TypeScript Index Signatures

An index signature types every key of a dictionary with one pattern. A value can hold any number of keys while each stored value keeps one type.

A dictionary built and read

The ScoreBoard type fixes no subject names in advance. Any string key is allowed, and every value must be a number.

interface ScoreBoard {
  [subject: string]: number;
}
const scores: ScoreBoard = { maths: 90, english: 84 };
scores.science = 77;
console.log(scores.maths);
console.log(scores.science);
console.log(scores.english);

A named member must fit the index type

The title member is a string, but the index type allows only numbers. The checker refuses the interface itself.

interface Scores {
  [subject: string]: number;
  title: string;
}

Number keys act as string keys

The same entry is read through a number key and through a string key. Both reads return the stored price.

interface PriceList {
  [item: string]: number;
}
const prices: PriceList = { 10: 250, 20: 400 };
console.log(prices[10]);
console.log(prices["10"]);
console.log(prices["20"]);

Example

interface WordCounts {
  [word: string]: number;
}
function countWords(words: string[]): WordCounts {
  const counts: WordCounts = {};
  for (const word of words) {
    counts[word] = (counts[word] ?? 0) + 1;
  }
  return counts;
}
const result = countWords(["pen", "book", "pen"]);
console.log(result.pen);
console.log(result.book);

Key points

An index signature is a pattern that types every key in an object at once.
Every named member of the type must fit the index type.
Number keys are string keys in disguise, because JavaScript converts them at runtime.
Record<string, number> is the alias spelling of the same dictionary idea.

In this example, the index signature lets countWords create keys as words appear, while every count stays a number.

Note

An index signature controls values by key pattern, not by a fixed list of names.

Try it Yourself

Edit the code, then press Run.

Running is simulated · TypeScript Playground

interface WordCounts {
  [word: string]: number;
}
function countWords(words: string[]): WordCounts {
  const counts: WordCounts = {};
  for (const word of words) {
    counts[word] = (counts[word] ?? 0) + 1;
  }
  return counts;
}
const result = countWords(["pen", "book", "pen"]);
console.log(result.pen);
console.log(result.book);

Exercise

Why does the checker refuse a string member inside a type whose index type is number?

String members are never allowed in interfaces Index signatures only work with arrays Every named member must fit the index type The member name is too short