Dynamic Keys

Sometimes, you won't know all of an object's property names in advance. For example, say you're building a customer management system where employees can add custom key/value pairs to customer records:

- favoriteColor: "blue"
- favoriteFood: "pizza"
- favoriteAnimal: "cat"
- etc

You can't know what the user will add ahead of time, but you still want to model the data in your program.

You can define dynamic keys using an index signature:

type UserMetrics = {
  [key: string]: number;
};

This type says "this object can have any number of properties if the keys are strings and the values are numbers."

const metrics: UserMetrics = {
  wordsPerMinute: 50,
  errors: 2,
  timeOnPage: 120,
};

metrics["refreshRate"] = 60; // OK
metrics["theme"] = "dark"; // Error: Type 'string' is not assignable to type 'number'