Index signatures for dynamic keys in Typescript

Introduction

Index signatures let you describe objects with keys you don't know in advance. This helps when you want to store many similar items with different names.

You want to store user settings where each setting name can vary.

You need to keep track of scores for players with unknown player names.

You want to create a dictionary where keys are dynamic and values have the same type.

You are working with JSON data that has flexible keys.

You want to access object properties using variables instead of fixed names.

Syntax

Typescript

interface MyObject {
  [key: string]: ValueType;
}

The key type is usually 'string' or 'number'.

All properties must match the ValueType declared.

Examples

This means any string key will have a number value, like player scores.

Typescript

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

Keys are setting names, values can be string or number.

Typescript

interface Settings {
  [settingName: string]: string | number;
}

Keys are numbers (user IDs), values are ages.

Typescript

const userAges: { [id: string]: number } = {
  101: 25,
  102: 30
};

Sample Program

This example creates a phone book where names are keys and phone numbers are values. It prints the numbers for Alice and Bob.

Typescript

interface PhoneBook {
  [name: string]: string;
}

const contacts: PhoneBook = {
  "Alice": "123-4567",
  "Bob": "987-6543"
};

console.log(contacts["Alice"]);
console.log(contacts["Bob"]);

Important Notes

Index signatures allow any number of properties with matching types.

You cannot mix incompatible types for values when using index signatures.

Using index signatures helps when keys are not fixed or known beforehand.

Summary

Index signatures describe objects with dynamic keys.

They ensure all keys have values of the same type.

Useful for flexible data like dictionaries or maps.