Index Signatures

TL;DR

An Index Signature is used when you don’t know all the names of a type’s properties, but you do know the shape of the values. It allows objects to act like dictionaries or hash maps.

Mental Model

How It Works

You define an index signature using square brackets.
The key type must be either string, number, symbol, or template literal strings.

Warning: Index signatures are very loose. If you access myDictionary["nonExistentKey"], TypeScript will tell you it returns a number, even though it actually returns undefined at runtime. (Use the noUncheckedIndexedAccess compiler flag to fix this).

Example

// Define an interface with an Index Signature
interface SalaryMap {
    // There can be any string key, but the value MUST be a number
    [employeeName: string]: number;
}

const salaries: SalaryMap = {
    "Alice": 100000,
    "Bob": 85000,
    // "Charlie": "Intern" // ERROR: Type 'string' is not assignable to type 'number'
};

// You can mix specific properties with an index signature!
interface Configuration {
    id: string; // Specific property
    [key: string]: string; // Index signature for remaining properties
}

const config: Configuration = {
    id: "123",
    theme: "dark",
    language: "en"
};

Common Interview Questions

Can an interface have a specific property that conflicts with the index signature?

No. If the index signature says [key: string]: number, you cannot have a property name: string. All specific properties must match the return type of the index signature. If you need mixed types, the index signature must be a union: [key: string]: number | string.

What is the difference between Record<K, V> and an Index Signature?

Record<string, number> is a built-in utility type that under the hood creates an index signature. They are functionally identical for simple use cases. However, Record is much easier to use when mapping over specific union types (e.g., Record<"admin" | "user", boolean>), which is impossible to do directly with a standard interface index signature.