; // "user" | "admin" | "anonymous"

Why `[number]` Works

The number type, when used as an index, represents a union of ALL possible numeric indices. TypeScript uses this as a shortcut to access all elements:

// This is conceptually equivalent to:
type AnyRole = (typeof roles)[0 | 1 | 2];
// But [number] handles any array length automatically

Pattern: Extract Array Element Types

const userAccessModel = {
  user: ["update-self", "view"],
  admin: ["create", "update-self", "update-any", "delete", "view"],
  anonymous: ["view"],
} as const;

type Role = keyof typeof userAccessModel;
// Type: "user" | "admin" | "anonymous"

// Get all values (arrays) as a union
type UserAccessModelValues = (typeof userAccessModel)[Role];
// Type: readonly ["update-self", "view"] | readonly ["create", ...] | readonly ["view"]

// Get all actions from all roles
type Action = (typeof userAccessModel)[Role][number];
// Type: "update-self" | "view" | "create" | "update-any" | "delete"

Difference Between Tuple and Array Access

// Tuple - fixed length, specific types at each position
const tuple = ["hello", 42, true] as const;
type TupleElements = (typeof tuple)[number]; // "hello" | 42 | true

// Array - variable length, single element type
const array: string[] = ["a", "b", "c"];
type ArrayElement = (typeof array)[number]; // string

Pattern: Extract Function Parameter Types

Combined with Parameters<>, you can get a union of all parameter types:

const funcWithManyParameters = (
  a: string,
  b: string,
  c: number,
  d: boolean
) => {
  return [a, b, c, d].join(" ");
};

// Get tuple of all parameter types
type ParamsTuple = Parameters<typeof funcWithManyParameters>;
// Type: [string, string, number, boolean]

// Get union of all parameter types
type ParamsUnion = Parameters<typeof funcWithManyParameters>[number];
// Type: string | number | boolean

When to Use This Pattern

Role-based access control: Extract all possible actions/permissions
Configuration validation: Get all possible config values
Event systems: Extract all possible event types from an array
Form fields: Get all field names from a fields array

Practical Example: Type-Safe Access Control

const userAccessModel = {
  user: ["update-self", "view"],
  admin: ["create", "update-self", "update-any", "delete", "view"],
  anonymous: ["view"],
} as const;

type Role = keyof typeof userAccessModel;
type Action = (typeof userAccessModel)[Role][number];

const canUserAccess = (role: Role, action: Action): boolean => {
  // Need to cast because TypeScript can't narrow the array type
  return (userAccessModel[role] as ReadonlyArray<Action>).includes(action);
};

// Type-safe usage
canUserAccess("admin", "delete"); // OK
canUserAccess("user", "delete"); // OK at compile time, false at runtime
canUserAccess("admin", "invalid"); // Error: "invalid" is not assignable to Action

Common Pitfalls

Forgetting `as const` on Arrays

// BAD - elements widened to string
const actions = ["view", "edit", "delete"];
type Action = (typeof actions)[number]; // string

// GOOD - literal types preserved
const actions = ["view", "edit", "delete"] as const;
type Action = (typeof actions)[number]; // "view" | "edit" | "delete"

ReadonlyArray Type Mismatch

When using .includes() on readonly arrays, you may need to cast:

const items = ["a", "b", "c"] as const;
type Item = (typeof items)[number];

// Error: Argument of type 'string' is not assignable to parameter of type '"a" | "b" | "c"'
const hasItem = items.includes(someString);

// Solution: Cast the array
const hasItem = (items as ReadonlyArray<Item>).includes(value as Item);

Advanced: Conditional Access

You can combine [number] with conditional types:

type ExtractArrayElements<T> = T extends readonly (infer U)[] ? U : never;

const permissions = ["read", "write", "admin"] as const;
type Permission = ExtractArrayElements<typeof permissions>;
// Type: "read" | "write" | "admin"

Array Index Access with [number]